313D Residential Air Conditioning Systems Mechanic — Questions and Answers

Question 1: What is the primary difference between refrigerant 'recovery,' 'recycling,' and 'reclaiming'?

Recycling and reclaiming are both done at certified facilities; recovery is done on-site
They are interchangeable terms with no technical distinction
Recovery=remove from system; recycling=oil separation/filter on-site; reclaiming=AHRI 700 purity at certified facility (Correct answer)
Recovery=AHRI 700 purity; recycling=on-site cleaning; reclaiming=remove from system

Correct answer: Recovery=remove from system; recycling=oil separation/filter on-site; reclaiming=AHRI 700 purity at certified facility

EPA defines recovery as removing refrigerant from a system, recycling as basic on-site cleaning (oil separation, filtering), and reclaiming as reprocessing to AHRI 700 purity at a certified facility.

Question 2: Why is it critical to keep moisture out of recovered R-410A refrigerant?

Moisture raises R-410A's boiling point, reducing system capacity
Moisture causes R-410A to fractionate into its component refrigerants
Moisture increases R-410A's GWP rating
Moisture reacts with POE oil to form acids that damage compressors (Correct answer)

Correct answer: Moisture reacts with POE oil to form acids that damage compressors

Moisture reacts with POE oil at high temperatures to form hydrolysis acids, which corrode metal components and break down the oil's lubricating properties.

Question 3: What action must a technician take under EPA Section 608 if a system with more than 50 pounds of refrigerant is found to be leaking above the applicable threshold rate?

Immediately evacuate all refrigerant and decommission the system
Repair the leak within 30 days or develop a retrofit or retirement plan (Correct answer)
Add refrigerant to compensate for the loss and monitor the system
Report the leak to the EPA within 24 hours

Correct answer: Repair the leak within 30 days or develop a retrofit or retirement plan

EPA Section 608 requires that leaks exceeding the threshold be repaired within 30 days of discovery; if repair is not feasible, a retrofit or retirement plan must be developed within 30 days.

Question 4: What is the key difference between sensible heat and latent heat in air conditioning?

There is no practical difference — both describe the same heat transfer mechanism
Sensible heat causes a measurable temperature change; latent heat involves phase change without temperature change (Correct answer)
Sensible heat relates to refrigerant flow; latent heat relates to air velocity
Sensible heat is removed at the condenser; latent heat is added at the compressor

Correct answer: Sensible heat causes a measurable temperature change; latent heat involves phase change without temperature change

Sensible heat changes the temperature of air (measurable with a thermometer), while latent heat is absorbed or released during phase changes like evaporation or condensation.

Question 5: A large wet-bulb depression (big difference between dry-bulb and wet-bulb temperatures) indicates what about the air?

The supply air temperature leaving the coil is too high
The system refrigerant charge is low and needs recharging
The air is relatively dry with low relative humidity (Correct answer)
The air is nearly saturated and close to 100% relative humidity

Correct answer: The air is relatively dry with low relative humidity

A large wet-bulb depression means significant evaporation is occurring from the wet wick, which only happens when the surrounding air has a low relative humidity.

Question 6: A technician servicing a residential split system needs to replace a leaking Schrader core on the suction line. They briefly vent a small amount of R-410A refrigerant to the atmosphere before replacing the core. According to Canadian federal and provincial regulations, what is the status of this action?

It is a violation of regulations, as any intentional venting of HFC refrigerants is prohibited. (Correct answer)
It is permissible as a 'de minimis' release necessary for service.
It is allowed only for certified technicians during emergency repairs.
It is acceptable as long as it is documented in the service log and is under 50 grams.

Correct answer: It is a violation of regulations, as any intentional venting of HFC refrigerants is prohibited.

Federal Halocarbon Regulations and provincial rules, such as Ontario's O. Reg. 463/10, strictly prohibit the intentional release of HFC refrigerants like R-410A into the environment. There is no provision that allows for 'de minimis' or small intentional releases during service; proper recovery procedures must always be used.

Question 7: A residential AC system is running, and the compressor is functioning correctly. What is the expected state of the refrigerant as it enters the compressor?

High-pressure, saturated liquid/vapor mix
Low-pressure, subcooled liquid
Low-pressure, superheated vapor (Correct answer)
High-pressure, superheated vapor

Correct answer: Low-pressure, superheated vapor

The refrigerant leaves the evaporator as a low-pressure vapor. It should be slightly superheated, meaning its temperature is above its boiling point, to ensure no liquid refrigerant enters and damages the compressor.

Question 8: Which electrical test would BEST confirm that a compressor motor winding has a ground fault?

Measure resistance between each winding terminal and the compressor shell/ground (Correct answer)
Measure voltage across the run capacitor terminals
Measure current draw with a clamp meter during startup
Measure resistance between the two run terminals

Correct answer: Measure resistance between each winding terminal and the compressor shell/ground

A megohmmeter or ohmmeter reading between a winding terminal and the grounded shell detects insulation breakdown indicating a ground fault.

Question 9: What is the primary function of a humidistat in a residential HVAC system?

To measure the temperature differential across the evaporator coil
To regulate blower motor speed based on duct static pressure
To measure refrigerant pressure on the suction line
To sense indoor relative humidity and signal the system to humidify or dehumidify (Correct answer)

Correct answer: To sense indoor relative humidity and signal the system to humidify or dehumidify

A humidistat detects relative humidity and cycles humidification or dehumidification equipment to maintain the set humidity level, functioning like a thermostat but for moisture.

Question 10: What does SEER stand for and what does it measure?

System Electrical Efficiency Review — compressor amp draw
Seasonal Evaporator Evaporation Rate — pounds of water removed per hour
Seasonal Energy Efficiency Ratio — cooling output per unit of electrical energy input over a season (Correct answer)
Standard Equipment Efficiency Rating — maximum BTU capacity

Correct answer: Seasonal Energy Efficiency Ratio — cooling output per unit of electrical energy input over a season

SEER measures the total cooling output in BTUs divided by the total electrical energy consumed in watt-hours over a typical cooling season.

Question 11: Which type of metering device maintains a constant evaporator outlet superheat regardless of load changes?

Capillary tube
Float valve
Thermostatic expansion valve (TXV) (Correct answer)
Fixed orifice

Correct answer: Thermostatic expansion valve (TXV)

A TXV uses a sensing bulb to modulate refrigerant flow and maintain a nearly constant superheat at the evaporator outlet across varying load conditions.

Question 12: When calculating latent heat loads, which source contributes the most latent load in a typical residential kitchen?

Window solar gain
Computers and electronics
Lighting fixtures
Cooking appliances and occupants (Correct answer)

Correct answer: Cooking appliances and occupants

Cooking appliances release significant steam and moisture, and occupants also produce latent heat through perspiration and respiration.

Question 13: A technician discovers a refrigerant leak during routine service and the system charge is less than 50 lbs. Under EPA Section 608, the technician must:

Inform the equipment owner but no EPA reporting is required for systems under 50 lbs (Correct answer)
Evacuate and recover all refrigerant immediately regardless of leak rate
Report the leak to the EPA within 24 hours
Repair the leak within 30 days and document the repair

Correct answer: Inform the equipment owner but no EPA reporting is required for systems under 50 lbs

EPA Section 608 mandatory leak repair requirements only apply to appliances with 50 or more pounds of refrigerant charge; smaller systems have no federal repair mandate.

Question 14: A technician measures a compressor drawing high amperage with low suction and high discharge pressure. What is the most likely diagnosis?

Refrigerant undercharge
Failed condenser fan motor
Refrigerant overcharge (Correct answer)
Liquid slugging

Correct answer: Refrigerant overcharge

Refrigerant overcharge increases system pressures and compressor workload, resulting in elevated amperage draw.

Question 15: During refrigerant recovery, what does a rising pressure on the recovery machine's discharge gauge while the suction side is in deep vacuum most likely indicate?

The recovery machine compressor is overheating
Non-condensable gases (air) are present in the recovered refrigerant (Correct answer)
The system expansion valve is restricting refrigerant flow
The recovery cylinder is nearly full

Correct answer: Non-condensable gases (air) are present in the recovered refrigerant

Non-condensable gases like air do not condense at normal recovery temperatures, causing discharge pressure to rise even after the suction side reaches deep vacuum.

Question 16: What does the AHRI Certification Program verify for residential air conditioning equipment?

Technician licensing requirements
Published capacity and efficiency ratings are accurate (Correct answer)
Refrigerant purity levels
Installation code compliance

Correct answer: Published capacity and efficiency ratings are accurate

The AHRI Certification Program independently tests and verifies that manufacturers' published capacity and efficiency ratings match actual performance.

Question 17: During a service call, a homeowner reports the system runs continuously but never reaches setpoint. The evaporator coil is partially frosted. What should the technician check first?

Condenser fan motor
Compressor amperage draw
Airflow across the evaporator coil (Correct answer)
Thermostat calibration

Correct answer: Airflow across the evaporator coil

Restricted airflow is the most common cause of evaporator coil frosting and inadequate cooling, and should be checked before electrical components.

Question 18: Which principle explains the heat absorption in the evaporator coil?

Latent heat transfer during phase change (Correct answer)
Adiabatic cooling
Thermal conduction
Sensible heat transfer

Correct answer: Latent heat transfer during phase change

The evaporator coil cools the air primarily through latent heat transfer. As the liquid refrigerant inside the coil absorbs heat from the indoor air, it changes phase from a liquid to a gas (vaporizes) without a significant change in its own temperature. This phase change efficiently removes a large amount of heat from the surrounding air.

Question 19: A technician notices the system cools adequately in the morning but loses capacity significantly in the afternoon heat. Which diagnosis is most consistent with this behavior?

Low refrigerant charge
Intermittent thermostat fault
Dirty evaporator coil
Dirty or blocked condenser coil (Correct answer)

Correct answer: Dirty or blocked condenser coil

A partially blocked condenser coil can handle lighter loads in cooler morning temperatures but cannot reject adequate heat when ambient temperatures peak in the afternoon.

Question 20: What does a high-pressure switch do in the electrical circuit of a residential air conditioner?

Closes the contactor at startup
Regulates compressor motor speed
Opens the control circuit if refrigerant pressure exceeds a safe limit (Correct answer)
Monitors low-side suction pressure

Correct answer: Opens the control circuit if refrigerant pressure exceeds a safe limit

A high-pressure switch is a normally closed safety device that opens the control circuit—de-energizing the compressor—when discharge pressure exceeds a preset limit.

Question 21: Why is it important to level the outdoor condenser unit during installation?

To allow proper airflow
To avoid refrigerant leaks
To align the refrigerant lines
To ensure proper drainage and reduce compressor strain (Correct answer)

Correct answer: To ensure proper drainage and reduce compressor strain

Leveling the outdoor condenser unit is important for several reasons. It ensures proper drainage of condensate, preventing water accumulation that could lead to rust or ice formation. More critically, it helps the compressor operate efficiently and without undue strain, as an unlevel unit can cause oil to pool improperly, potentially shortening the compressor's lifespan.

Question 22: While diagnosing a system, a technician observes a very high superheat reading at the evaporator outlet. Which of the following is a potential cause?

Restricted liquid line filter drier (Correct answer)
Non-condensables in the system
Dirty condenser coil
Refrigerant overcharge

Correct answer: Restricted liquid line filter drier

A restricted liquid line filter drier will starve the metering device of refrigerant. This causes the refrigerant to flash off too early in the evaporator, leading to an excessive amount of superheat as the vapor travels through the remainder of the coil.

Question 23: A run capacitor shown in a schematic is connected in series with which motor winding?

Neither winding — it is connected across the power supply
Main run winding
Start winding (auxiliary winding) (Correct answer)
Both windings simultaneously

Correct answer: Start winding (auxiliary winding)

The run capacitor is wired in series with the start (auxiliary) winding to create a phase shift that maintains running torque and efficiency in single-phase motors.

Question 24: In load calculations, what does 'sensible heat' refer to?

Heat that changes the moisture content of air
Heat transferred by radiation from the sun
Total heat content of air including moisture
Heat that changes air temperature without changing moisture content (Correct answer)

Correct answer: Heat that changes air temperature without changing moisture content

Sensible heat causes a measurable temperature change in the air without altering its moisture (humidity) content.

Question 25: What is the required minimum depth of a P-trap installed on a condensate drain line for an air handler with 0.5 inches of static pressure in the negative pressure section?

0.5 inches
2 inches
3 inches
1 inch (Correct answer)

Correct answer: 1 inch

The trap depth must exceed the static pressure to prevent air bypass; for 0.5 inches of negative pressure, a minimum 1-inch trap depth is required to maintain the water seal.

Question 26: During a maintenance visit, a technician finds the evaporator coil heavily coated with mold and biological growth. What is the recommended maintenance procedure?

Clean the coil with an EPA-registered coil cleaner and treat with an antimicrobial solution (Correct answer)
Run the system at maximum cooling to freeze off the growth
Spray the coil with bleach and rinse with a garden hose
Replace the entire air handler immediately

Correct answer: Clean the coil with an EPA-registered coil cleaner and treat with an antimicrobial solution

Evaporator coils with mold growth should be cleaned with an EPA-registered coil cleaner and treated with an approved antimicrobial solution to eliminate biological contamination safely.

Question 27: What indoor relative humidity range does ASHRAE recommend for occupant comfort in residential buildings?

30–60% (Correct answer)
70–80%
85–95%
10–20%

Correct answer: 30–60%

ASHRAE recommends maintaining indoor RH between 30% and 60% to ensure occupant comfort and prevent mold growth or dry-air discomfort.

Question 28: On a schematic, the thermostat wires are typically represented by which of the following?

Dotted lines connecting to the refrigerant piping.
Thin, solid lines connecting low-voltage components like the thermostat, control board, and contactor. (Correct answer)
A single line labeled 'COMM' for communication.
Heavy, solid lines indicating high voltage.

Correct answer: Thin, solid lines connecting low-voltage components like the thermostat, control board, and contactor.

Schematics use different line types to represent different things. Low-voltage control wiring, such as that running from the thermostat to the air handler and condenser, is typically shown as thin, solid lines to differentiate it from the thicker lines used for high-voltage power wiring.

Question 29: A residential system is equipped with a TXV (thermostatic expansion valve). The technician finds high superheat and low suction pressure. What should be inspected?

Compressor discharge valve
Condenser inlet temperature
Discharge line temperature
TXV bulb mounting and charge, and refrigerant charge level (Correct answer)

Correct answer: TXV bulb mounting and charge, and refrigerant charge level

A TXV that is underfeeding the evaporator (due to a faulty bulb, lost bulb charge, or low system charge) causes high superheat and low suction pressure.

Question 30: What is the purpose of the crankcase heater on a compressor?

To maintain discharge gas temperature above ambient
To warm refrigerant vapor before compression
To prevent refrigerant migration and oil dilution in the crankcase during off cycles (Correct answer)
To heat the suction line during low ambient operation

Correct answer: To prevent refrigerant migration and oil dilution in the crankcase during off cycles

The crankcase heater keeps the oil warm so refrigerant does not migrate into and dilute the compressor oil during the off cycle, protecting bearings on startup.

Question 31: On blueprints, what does the scale '1/4" = 1'-0"' mean?

One quarter inch on the drawing equals one foot in reality (Correct answer)
The wall thickness is one quarter inch
The drawing is one-quarter actual size overall
One foot on the drawing equals a quarter inch in reality

Correct answer: One quarter inch on the drawing equals one foot in reality

A scale of 1/4" = 1'-0" means every quarter inch measured on the drawing represents one actual foot in the field.

Question 32: A kinked refrigerant line during installation should be:

Heated and straightened with a torch
Cut out and replaced with a new section of properly routed tubing (Correct answer)
Brazed over to reinforce the kink
Straightened carefully with pliers and reused

Correct answer: Cut out and replaced with a new section of properly routed tubing

A kinked tube has a weakened wall that can fail under pressure, so the kinked section must be cut out and replaced with new tubing.

Question 33: Why is refrigerant with a low boiling point ideal for air conditioning systems?

It ensures efficient heat absorption at room temperature (Correct answer)
It prevents overcooling
It condenses easily in the evaporator
It minimizes energy use

Correct answer: It ensures efficient heat absorption at room temperature

Refrigerants with low boiling points are ideal because they can easily vaporize and absorb heat from relatively cool indoor air (around room temperature). This phase change from liquid to gas within the evaporator coil is how the system efficiently extracts heat from the living space. A higher boiling point would mean the refrigerant wouldn't vaporize effectively at typical indoor temperatures.

Question 34: When routing refrigerant lines through a wall or floor, the technician should:

Leave lines bare to allow visual inspection
Wrap lines tightly with duct tape only
Install a protective sleeve or grommet to prevent abrasion (Correct answer)
Solder lines to the framing for stability

Correct answer: Install a protective sleeve or grommet to prevent abrasion

A sleeve or grommet protects the refrigerant lines from abrasion against sharp edges, which can cause premature leaks.

Question 35: What does an elevated wet-bulb temperature at the evaporator inlet indicate?

Low indoor humidity
High indoor humidity or high sensible heat ratio (Correct answer)
Blocked condensate drain
Refrigerant overcharge in the evaporator

Correct answer: High indoor humidity or high sensible heat ratio

A higher wet-bulb temperature at the coil inlet indicates more moisture (latent heat) in the air, increasing the total load on the evaporator.

Question 36: A homeowner complains that one bedroom is consistently warmer than others in cooling season. The technician measures lower-than-design CFM at that register. What is the FIRST component to inspect?

The duct run serving that room for restrictions, kinks, or disconnections (Correct answer)
The air handler evaporator coil
The condenser unit refrigerant charge
The thermostat calibration

Correct answer: The duct run serving that room for restrictions, kinks, or disconnections

Low airflow at a specific register most commonly results from a duct restriction, kink, or disconnection in the branch serving that room.

Question 37: What is the bypass factor of a cooling coil?

The fraction of airflow that passes through the coil without making effective thermal contact (Correct answer)
The amount of refrigerant bypassed around the expansion valve at part load
The heat transfer rate through the outer casing of the air handler
The ratio of air that bypasses the filter media to total airflow

Correct answer: The fraction of airflow that passes through the coil without making effective thermal contact

Bypass factor is the fraction of air that passes through the coil without meaningfully contacting the coil surface, reducing cooling and dehumidification effectiveness.

Question 38: A technician is diagnosing a no-cool situation. The thermostat is calling for cooling, and the low-voltage fuse on the indoor unit's control board is blown. What is a common cause for this issue?

A stuck-closed contactor.
A short circuit in the low-voltage wiring between the indoor and outdoor units. (Correct answer)
An overcharged refrigerant system.
A clogged air filter.

Correct answer: A short circuit in the low-voltage wiring between the indoor and outdoor units.

A blown low-voltage fuse indicates an overcurrent condition in the control circuit. This is frequently caused by a short to ground, where a low-voltage wire (like the 'Y' or 'C' wire) has its insulation damaged and is touching the chassis or another wire, creating a direct path for current to flow, which blows the protective fuse.

Question 39: What does a megohmmeter (megger) test on a compressor motor measure?

Supply voltage quality
Winding resistance in ohms
Insulation resistance between windings and ground (Correct answer)
Capacitor microfarad rating

Correct answer: Insulation resistance between windings and ground

A megohmmeter applies high DC voltage to measure insulation resistance, revealing moisture or degradation in motor windings.

Question 40: Which document must an HVAC technician have to legally purchase refrigerants regulated under EPA Section 608?

A manufacturer's authorization letter
A state contractor's license
An EPA Section 608 technician certification (Correct answer)
An OSHA 10-hour card

Correct answer: An EPA Section 608 technician certification

EPA Section 608 requires technicians who purchase refrigerants in containers larger than 2 lbs to hold a valid Section 608 certification.

Question 41: Which component in a residential AC system provides low-voltage power to the thermostat and control board?

The outdoor unit contactor
The 24V step-down control transformer (Correct answer)
The run capacitor
The main disconnect switch

Correct answer: The 24V step-down control transformer

The control transformer steps 120V or 240V primary voltage down to 24V AC to power the thermostat, control board, and relay coils.

Question 42: What is the correct method for charging a new R-410A system that was evacuated and requires a full charge per manufacturer nameplate?

Charge liquid refrigerant through the high-side service port with the system off (Correct answer)
Charge liquid through the low-side port slowly
Charge vapor into the suction service port with the system running
Charge vapor through the high-side port until target weight is reached

Correct answer: Charge liquid refrigerant through the high-side service port with the system off

R-410A is a near-azeotrope that must be charged as liquid to maintain proper composition; liquid is introduced through the high-side with the system off for a full initial charge.

Question 43: What does the term 'psychrometrics' refer to in HVAC?

The study of thermodynamic properties of moist air (Correct answer)
The analysis of duct static pressure losses
The measurement of refrigerant pressures in a system
The study of chemical reactions in refrigerants

Correct answer: The study of thermodynamic properties of moist air

Psychrometrics is the engineering science that studies the thermodynamic properties of moist air — mixtures of dry air and water vapor.

Question 44: Which of the following is a critical reason for installing a P-trap at the base of a long vertical suction line riser in a split-system air conditioner installation?

To prevent liquid refrigerant from flooding the evaporator coil on startup.
To provide a location for future system access and service.
To reduce unwanted noise and vibration during operation.
To ensure proper oil return to the compressor. (Correct answer)

Correct answer: To ensure proper oil return to the compressor.

During operation, a small amount of compressor oil circulates with the refrigerant. In a vertical suction line, gravity can work against the flow of this oil back to the compressor. A P-trap at the base of the riser collects oil, which is then pushed up the riser in slugs by the refrigerant vapor velocity, ensuring the compressor remains properly lubricated. Lack of proper oil return can lead to compressor failure.

Question 45: A technician uses a self-sealing valve recovery cylinder. What is the key safety advantage of this cylinder design?

It eliminates the need for a pressure relief valve
The valve automatically closes when the hose is disconnected, preventing accidental release (Correct answer)
It allows mixing of different refrigerant types safely
It allows refrigerant to be vented safely if overfilled

Correct answer: The valve automatically closes when the hose is disconnected, preventing accidental release

Self-sealing (Schrader-type) valve cylinders automatically close when the service hose is removed, preventing refrigerant escape during connection and disconnection.

Question 46: During system commissioning, measured suction pressure is within range, but subcooling is 3°F when the spec calls for 10–15°F. What does this indicate?

Refrigerant undercharge (Correct answer)
Condenser coil is too efficient
Refrigerant overcharge
Restriction before the metering device

Correct answer: Refrigerant undercharge

Low subcooling indicates insufficient liquid refrigerant in the condenser, which is a sign of refrigerant undercharge.

Question 47: When transferring liquid refrigerant from a recovery cylinder to a system, the technician should:

Invert the cylinder or use the liquid valve to push liquid into the low side (Correct answer)
Connect the cylinder liquid port to the system low side with the cylinder upright
Use the recovery machine in reverse to push liquid under pressure
Connect the cylinder vapor port to the system high side

Correct answer: Invert the cylinder or use the liquid valve to push liquid into the low side

Inverting the cylinder or using the dip-tube liquid valve ensures liquid refrigerant flows out; connecting to the high side helps push the charge into the system.

Question 48: A residential thermostat's R terminal is energized and the Y terminal is calling for cooling, but the contactor does not pull in. Voltage measured at the contactor coil terminals is 0V. The MOST likely cause is:

A failed contactor coil
Low refrigerant pressure
An open safety switch or broken wire in the Y control circuit (Correct answer)
A bad thermostat subbase

Correct answer: An open safety switch or broken wire in the Y control circuit

Zero volts at the coil means the 24V signal never reached the contactor, pointing to an open in the control circuit such as a tripped safety switch.

Question 49: A motor nameplate shows FLA = 8A and SF = 1.15. What does the service factor (SF) indicate?

The motor efficiency at full load is 115%
The motor starts at 115% of rated voltage
The motor requires a capacitor rated 15% above the FLA
The motor can be operated continuously at 115% of its rated load without damage (Correct answer)

Correct answer: The motor can be operated continuously at 115% of its rated load without damage

A service factor of 1.15 means the motor can handle 15% above its rated load on a continuous basis without exceeding its thermal limits.

Question 50: Under current EPA venting prohibition rules, which substance may still legally be vented to atmosphere under specific conditions?

Nitrogen used to pressurize a system for leak testing (Correct answer)
R-22
R-134a
R-410A

Correct answer: Nitrogen used to pressurize a system for leak testing

Nitrogen is not a refrigerant and is not subject to the EPA venting prohibition; it may be vented after leak testing or pressurization procedures.

Question 51: How many grains of moisture are in one pound of water?

1,000 grains
7,000 grains (Correct answer)
10,000 grains
5,040 grains

Correct answer: 7,000 grains

There are exactly 7,000 grains per pound of water; HVAC technicians express air moisture content in grains per pound (GPP) of dry air.

Question 52: When charging an R-410A system, the refrigerant cylinder must be kept upright or inverted so that:

Liquid is drawn from the cylinder because R-410A is a near-azeotrope blend (Correct answer)
The pressure gauge reads accurately at all times
Vapor is drawn from the cylinder to avoid liquid slugging the compressor
The desiccant in the cylinder stays active

Correct answer: Liquid is drawn from the cylinder because R-410A is a near-azeotrope blend

R-410A is a near-azeotropic blend that must be charged as a liquid to maintain the correct refrigerant composition ratio.

Question 53: What is the purpose of a transformer in the residential AC control circuit?

To step up 24V to 240V for the compressor
To convert AC power to DC for the control board
To step down 240V line voltage to 24V for the low-voltage control circuit (Correct answer)
To regulate current flow to the condenser fan motor

Correct answer: To step down 240V line voltage to 24V for the low-voltage control circuit

The control transformer steps down 240V (or 120V) line voltage to 24V AC, which powers the thermostat and control circuit components safely.

Question 54: Under what condition does condensation form on interior window surfaces?

Whenever indoor relative humidity exceeds 50%
Only when outdoor temperatures exceed indoor temperatures
Whenever indoor relative humidity exceeds 30%
When the window surface temperature drops below the dew point of the indoor air (Correct answer)

Correct answer: When the window surface temperature drops below the dew point of the indoor air

Condensation forms on any surface whose temperature falls below the dew point of the surrounding air, regardless of a single fixed RH percentage.

Question 55: What is the purpose of a duct transition fitting that changes from a rectangular trunk to a round branch?

To filter the air before distribution
To smoothly convert airflow from one duct geometry to another with minimal pressure loss (Correct answer)
To add an access point for duct cleaning
To increase system static pressure

Correct answer: To smoothly convert airflow from one duct geometry to another with minimal pressure loss

Transition fittings are designed to convert duct geometry while minimizing turbulence and pressure drop through gradual shape changes.

Question 56: The liquid line in a residential AC system is typically NOT insulated because:

It carries high-pressure vapor that needs to dissipate heat
The liquid refrigerant temperature is near ambient and condensation is not a concern (Correct answer)
Insulation would restrict refrigerant flow
Code prohibits insulating liquid lines

Correct answer: The liquid refrigerant temperature is near ambient and condensation is not a concern

The liquid line carries subcooled refrigerant near ambient temperature, so condensation does not form and insulation provides negligible benefit.

Question 57: What is the primary purpose of adding superheat to the refrigerant vapor before it leaves the evaporator?

To ensure only vapor enters the compressor, preventing damage. (Correct answer)
To increase the efficiency of the condenser coil.
To increase the cooling capacity of the system.
To lower the pressure of the refrigerant before condensation.

Correct answer: To ensure only vapor enters the compressor, preventing damage.

Superheat is the heat added to the refrigerant after it has completely boiled into a vapor. Its main purpose is to ensure that no liquid droplets travel back to the compressor, as liquid cannot be compressed and can cause severe mechanical damage (slugging).

Question 58: Which of the following is a safety control designed to shut down the compressor if the refrigerant pressure on the high side becomes dangerously high?

High-pressure switch (Correct answer)
Thermostatic expansion valve (TXV)
Crankcase heater
Low-pressure switch

Correct answer: High-pressure switch

The high-pressure switch is a safety device that monitors the discharge pressure of the compressor. If the pressure exceeds a preset limit (often due to a dirty condenser coil or failed fan motor), the switch opens the control circuit, de-energizing the contactor and stopping the compressor to prevent catastrophic damage.

Question 59: Before replacing a faulty blower motor in a residential air handler, which of the following represents the most critical and complete safety procedure?

Isolate, lock out, and tag the circuit breaker controlling the air handler. (Correct answer)
Wear insulated gloves and use non-conductive tools.
Turn off the circuit breaker for the air handler at the main electrical panel.
Turn off the thermostat and pull the outdoor disconnect.

Correct answer: Isolate, lock out, and tag the circuit breaker controlling the air handler.

The most critical safety procedure is to de-energize the equipment and ensure it cannot be accidentally re-energized. This is accomplished by turning off the correct circuit breaker, applying a personal lock (lockout), and attaching a tag (tagout) to inform others that the circuit is being serviced. This prevents accidental startup and electrical shock.

Question 60: What is the primary purpose of a metering device in a refrigeration circuit?

To transfer heat from the refrigerant to outdoor air
To compress the refrigerant vapor
To reduce refrigerant pressure and control flow into the evaporator (Correct answer)
To filter moisture from the refrigerant

Correct answer: To reduce refrigerant pressure and control flow into the evaporator

The metering device drops refrigerant pressure and regulates flow into the evaporator, enabling it to absorb heat effectively.

Question 61: After a thunderstorm, a residential system has no cooling. The indoor fan operates, but the outdoor unit does not run. What is the most likely first item to check?

Refrigerant charge
Outdoor unit disconnect and fuses or circuit breaker (Correct answer)
Thermostat wiring
Contactor coil voltage

Correct answer: Outdoor unit disconnect and fuses or circuit breaker

Power surges from storms frequently blow fuses or trip breakers to the outdoor unit, making electrical supply the first item to verify.

Question 62: A residential split system has the evaporator coil located in the:

Compressor compartment
Outdoor condenser cabinet
Indoor air handler or furnace plenum (Correct answer)
Condensate drain pan only

Correct answer: Indoor air handler or furnace plenum

In a split system, the evaporator coil is installed indoors in the air handler or above the furnace to cool circulating air.

Question 63: When a residential air conditioner produces adequate cooling but the homeowner reports unusually high humidity indoors, what should the technician evaluate?

Compressor valve efficiency
Condenser fan blade pitch
Refrigerant charge level
Evaporator coil temperature and airflow rate (Correct answer)

Correct answer: Evaporator coil temperature and airflow rate

Excessive airflow or elevated evaporator coil temperature reduces the system's ability to condense and drain moisture from the air.

Question 64: Which safety standard focuses on preventing exposure to hazardous substances in the workplace?

OSHA Hazard Communication Standard (Correct answer)
EPA Clean Air Act
NFPA 90A
ASHRAE Standard 90.1

Correct answer: OSHA Hazard Communication Standard

The OSHA Hazard Communication Standard (HCS) requires employers to provide information and training to employees about hazardous chemicals they may encounter in the workplace. This includes labeling containers, providing Safety Data Sheets (SDS), and training workers on safe handling, storage, and emergency procedures. Its primary goal is to ensure employees understand the risks associated with chemicals and how to protect themselves from exposure.

Question 65: What is the correct procedure for sealing a connection between a flexible duct collar and a sheet metal fitting?

Use duct tape rated for duct use (UL 181B-FX) and pull the outer jacket over the collar (Correct answer)
Apply spray foam insulation around the collar and cover with foil tape
Self-tapping screws alone are sufficient
Use zip ties only; tape is not code-compliant

Correct answer: Use duct tape rated for duct use (UL 181B-FX) and pull the outer jacket over the collar

UL 181B-FX tape must be used to seal the inner liner to the collar, and the outer jacket and insulation should then be pulled over and secured with a tie or clamp.

Question 66: A homeowner reports the AC runs but does not cool. You measure 240V at the contactor load side and hear the compressor hum briefly then shut off. The MOST likely cause is:

Low refrigerant charge
Open start capacitor (Correct answer)
Faulty thermostat
Shorted run capacitor

Correct answer: Open start capacitor

A failed start capacitor prevents the start winding from producing the torque needed to start the compressor, causing it to hum and trip on thermal overload.

Question 67: What happens to the boiling point of a refrigerant when pressure increases?

The boiling point increases (Correct answer)
The refrigerant solidifies
The boiling point decreases
The boiling point stays the same

Correct answer: The boiling point increases

Higher pressure raises the saturation (boiling) temperature of a refrigerant, which is why condenser pressures are higher than evaporator pressures.

Question 68: A technician suspects that a system has been contaminated with non-condensables, such as air. Which of the following is the most reliable method to confirm this diagnosis?

Measuring a very low amp draw on the compressor while the unit is running.
Noticing that the suction line is colder than usual.
Comparing the condenser's saturation temperature (from the gauge) to the ambient temperature after the system has been off for several hours. (Correct answer)
Observing an unusually high superheat reading at the evaporator outlet.

Correct answer: Comparing the condenser's saturation temperature (from the gauge) to the ambient temperature after the system has been off for several hours.

After a system has been off long enough for its temperature to equalize with the surrounding air, the pressure inside should correspond to the saturation pressure of the pure refrigerant at that ambient temperature. Non-condensables exert their own partial pressure, resulting in a total system pressure that is higher than the refrigerant's saturation pressure for that ambient temperature. This discrepancy is a definitive sign of contamination.

Question 69: A residential AC system uses a dual run capacitor. Which terminals are typically labeled on this component?

Start, run, and common
High, low, and neutral
C (common), FAN, HERM (Correct answer)
L1, L2, and ground

Correct answer: C (common), FAN, HERM

A dual run capacitor has three terminals: C (common), HERM (compressor/hermetic), and FAN (condenser fan motor).

Question 70: What does a high superheat AND high subcooling reading together most likely indicate?

Refrigerant overcharge
Refrigerant undercharge
Restriction in the liquid line or metering device (Correct answer)
Compressor valve failure

Correct answer: Restriction in the liquid line or metering device

High subcooling indicates liquid refrigerant is backing up before a restriction, while high superheat indicates insufficient refrigerant reaching the evaporator.

Question 71: A residential system has correct refrigerant charge but the supply air temperature differential (delta-T) is only 10°F instead of the expected 16–22°F. What is the most likely cause?

Overcharge of refrigerant
Excessive airflow across the evaporator (Correct answer)
Liquid line restriction
Refrigerant undercharge

Correct answer: Excessive airflow across the evaporator

Too much airflow across the evaporator reduces the time the air has to exchange heat, resulting in a lower temperature differential.

Question 72: What happens to a run capacitor's capacitance value as it ages and begins to fail?

It increases above its rated MFD
It remains stable but its voltage rating drops
It decreases below its rated MFD (Correct answer)
It fluctuates randomly between high and low values

Correct answer: It decreases below its rated MFD

As a run capacitor ages, its dielectric deteriorates and capacitance decreases, reducing its ability to maintain motor efficiency and causing overheating.

Question 73: A technician is diagnosing a system and finds the measured evaporator saturation temperature is 45°F but the suction line temperature at the evaporator outlet is 75°F. What is the superheat?

75°F
30°F (Correct answer)
45°F
20°F

Correct answer: 30°F

Superheat equals the actual suction line temperature minus the saturation (boiling) temperature: 75°F − 45°F = 30°F.

Question 74: Which of the following refrigerants is classified as an A2L (mildly flammable) refrigerant?

R-134a
R-410A
R-22
R-32 (Correct answer)

Correct answer: R-32

R-32 is classified A2L — lower flammability limit above 3.5% by volume and a burning velocity ≤10 cm/s — unlike the A1 refrigerants R-22 and R-410A.

Question 75: During equipment startup, the compressor short-cycles off on the high-pressure switch after 2 minutes. What should the technician check FIRST?

Condenser fan motor operation and airflow restriction (Correct answer)
Thermostat set point
Refrigerant charge level
Suction line insulation integrity

Correct answer: Condenser fan motor operation and airflow restriction

Condenser fan failure or blocked airflow is the most common cause of high-pressure trips because discharge heat cannot be rejected, causing pressure to spike.

Question 76: In rectangular duct design, what does the term 'aspect ratio' describe?

The ratio of the duct's total length to its total CFM.
The ratio of supply airflow to return airflow.
The ratio of static pressure to velocity pressure.
The ratio of the duct's longer side to its shorter side. (Correct answer)

Correct answer: The ratio of the duct's longer side to its shorter side.

Aspect ratio is a fundamental term in duct design that refers to the ratio of the long side to the short side of a rectangular duct. For example, a 16x8 inch duct has an aspect ratio of 2:1. Lower aspect ratios (closer to 1:1, or square) are generally more efficient for airflow and minimize friction loss, while very high aspect ratios can lead to increased static pressure and potential for noise.

Question 77: A technician uses a clamp meter to measure compressor amperage. The reading is 20% below the rated load amps (RLA). What does this suggest?

The system is undercharged with refrigerant
The capacitor has failed
The compressor is healthy and operating efficiently
The compressor has internal valve leakage reducing workload (Correct answer)

Correct answer: The compressor has internal valve leakage reducing workload

A compressor drawing significantly less than its RLA may indicate worn or leaking internal valves that reduce compression efficiency.

Question 78: A technician is preparing to recover refrigerant from a residential split system. To significantly increase the speed of the recovery process, which of the following actions is most effective?

Recovering only in the vapor phase to protect the recovery machine.
Ensuring the recovery machine is placed above the outdoor unit.
Removing the Schrader valve cores from the service ports. (Correct answer)
Using the longest possible hoses to reach the service vehicle.

Correct answer: Removing the Schrader valve cores from the service ports.

The Schrader valve core is the most significant restriction in the service port. Removing it with a core removal tool before starting the recovery process dramatically increases the flow rate of both liquid and vapor refrigerant, significantly speeding up the job.

Question 79: Which of the following components is responsible for creating a pressure differential that allows the refrigerant to change from a high-pressure liquid to a low-pressure liquid?

Evaporator
Condenser
Metering Device (Correct answer)
Compressor

Correct answer: Metering Device

The metering device (such as a TXV or capillary tube) creates a restriction in the flow of refrigerant. This restriction causes a significant drop in pressure, which in turn lowers the refrigerant's boiling point before it enters the evaporator.

Question 80: A technician suspects a refrigerant leak. After recovering and recharging, what is the correct procedure before returning the system to service?

Immediately evacuate to 500 microns and recharge
Pressurize with nitrogen and perform a leak test (Correct answer)
Add UV dye and charge with refrigerant to find the leak location
Run the system and monitor pressures for 30 minutes

Correct answer: Pressurize with nitrogen and perform a leak test

After any repair, the system must be pressure-tested with nitrogen to confirm the repair is leak-free before evacuation and recharge.

Question 81: What is the primary purpose of a capacitor in an air conditioning system?

To stabilize voltage fluctuations
To reduce energy consumption
To provide a start-up boost for motors (Correct answer)
To convert AC to DC

Correct answer: To provide a start-up boost for motors

Capacitors in air conditioning systems, particularly start and run capacitors, store electrical energy and release it to provide an initial surge of power to motors like the compressor and fan motors. This boost helps overcome the initial inertia and allows the motors to start efficiently and run smoothly. Without them, motors might struggle to start or draw excessive current.

Question 82: What is the purpose of an anti-short-cycle timer integrated into a modern AC control board?

To prevent the blower from starting before the compressor
To limit the maximum run time of the compressor per hour
To delay thermostat response during setpoint changes
To enforce a minimum off-time between compressor starts to protect it from pressure equalization issues (Correct answer)

Correct answer: To enforce a minimum off-time between compressor starts to protect it from pressure equalization issues

An anti-short-cycle timer ensures the compressor remains off for a minimum period (typically 3–5 minutes) before restarting, allowing refrigerant pressures to equalize and preventing motor damage.

Question 83: Which ASHRAE standard provides guidance on acceptable duct leakage rates for residential HVAC systems?

ASHRAE 15
ASHRAE 62.2
ASHRAE Standard 193
ASHRAE 90.1 (Correct answer)

Correct answer: ASHRAE 90.1

ASHRAE 90.1 (and its residential counterpart 90.2) establishes duct leakage requirements as part of energy efficiency standards.

Question 84: Which refrigerant classification under ASHRAE 34 indicates the highest flammability level?

Class 1
Class 3 (Correct answer)
Class 2L
Class 2

Correct answer: Class 3

ASHRAE 34 Class 3 refrigerants are highly flammable, with a flame propagation rate greater than 10 cm/s and heat of combustion ≥19 kJ/g.

Question 85: When a compressor is tested and found to have equal resistance between all three terminals (C, S, R), this indicates:

The thermal overload has opened
The compressor is in good condition
The compressor windings are internally shorted (Correct answer)
The start capacitor has failed

Correct answer: The compressor windings are internally shorted

In a healthy single-phase compressor, R-to-S resistance should equal C-to-R plus C-to-S; equal readings on all three terminals indicates a winding short.

Question 86: A technician is called for a no-cool complaint and finds the suction pressure is unusually low, near 0 psig, and the head pressure is also significantly lower than normal. The suction line at the outdoor unit feels warm to the touch. What is the most likely cause?

A major system overcharge
Severe refrigerant leak or empty system (Correct answer)
Non-condensables in the system
Failed condenser fan motor

Correct answer: Severe refrigerant leak or empty system

When a system has lost most or all of its refrigerant, there is very little medium to absorb heat in the evaporator or reject it in the condenser. This results in extremely low pressures on both the high and low sides. The lack of refrigerant means no cooling is occurring, leading to a warm suction line.

Question 87: Why is it important that the disconnect box for a residential condensing unit be located within sight of the unit?

To allow the condensate drain to be observed during operation
To allow service technicians to lock out the unit while remaining in visual contact with it (Correct answer)
To meet refrigerant recovery regulations
To reduce the wire length between the panel and the unit

Correct answer: To allow service technicians to lock out the unit while remaining in visual contact with it

NEC 440.14 requires a disconnect within sight of the unit so technicians can verify the unit is de-energized and prevent unexpected re-energization while servicing.

Question 88: A technician is troubleshooting a compressor that hums but fails to start. After safely discharging the capacitors, a multimeter with a capacitance function is used to test the dual-run capacitor, which is rated for 45/5 MFD. The reading between the 'C' (Common) and 'HERM' (Hermetic) terminals is 22 MFD. What is the correct course of action?

Check for a grounded compressor winding
Add a hard start kit to assist the compressor
Replace the compressor contactor
Replace the dual-run capacitor (Correct answer)

Correct answer: Replace the dual-run capacitor

The 'HERM' terminal on a dual-run capacitor serves the compressor's start winding. The measured value of 22 MFD is significantly below the required 45 MFD, falling far outside the typical +/- 6% tolerance. This weak capacitor cannot provide the necessary phase shift and starting torque for the compressor motor, causing it to hum without starting. The capacitor has failed and must be replaced.

Question 89: A technician is troubleshooting a system where the indoor fan runs continuously, but the outdoor unit never starts. A multimeter shows 24V at the thermostat's 'Y' terminal when calling for cooling, but the contactor coil is not energized. Which of the following is the MOST likely cause?

The thermostat is improperly configured.
A failed indoor blower motor.
A faulty run capacitor for the compressor.
A tripped high-pressure safety switch. (Correct answer)

Correct answer: A tripped high-pressure safety switch.

The 24V signal from the 'Y' terminal travels in series through safety switches, such as high and low-pressure switches, before reaching the contactor coil. If a safety switch is open (tripped), it will interrupt the circuit, preventing the contactor from energizing and starting the outdoor unit.

Question 90: Which document must a refrigerant reclaimation facility provide to verify that recovered refrigerant meets AHRI 700 purity standards?

Manufacturer's Safety Data Sheet (SDS)
EPA Form 7630
Certificate of Reclaim (analysis report) (Correct answer)
DOT Form HM-181

Correct answer: Certificate of Reclaim (analysis report)

A certified reclaimer issues a certificate of reclaim (or analysis report) confirming the refrigerant meets AHRI Standard 700 purity requirements.

Question 91: A technician is recovering R-410A from a residential system using a recovery machine. The recovery cylinder, which has a water capacity (WC) of 47.6 lbs and a tare weight (TW) of 18 lbs, is placed on a digital scale. What is the maximum final weight on the scale before the recovery process must be stopped?

38.1 lbs
65.6 lbs
56.1 lbs (Correct answer)
47.6 lbs

Correct answer: 56.1 lbs

To prevent dangerous hydrostatic pressure from liquid expansion, recovery cylinders must not be filled with liquid refrigerant beyond 80% of their water capacity. First, calculate the maximum refrigerant weight: 47.6 lbs (WC) * 0.80 = 38.08 lbs. Then, add the cylinder's tare weight (empty weight) to find the maximum gross weight: 38.08 lbs + 18 lbs (TW) = 56.08 lbs. The closest and safest answer is 56.1 lbs.

Question 92: Which condition would MOST likely cause a recovery machine to short-cycle (start and stop rapidly) during vapor-phase recovery?

A refrigerant system with very low remaining charge and near-vacuum pressures (Correct answer)
An oversized recovery cylinder creating too much pressure differential
Refrigerant oil contaminating the recovery machine's suction port
High ambient temperature reducing condenser efficiency

Correct answer: A refrigerant system with very low remaining charge and near-vacuum pressures

When system pressure drops near vacuum as the charge is depleted, the recovery machine's low-pressure cutout trips repeatedly, causing short-cycling.

Question 93: During installation, which type of pipe insulation is most suitable for residential refrigerant suction lines?

Rubber pipe wrap tape
Closed-cell foam insulation (Correct answer)
Open-cell foam insulation
Fiberglass wrap

Correct answer: Closed-cell foam insulation

Closed-cell foam insulation is preferred for suction lines because it resists moisture absorption and provides effective thermal insulation.

Question 94: In a vapor-compression cycle, which component raises refrigerant pressure and temperature?

Filter-drier
Compressor (Correct answer)
Thermostatic expansion valve
Evaporator

Correct answer: Compressor

The compressor draws in low-pressure vapor and raises it to high pressure and temperature before sending it to the condenser.

Question 95: When measuring voltage across a closed contactor's load-side terminals, a technician reads 5V instead of 0V. This most likely indicates:

The contactor coil is energized
Incorrect meter lead placement
Normal voltage drop across a good contactor
Pitted or burned contacts causing excessive resistance (Correct answer)

Correct answer: Pitted or burned contacts causing excessive resistance

A voltage drop across a closed contactor indicates high contact resistance from pitting or burning, which causes heat and inefficient power delivery to the compressor.

Question 96: A rectangular duct measures 12 inches by 8 inches. What is its equivalent round duct diameter?

8.8 inches
11.3 inches
10.7 inches
9.6 inches (Correct answer)

Correct answer: 9.6 inches

The equivalent diameter for a 12×8 rectangular duct using the hydraulic diameter formula is approximately 9.6 inches.

Question 97: When recovering refrigerant from a system with a known motor burn, the technician should use:

Recovery equipment equipped with an acid-rated filter to capture contaminants (Correct answer)
Standard recovery equipment with a new filter-drier after recovery
The existing system compressor to push refrigerant into recovery cylinders
A direct-connect hose to the recovery cylinder only — no machine needed

Correct answer: Recovery equipment equipped with an acid-rated filter to capture contaminants

Acid-laden refrigerant from a burned compressor requires an acid-rated filter in the recovery machine to protect its components and prevent cross-contamination.

Question 98: A residential condenser fan motor runs in the wrong direction after replacement. The MOST likely cause is:

The start and run capacitor leads were swapped on the new motor (Correct answer)
A failed run capacitor
The contactor contacts are worn
The motor is wired to 208V instead of 240V

Correct answer: The start and run capacitor leads were swapped on the new motor

Swapping the start winding leads relative to the run winding reverses the phase shift and changes motor rotation direction.

Question 99: A technician finds that a condenser fan motor runs but the compressor does not start and hums loudly. The MOST likely electrical cause is:

Failed high-pressure switch
Open run capacitor for the compressor (Correct answer)
Loose thermostat R wire
Faulty contactor holding coil

Correct answer: Open run capacitor for the compressor

A failed run capacitor reduces starting torque, causing the compressor to hum and fail to start while still drawing locked-rotor amperage.

Question 100: What is the function of a hard start kit (potential relay and start capacitor) shown in a schematic?

To boost starting torque and reduce locked-rotor amperage duration (Correct answer)
To provide surge protection during lightning storms
To regulate refrigerant pressure during startup
To soft-start the condenser fan motor

Correct answer: To boost starting torque and reduce locked-rotor amperage duration

A hard start kit adds extra capacitance to boost starting torque, allowing the compressor to start under load and reducing the time spent at high locked-rotor amperage.

Question 101: A residential split system's outdoor disconnect must be located within sight of and within how many feet of the equipment?

25 feet
100 feet
75 feet
50 feet (Correct answer)

Correct answer: 50 feet

The NEC requires the disconnecting means to be within sight of and within 50 feet of the air conditioning equipment it serves.

Question 102: A run capacitor rated 45+5 MFD is used with a condenser fan motor and compressor. What do the two capacitance values represent?

Startup and shutdown capacitance
Compressor and fan motor capacitance respectively (Correct answer)
Primary and secondary winding values
Capacitance at high and low ambient temperatures

Correct answer: Compressor and fan motor capacitance respectively

In a dual-run capacitor, the larger value (45 MFD) serves the compressor and the smaller value (5 MFD) serves the condenser fan motor.

Question 103: In residential AC electrical schematics, a dashed line between two components typically indicates:

A ground wire
A field-installed wire by the installer
A high-voltage conductor
A mechanical linkage or shared enclosure (Correct answer)

Correct answer: A mechanical linkage or shared enclosure

Dashed lines in schematics represent mechanical connections or components physically housed together, not electrical conductors.

Question 104: While diagnosing a system with a TXV, a technician measures a consistently high superheat and low suction pressure. The subcooling measured at the condenser outlet is found to be normal to high. Which of the following is the most probable fault?

The condenser coil is dirty and restricted
The system is overcharged with refrigerant
The TXV sensing bulb has lost its charge (Correct answer)
Inefficient compressor valves

Correct answer: The TXV sensing bulb has lost its charge

If the TXV's sensing bulb loses its refrigerant charge, the opening force on the valve's diaphragm is lost. The valve's spring pressure will then overcome the evaporator pressure and force the valve to a nearly closed position. This starves the evaporator of refrigerant, causing low suction pressure and high superheat. The refrigerant that cannot enter the evaporator backs up in the condenser, leading to normal or high subcooling.

Question 105: During the refrigeration cycle, where does the refrigerant absorb heat from the space being cooled, causing it to change from a liquid to a vapor?

In the metering device
In the evaporator (Correct answer)
In the compressor
In the condenser

Correct answer: In the evaporator

The evaporator is the component located inside the space being cooled. As the low-pressure, low-temperature liquid refrigerant flows through the evaporator coil, it absorbs heat from the indoor air, causing the refrigerant to boil and change into a low-pressure vapor.

Question 106: In a standard residential air conditioning system, what is the primary function of the evaporator coil?

To absorb heat from the indoor air, causing the refrigerant to boil. (Correct answer)
To reject heat from the refrigerant to the outdoor air.
To reduce the pressure of the high-pressure liquid refrigerant.
To compress low-pressure refrigerant gas into a high-pressure gas.

Correct answer: To absorb heat from the indoor air, causing the refrigerant to boil.

The evaporator coil is located indoors and is responsible for absorbing heat from the indoor air. As the warm indoor air is blown across the cold evaporator coil, the liquid refrigerant inside absorbs this heat and boils, changing its state from a liquid to a low-pressure gas.

Question 107: What is humidity ratio (specific humidity) as used in HVAC psychrometrics?

The ratio of latent heat to sensible heat in the conditioned airstream
The percentage of moisture in air relative to its maximum holding capacity at that temperature
The temperature at which the air sample becomes fully saturated
The mass of water vapor per unit mass of dry air, expressed in lbs/lb or grains/lb (Correct answer)

Correct answer: The mass of water vapor per unit mass of dry air, expressed in lbs/lb or grains/lb

Humidity ratio (W) is the actual mass of water vapor per unit mass of dry air — an absolute measure of moisture content independent of temperature.

Question 108: When installing a new thermostat wire for a split system with a variable-speed air handler, which wire gauge is typically required for the communication wiring?

6 AWG aluminum wire
14 AWG THHN wire
10 AWG solid copper wire
22 AWG solid or stranded wire (Correct answer)

Correct answer: 22 AWG solid or stranded wire

Low-voltage thermostat and communication wiring for residential HVAC systems typically uses 18–22 AWG stranded or solid wire.

Question 109: A residential AC system is short-cycling on high pressure. Which maintenance action should the technician perform FIRST?

Add refrigerant to the system
Check and clean the condenser coil for blockage or dirt (Correct answer)
Replace the compressor
Replace the high-pressure switch

Correct answer: Check and clean the condenser coil for blockage or dirt

A dirty or blocked condenser coil reduces heat rejection, causing high-side pressure to rise and the system to trip on the high-pressure limit.

Question 110: What is the first step when installing a new residential air conditioning system?

Conducting a site inspection (Correct answer)
Testing the airflow
Connecting the electrical wiring
Charging the refrigerant system

Correct answer: Conducting a site inspection

Before installing a new AC system, a thorough site inspection is the crucial first step. This involves assessing the home's layout, existing ductwork, electrical capacity, and potential locations for indoor and outdoor units. A proper inspection ensures the correct system size is chosen, identifies any necessary modifications, and plans for an efficient and safe installation.

Question 111: What is the maximum amount of R-410A that can be stored in a single control area of a building under IFC (International Fire Code) limits before additional safeguards are required?

100 pounds
1,000 pounds
220 pounds (Correct answer)
400 pounds

Correct answer: 220 pounds

The IFC limits the storage of R-410A (a nonflammable liquefied gas) to 220 pounds per control area without additional hazardous materials controls.

Question 112: What is the result of non-condensable gases (such as air) in a refrigerant system?

Lower-than-normal head pressure and reduced compressor amperage
Flooding of the evaporator with liquid refrigerant
Higher-than-normal head pressure, elevated condenser temperature, and reduced efficiency (Correct answer)
A drop in suction pressure causing evaporator freeze-up

Correct answer: Higher-than-normal head pressure, elevated condenser temperature, and reduced efficiency

Non-condensable gases occupy condenser volume and cannot condense, raising head pressure and reducing system efficiency and capacity.

Question 113: What is the role of the contactor in an air conditioning system?

Measure system pressure
Control the power supply to motors (Correct answer)
Start the system's cooling cycle
Regulate refrigerant flow

Correct answer: Control the power supply to motors

The contactor acts as an electrically operated switch that controls the flow of high-voltage power to the major components of the air conditioning system, such as the compressor and outdoor fan motor. When the thermostat calls for cooling, it sends a low-voltage signal to the contactor, which then closes its contacts to supply power to these motors.

Question 114: A technician measures a very high condenser entering air temperature of 115°F on a rooftop-adjacent unit. How does this affect system diagnosis?

It lowers subcooling and reduces system capacity (Correct answer)
It raises suction pressure and improves efficiency
It causes the evaporator to frost over
It has no effect on system pressures

Correct answer: It lowers subcooling and reduces system capacity

High entering air temperature to the condenser reduces heat rejection efficiency, raising head pressure and reducing subcooling and overall system capacity.

Question 115: On a psychrometric chart, wet-bulb temperature lines run in which direction?

Diagonally downward from the saturation curve toward the lower right (Correct answer)
Vertically parallel to the dry-bulb temperature axis
Horizontally across the chart
In concentric arcs around the origin point

Correct answer: Diagonally downward from the saturation curve toward the lower right

Wet-bulb temperature lines slope diagonally downward from upper left to lower right on a psychrometric chart and nearly coincide with lines of constant enthalpy.

Question 116: A homeowner complains of a persistent whistling noise coming from a supply air register, which becomes louder when other registers in the house are closed. What is the most probable cause of this issue?

The air velocity through the register is too high. (Correct answer)
The ductwork is oversized for the system's CFM output.
The thermostat is located too close to the supply register.
The return air filter is oversized.

Correct answer: The air velocity through the register is too high.

A whistling noise at a register is a classic sign of excessive air velocity. When air is forced through a small opening (the register) at a high speed, it creates turbulence and noise. Closing other registers increases the static pressure in the system, forcing even more air through the remaining open registers at a higher velocity, which explains why the noise gets louder.

Question 117: A technician uses a clamp meter to measure current on one hot leg feeding a 240V condensing unit and reads 0 amps while the unit should be running. What should be checked FIRST?

Refrigerant charge level
Whether one leg of the breaker has tripped or lost power (Correct answer)
Low-voltage thermostat signal
Compressor motor winding resistance

Correct answer: Whether one leg of the breaker has tripped or lost power

Zero current on one leg of a 240V circuit most often indicates that leg's breaker has tripped or the utility has lost one phase at the panel.

Question 118: What is the standard control voltage used in most modern residential air conditioning systems for the thermostat and safety circuits?

240 VAC
24 VAC (Correct answer)
12 VDC
120 VAC

Correct answer: 24 VAC

Residential HVAC systems use a step-down transformer to convert line voltage (120V or 240V) to 24 VAC for the control circuit. This lower voltage is safer for the wiring that runs to the thermostat and through various safety switches.

Question 119: How often should the condenser coils be cleaned to maintain optimal performance?

Only when the system fails
Once a year (Correct answer)
Once every five years
Every two months

Correct answer: Once a year

The outdoor condenser coils are exposed to dirt, leaves, grass clippings, and other debris, which can accumulate and reduce their ability to dissipate heat. Cleaning them at least once a year, typically in the spring before the cooling season begins, ensures optimal heat transfer efficiency, prevents system strain, and extends the unit's lifespan.

Question 120: A technician measures 0.80 inches W.C. total external static pressure on a system rated for 0.50 inches W.C. What corrective action should be taken first?

Inspect the duct system for restrictions, undersized ducts, or dirty filters (Correct answer)
Add refrigerant to lower head pressure
Replace the air handler with a higher-capacity unit
Increase blower speed immediately

Correct answer: Inspect the duct system for restrictions, undersized ducts, or dirty filters

Elevated external static pressure most often results from duct restrictions, undersizing, or a clogged filter rather than a blower or refrigerant issue.

Question 121: What are the primary negative effects of high indoor relative humidity on occupants?

It increases comfort by preventing static electricity buildup in dry climates
It promotes mold growth, makes occupants feel warmer, and can cause respiratory irritation (Correct answer)
It reduces the sensible cooling load by lowering the sensible heat ratio
It improves air quality by causing airborne dust particles to clump and settle

Correct answer: It promotes mold growth, makes occupants feel warmer, and can cause respiratory irritation

High indoor humidity fosters mold and mildew, makes occupants feel hotter because perspiration evaporates more slowly, and can aggravate asthma and allergies.

Question 122: A duct system uses a trunk-and-branch layout. What is the primary advantage of this design?

Eliminates the need for dampers entirely
Simplest to fabricate and requires less material than extended plenum (Correct answer)
Easiest to balance airflow to individual rooms
Provides the lowest static pressure of any layout

Correct answer: Simplest to fabricate and requires less material than extended plenum

Trunk-and-branch systems use a central trunk with smaller branch takeoffs, reducing material compared to running full-size ducts to every outlet.

Question 123: A contactor coil measures infinite resistance with an ohmmeter. This indicates the coil is:

Functioning normally
Grounded to the cabinet
Shorted and drawing excess current
Open and will not energize (Correct answer)

Correct answer: Open and will not energize

Infinite resistance on a coil indicates an open circuit, meaning no current can flow through the coil and the contactor will not pull in.

Question 124: What is the effect of an oversized supply duct on room airflow distribution?

Reduces air velocity causing poor throw and stratification (Correct answer)
Has no effect on comfort
Increases static pressure at the register
Improves throw and mixing in the room

Correct answer: Reduces air velocity causing poor throw and stratification

An oversized duct lowers air velocity, reducing register throw and leaving the room with poor air mixing and temperature stratification.

Question 125: Which Manual D concept defines the path through the duct system with the greatest pressure drop?

Critical path (Correct answer)
Trunk index
Longest equivalent length run
Primary duct run

Correct answer: Critical path

The critical path (or index circuit) is the duct run with the highest total effective length and determines the required system static pressure.

Question 126: What type of electrical fault would cause a ground fault circuit interrupter (GFCI) to trip on an AC circuit?

A short circuit between two hot conductors
Voltage drop in the supply conductors
Current leaking from a hot conductor to ground exceeding ~5 mA (Correct answer)
Overload current exceeding the breaker rating

Correct answer: Current leaking from a hot conductor to ground exceeding ~5 mA

A GFCI detects an imbalance between hot and neutral (or ground path) current as small as 4–6 mA, indicating current is leaking to ground through an unintended path.

Question 127: Which of the following describes the state of the refrigerant as it enters the metering device in a properly functioning air conditioning system?

Low-pressure, low-temperature liquid
Low-pressure, superheated vapor
High-pressure, warm liquid (Correct answer)
High-pressure, high-temperature vapor

Correct answer: High-pressure, warm liquid

After leaving the condenser, the refrigerant has rejected heat to the outdoors and has condensed from a high-pressure vapor into a high-pressure, warm liquid. This subcooled liquid then flows to the metering device.

Question 128: Which type of fire damper is required in a duct penetrating a fire-rated wall and also a smoke barrier?

Smoke damper only
No damper is required if duct is metal
Type A fire damper only
Combination fire/smoke damper (Correct answer)

Correct answer: Combination fire/smoke damper

Where a duct penetrates both a fire-rated wall and a smoke barrier, a combination fire/smoke damper satisfying both requirements must be installed.

Question 129: What is relative humidity (RH)?

The actual mass of water vapor in air measured in grains per pound of dry air
The temperature at which condensation forms on interior surfaces
The ratio of actual moisture content to the maximum moisture air can hold at that temperature (Correct answer)
The temperature difference between wet-bulb and dry-bulb readings

Correct answer: The ratio of actual moisture content to the maximum moisture air can hold at that temperature

Relative humidity is the ratio of actual water vapor pressure to saturation water vapor pressure at the same temperature, expressed as a percentage.

Question 130: A residential air conditioning system uses a 240V/24V step-down transformer. If the primary side shows 240V but the secondary shows 0V, the MOST likely cause is:

Incorrect transformer sizing
Open primary winding or blown fuse (Correct answer)
Overloaded control circuit
Shorted secondary winding

Correct answer: Open primary winding or blown fuse

An open primary winding or a blown fuse on the primary side will prevent current flow and result in zero secondary voltage output.

Question 131: A technician needs to verify the airflow (CFM) coming out of a specific supply register to help balance a system. Which of the following instruments is most suitable for this task?

A static pressure probe (manometer)
A combustion analyzer
A refrigerant leak detector
An airflow capture hood (balometer) (Correct answer)

Correct answer: An airflow capture hood (balometer)

An airflow capture hood, or balometer, is specifically designed to fit over a register or grille and directly measure the volume of air in CFM. While an anemometer can measure velocity (which can then be used to calculate CFM), a capture hood provides a direct and more accurate reading for this purpose.

Question 132: During a new split system installation, the technician notes the liquid line is 3/8" and the suction line is 7/8". The line set length is 50 feet. The manufacturer's instructions state the outdoor unit comes pre-charged for a 15-foot line set. What is the most critical next step for the technician regarding the refrigerant charge?

Release the factory charge and allow the system to run for 30 minutes before checking pressures.
Add a calculated amount of refrigerant based on the additional line set length and diameter. (Correct answer)
Recover the factory charge and weigh in a new charge based on the nameplate value only.
Assume the factory charge is sufficient for the longer line set length.

Correct answer: Add a calculated amount of refrigerant based on the additional line set length and diameter.

Most outdoor units are pre-charged for a standard length, often 15 feet. When the line set exceeds this length, additional refrigerant must be added. The amount is calculated based on the manufacturer's specification (e.g., ounces per additional foot) for the specific liquid line diameter. Failing to add the correct amount will result in an undercharged system, leading to poor performance and potential equipment damage.

Question 133: A technician measures the total external static pressure (TESP) of a residential air handler and gets a reading of 0.95 in. w.c. (inches of water column). The equipment's data plate specifies a maximum TESP of 0.5 in. w.c. Which of the following is a direct consequence of this high static pressure?

The blower motor will be strained, leading to reduced airflow and potential premature failure. (Correct answer)
The compressor will short-cycle due to rapid heat exchange.
The system will provide excess airflow, overcooling the space.
The blower motor will work less, consuming less electricity.

Correct answer: The blower motor will be strained, leading to reduced airflow and potential premature failure.

High external static pressure is resistance against which the blower motor must push air. When the TESP exceeds the manufacturer's maximum rating, the blower cannot move the intended volume of air (CFM). This strains the blower motor, increases its energy consumption, reduces its lifespan, and results in insufficient airflow to the conditioned spaces, leading to comfort issues and reduced system efficiency.

Question 134: When using an oxyacetylene torch for brazing refrigerant lines, the flame should be adjusted to a:

Oxidizing flame
Turbulent blue flame
Neutral flame (Correct answer)
Carburizing (reducing) flame

Correct answer: Neutral flame

A neutral flame provides the correct heat balance for brazing copper without introducing excess oxygen or carbon to the joint.

Question 135: What is the function of the crankcase heater in an air conditioning system, and how is it typically wired?

Cools refrigerant oil; wired in parallel with the compressor
Heats the suction line; wired in series with the thermostat
Warms the evaporator coil; wired to the defrost board
Keeps oil warm to prevent refrigerant migration; wired to be energized when the compressor is OFF (Correct answer)

Correct answer: Keeps oil warm to prevent refrigerant migration; wired to be energized when the compressor is OFF

A crankcase heater prevents liquid refrigerant from migrating into compressor oil during off cycles; it is wired so it energizes whenever the compressor contactor is open.

Question 136: A residential heat pump defrost control board initiates defrost based on:

Refrigerant charge level
Indoor thermostat demand only
Time and temperature, or time and pressure, depending on board design (Correct answer)
Outdoor humidity level only

Correct answer: Time and temperature, or time and pressure, depending on board design

Most defrost boards use a time-and-temperature or time-and-pressure strategy to determine when frost buildup warrants a defrost cycle.

Question 137: A technician discovers a system using R-22. Under current EPA rules, what refrigerant options exist for topping off this system?

Virgin R-22 only
Any HFC refrigerant as a drop-in replacement
The system must be immediately decommissioned
Reclaimed R-22 or approved substitute refrigerants (Correct answer)

Correct answer: Reclaimed R-22 or approved substitute refrigerants

As of 2020, only reclaimed or recycled R-22 may be used for servicing existing R-22 systems; virgin R-22 production has been phased out.

Question 138: A technician measures the refrigerant leaving the condenser and finds it is a high-pressure liquid at a temperature below its saturation point. What is this condition called?

Superheat
Subcooling (Correct answer)
Flashing
Saturation

Correct answer: Subcooling

Subcooling is the process of cooling a liquid refrigerant below its condensation/saturation temperature. This ensures that only liquid refrigerant enters the metering device, which is crucial for system efficiency and performance.

Question 139: What is the primary advantage of a radial duct layout over an extended plenum system?

Uses less total duct material
Allows higher static pressure operation
Each branch has equal length for more balanced airflow (Correct answer)
Requires no balancing dampers

Correct answer: Each branch has equal length for more balanced airflow

A radial (spider) layout routes individual ducts directly from the air handler plenum to each room, making branch lengths more equal and airflow more balanced.

Question 140: Which refrigerant cylinder color code is used to identify R-410A?

Rose/pink (Correct answer)
Tan/beige
Orange
Sky blue

Correct answer: Rose/pink

R-410A cylinders are color-coded rose/pink per ARI Standard 700 to distinguish them from other refrigerants.

Question 141: A technician is installing a new split system and must size the refrigerant line set. Which factor is MOST critical when determining the suction line diameter?

The brand of refrigerant being used
The tonnage of the system and the equivalent length of the run (Correct answer)
The color of the insulation jacket
The distance from the outdoor unit to the electrical panel

Correct answer: The tonnage of the system and the equivalent length of the run

Suction line diameter must be sized based on system tonnage and total equivalent line length to maintain adequate refrigerant velocity and minimize pressure drop.

Question 142: When installing a split-system air conditioner, the indoor unit's condensate drain line should be pitched at a minimum slope of:

1/2 inch per foot
1/4 inch per foot (Correct answer)
1/8 inch per foot
1 inch per foot

Correct answer: 1/4 inch per foot

A minimum slope of 1/4 inch per foot ensures proper gravity drainage of condensate without pooling.

Question 143: What duct material is NOT recommended for use in return air systems because it can off-gas harmful compounds when exposed to the airstream?

Galvanized spiral duct
Asbestos-containing duct insulation (Correct answer)
Fiberglass duct board
Sheet metal

Correct answer: Asbestos-containing duct insulation

Asbestos-containing duct insulation is prohibited because asbestos fibers can be entrained in the airstream and inhaled by occupants.

Question 144: What happens to airflow velocity when a duct transitions from a smaller to a larger cross-section?

Velocity stays the same but turbulence increases
Velocity decreases and static pressure partially recovers (Correct answer)
Velocity increases due to the pressure buildup
Velocity doubles and static pressure drops proportionally

Correct answer: Velocity decreases and static pressure partially recovers

When a duct expands, the air slows down (by continuity) and some velocity pressure converts back to static pressure—this is called static regain.

Question 145: Which refrigerant cycle process occurs in the condenser?

Pressure reduction through a fixed orifice
Condensation and heat rejection to outdoor air (Correct answer)
Evaporation at low pressure
Heat absorption from indoor air

Correct answer: Condensation and heat rejection to outdoor air

In the condenser, high-pressure refrigerant vapor releases heat to the outside air and condenses into a liquid.

Question 146: When sizing a condensate pump for a residential air handler installed in a basement without gravity drain access, which factor is MOST important?

The amperage draw of the pump motor
The pump's lift capacity relative to the required discharge height (Correct answer)
The length of the discharge tubing only
The color of the pump housing

Correct answer: The pump's lift capacity relative to the required discharge height

The condensate pump must have sufficient lift capacity to pump condensate to the required discharge height (vertical rise) where it can drain by gravity.

Question 147: An AC technician measures 127V at L1-to-ground and 121V at L2-to-ground on a 240V circuit. The voltage imbalance concern is because:

Ground voltage should always be zero
Voltage imbalance between phases causes disproportionately higher current imbalance, stressing compressor windings (Correct answer)
The total voltage of 248V exceeds equipment rating
Only the leg-to-leg voltage of 248V matters for compressor operation

Correct answer: Voltage imbalance between phases causes disproportionately higher current imbalance, stressing compressor windings

A small voltage imbalance creates a much larger current imbalance (roughly 6–10× the percentage), causing uneven heating in motor windings and premature failure.

Question 148: A zoning system with multiple motorized dampers closes all zones simultaneously. What device prevents damage to the air handler from this condition?

A low-pressure cutout on the evaporator
An outdoor air economizer damper
A barometric relief damper or bypass damper (Correct answer)
A differential pressure switch on the refrigerant circuit

Correct answer: A barometric relief damper or bypass damper

A bypass or barometric relief damper allows air to recirculate when all zone dampers are closed, preventing excessively high static pressure from damaging the blower.

Question 149: When installing ductwork for a new residential AC system, what is the purpose of using mastic sealant instead of standard duct tape?

Mastic is required only on return ducts, not supply ducts
Mastic improves thermal insulation of the ducts
Mastic provides a durable, long-lasting airtight seal that duct tape cannot maintain over time (Correct answer)
Mastic is easier to apply in tight spaces

Correct answer: Mastic provides a durable, long-lasting airtight seal that duct tape cannot maintain over time

Mastic sealant creates a permanent, flexible, airtight bond at duct joints, whereas standard duct tape degrades and loses adhesion over time, allowing air leaks.

Question 150: After completing a residential system installation and commissioning, which parameter should the technician record on the startup documentation for future reference?

Homeowner's preferred thermostat schedule
Serial number of the vacuum pump used
Actual measured suction pressure, discharge pressure, superheat, subcooling, and Delta-T at design conditions (Correct answer)
Date of the last refrigerant purchase receipt

Correct answer: Actual measured suction pressure, discharge pressure, superheat, subcooling, and Delta-T at design conditions

Recording actual operating pressures, superheat, subcooling, and Delta-T at commissioning establishes a baseline for diagnosing future performance changes.

Question 151: The global warming potential (GWP) of R-410A compared to R-22 is:

Lower — R-410A is more environmentally friendly
Irrelevant — only ODP matters under the Clean Air Act
The same — both are HCFCs with equivalent GWP
Higher — R-410A has a GWP of approximately 2088 vs. R-22's 1810 (Correct answer)

Correct answer: Higher — R-410A has a GWP of approximately 2088 vs. R-22's 1810

R-410A has a GWP of approximately 2088, which is higher than R-22's GWP of 1810, although R-410A has zero ozone depletion potential.

Question 152: When replacing a TXV, the technician must ensure the sensing bulb is:

Clamped firmly to the suction line and insulated from ambient air (Correct answer)
Submerged in the condensate drain pan
Installed on the discharge line near the compressor
Positioned inside the return air duct

Correct answer: Clamped firmly to the suction line and insulated from ambient air

The TXV sensing bulb must make good thermal contact with the suction line and be insulated so it accurately senses refrigerant vapor temperature, not ambient temperature.

313D Residential Air Conditioning Systems Mechanic

The 313D Residential Air Conditioning Systems Mechanic Certificate of Qualification exam, administered by Skilled Trades Ontario, validates competency in installing, servicing, and maintaining residential air conditioning and heat pump systems.

150

Questions

240 min

Time Limit

70%

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