Furnace Installation and Maintenance Certification (FC) — Questions and Answers

Question 1: A technician finds that the condensate drain on a 96% AFUE furnace is blocked. What is the MOST serious consequence if it is not cleared?

Condensate backs up into the secondary heat exchanger, causing a pressure switch fault and lockout (Correct answer)
Flue gases reverse into the living space immediately
Reduced combustion efficiency only
The blower motor overheats

Correct answer: Condensate backs up into the secondary heat exchanger, causing a pressure switch fault and lockout

Blocked condensate backs up into the secondary heat exchanger and pressure switch tubing, tripping the pressure switch and shutting down the furnace.

Question 2: What is the minimum upward slope required for horizontal sections of Category I single-wall vent connector pipe?

No slope; vertical sections only
1/2 inch per foot rise
Level is acceptable
1/4 inch per foot rise toward the chimney or vertical vent (Correct answer)

Correct answer: 1/4 inch per foot rise toward the chimney or vertical vent

Horizontal vent connectors must rise at least 1/4 inch per foot toward the chimney to help flue gases rise by natural draft.

Question 3: According to NFPA 54, when must gas piping inside a building be tested for leakage before a new furnace is placed into operation?

Only if visible corrosion is present on the piping
Only when the furnace BTU input exceeds 100,000 BTU/hr
Only when the local authority having jurisdiction (AHJ) requires it
After any repair, addition, or modification to the piping system (Correct answer)

Correct answer: After any repair, addition, or modification to the piping system

NFPA 54 requires leak testing of gas piping after any repair, addition, or modification before placing the system back into service.

Question 4: What is the purpose of a burner orifice in a gas furnace?

To meter the correct amount of gas into the burner (Correct answer)
To sense flame presence
To mix air with gas
To ignite the gas

Correct answer: To meter the correct amount of gas into the burner

The orifice spud meters the precise amount of gas needed to produce the correct BTU output at the burner.

Question 5: How far must a direct-vent (concentric) termination kit terminate above the ground in a US snow region?

24 inches
12 inches
18 inches (Correct answer)
6 inches

Correct answer: 18 inches

In areas subject to snow accumulation, direct-vent terminations must be at least 18 inches above grade to prevent obstruction by snow.

Question 6: A high-efficiency furnace is installed in a confined space. The technician notes combustion air is supplied by a single PVC pipe. What problem could arise from inadequate combustion air supply?

Overheating of the blower motor compartment
Negative pressure in the combustion chamber causing incomplete combustion and CO production (Correct answer)
Premature heat exchanger corrosion from excess moisture
Excessive condensate production

Correct answer: Negative pressure in the combustion chamber causing incomplete combustion and CO production

Inadequate combustion air creates negative pressure and starves the burners of oxygen, resulting in incomplete combustion and carbon monoxide generation.

Question 7: ENERGY STAR certification for gas furnaces requires a minimum AFUE of:

95%
85%
80%
90% (Correct answer)

Correct answer: 90%

ENERGY STAR certified gas furnaces must achieve at least 90% AFUE, signifying substantially higher efficiency than the federal minimum standard.

Question 8: What causes delayed ignition (a loud boom or puff) when a furnace starts up?

Excessive gas pressure
Gas accumulating in the combustion chamber before igniting (Correct answer)
A faulty limit switch
Low blower speed

Correct answer: Gas accumulating in the combustion chamber before igniting

Delayed ignition occurs when gas builds up before the igniter lights it, causing a small explosion when combustion finally starts.

Question 9: Under the International Mechanical Code (IMC), what is the maximum length allowed for a dryer exhaust duct before a length reduction factor must be applied for each 90-degree elbow?

25 feet total equivalent length (Correct answer)
35 feet total equivalent length
35 feet straight run
25 feet straight run

Correct answer: 25 feet total equivalent length

The IMC allows a maximum 25-foot equivalent duct length for dryer exhausts, with each 90-degree elbow reducing the allowable length by 5 feet.

Question 10: In a zoned HVAC system, what device prevents excessive static pressure from building up when zone dampers close?

Pressure relief valve
Zone control board
Variable-speed ECM blower
Bypass damper (Correct answer)

Correct answer: Bypass damper

A bypass damper (or a variable-speed blower) relieves excess static pressure by recirculating air when multiple zone dampers are closed.

Question 11: What should a technician do if they encounter a safety violation during furnace installation?

Ignore the violation and complete the work.
Continue the installation and fix the issue later.
Ask the client to resolve the issue.
Stop the installation and report the violation (Correct answer)

Correct answer: Stop the installation and report the violation

Encountering a safety violation during furnace installation demands immediate action to prevent potential hazards. Continuing work could exacerbate the risk, while ignoring it is irresponsible and illegal. Stopping the installation and reporting the violation ensures proper rectification, protecting both the technician and the future occupants of the building.

Question 12: What is the primary purpose of a draft inducer motor on a high-efficiency furnace?

To circulate warm air through the ducts
To pre-heat incoming combustion air
To control burner ignition timing
To pull combustion gases through the heat exchanger and exhaust them outdoors (Correct answer)

Correct answer: To pull combustion gases through the heat exchanger and exhaust them outdoors

The draft inducer creates a negative pressure to draw flue gases through the heat exchanger and out the venting system.

Question 13: What is the minimum R-value required by most US energy codes for ducts located in unconditioned attic spaces?

R-8 (Correct answer)
R-6
R-11
R-4

Correct answer: R-8

Most current US energy codes (IECC) require a minimum of R-8 insulation for ducts in unconditioned attic spaces.

Question 14: What is the primary cause of high static pressure in a residential duct system?

A dirty heat exchanger
Low gas pressure
Restrictive air filter, undersized ducts, or excessive elbows (Correct answer)
Oversized ductwork

Correct answer: Restrictive air filter, undersized ducts, or excessive elbows

Clogged filters, undersized supply or return ducts, and numerous sharp elbows are the most common causes of excessive static pressure.

Question 15: A technician measures a static pressure drop of 1.2 in. W.C. across a 1-inch pleated filter rated at 0.5 in. W.C. maximum. What should be done?

Increase blower motor speed
Replace the filter immediately (Correct answer)
Clean the filter with compressed air
Install a bypass duct around the filter

Correct answer: Replace the filter immediately

A pressure drop more than double the filter's rated maximum indicates a severely clogged filter that must be replaced to restore proper airflow.

Question 16: What is the correct procedure before replacing an ECM blower motor module?

Reset the control board first
Replace the motor and module together immediately
Measure incoming line voltage only
Cycle power and wait 5 minutes to discharge capacitors in the module (Correct answer)

Correct answer: Cycle power and wait 5 minutes to discharge capacitors in the module

ECM modules contain large capacitors that retain dangerous charge; power must be off and a 5-minute wait observed before handling the module.

Question 17: A furnace pressure switch trips on each heating call but resets when the draft inducer is turned off and back on manually. What should the technician inspect?

Thermostat wiring for shorts
Gas valve solenoid voltage
Inducer motor RPM, venting for blockages, and pressure switch hose for cracks (Correct answer)
Igniter resistance

Correct answer: Inducer motor RPM, venting for blockages, and pressure switch hose for cracks

Intermittent pressure switch faults often point to weak inducer motor performance, blocked venting, or cracked pressure switch hoses that cause loss of differential pressure.

Question 18: A furnace installation requires a horizontal run of flue vent pipe. What is the maximum allowable slope per NFPA 54 for a non-condensing appliance vent?

Level (0 inches per foot)
1/4 inch rise per foot toward the appliance
1 inch rise per foot toward the outdoor termination
1/4 inch rise per foot toward the outdoor termination (Correct answer)

Correct answer: 1/4 inch rise per foot toward the outdoor termination

Non-condensing appliance vents must slope upward at least 1/4 inch per foot toward the outdoor termination to allow flue gases to rise and prevent condensate from flowing back toward the furnace.

Question 19: Under the IMC, what type of opening is required between adjacent rooms to count toward unconfined space combustion air requirements?

Any open doorway regardless of size
Permanent openings with a combined free area of at least 1 sq in per 1,000 BTU/hr of all appliances (Correct answer)
A grille with a minimum free area of 100 sq in
A louvered door with a minimum 1 sq in per 1,000 BTU/hr

Correct answer: Permanent openings with a combined free area of at least 1 sq in per 1,000 BTU/hr of all appliances

Permanent openings must provide a combined free area of at least 1 square inch per 1,000 BTU/hr of total appliance input to supply adequate combustion air.

Question 20: Which ACCA manual provides guidance specifically for residential duct system design?

Manual D (Correct answer)
Manual J
Manual T
Manual S

Correct answer: Manual D

ACCA Manual D is the standard for designing residential duct systems, ensuring proper airflow to match the calculated loads from Manual J.

Question 21: A homeowner reports that one room is significantly colder than others even though the furnace runs continuously. Which maintenance item should be checked first?

Thermostat calibration
Gas valve pressure setting
Duct damper positions and duct integrity in that zone (Correct answer)
Furnace heat exchanger condition

Correct answer: Duct damper positions and duct integrity in that zone

Closed or leaking duct dampers and duct disconnections are the most common cause of uneven heating in a specific room.

Question 22: What is the primary danger of a blocked or restricted furnace flue vent?

The furnace will short cycle on the thermostat
The gas valve will fail to open
Combustion gases including carbon monoxide will spill into the living space (Correct answer)
The heat exchanger will overheat and crack

Correct answer: Combustion gases including carbon monoxide will spill into the living space

A blocked flue prevents combustion gases from exhausting, causing them to spill into the living space and creating a potentially fatal CO hazard.

Question 23: Under the International Residential Code (IRC), what is the minimum height of a furnace room door that opens into a garage where the furnace contains an ignition source?

A 1-hour fire-rated door is required
No special door is required if the furnace is sealed combustion
The furnace must be elevated 18 inches above the garage floor (Correct answer)
The door must be self-closing and self-latching

Correct answer: The furnace must be elevated 18 inches above the garage floor

The IRC requires that appliances with ignition sources installed in garages be elevated at least 18 inches above the floor to keep ignition sources above potential fuel vapors.

Question 24: When adding a new supply run to an existing duct system, what must be verified to prevent degrading other rooms?

That the new run uses metal duct only
That the new run is the same diameter as all existing runs
That the new register faces north
That the existing system has sufficient blower capacity to handle the added airflow load (Correct answer)

Correct answer: That the existing system has sufficient blower capacity to handle the added airflow load

Adding supply runs increases system airflow demand; the blower and duct system must have sufficient capacity to supply the new run without starving existing registers.

Question 25: Which condition most commonly causes a furnace burner to produce a lifting or floating flame?

Too little gas pressure
Excess primary air at the burner (Correct answer)
Faulty thermocouple
Cracked heat exchanger

Correct answer: Excess primary air at the burner

Excess primary air causes the flame to lift off the burner ports because the mixture velocity exceeds the flame propagation speed.

Question 26: Supply registers in a forced-air system are typically located near which area for best comfort in heating climates?

Low on exterior walls under windows (Correct answer)
Near the ceiling on interior walls
In the center of the room ceiling
High on interior walls

Correct answer: Low on exterior walls under windows

Placing supply registers low on exterior walls under windows counteracts cold downdrafts and creates an even floor-to-ceiling temperature.

Question 27: What is the primary purpose of balancing airflow in a multi-zone forced-air system?

To ensure each conditioned space receives the correct volume of air to meet its load (Correct answer)
To maximize the furnace's AFUE rating
To prevent the heat exchanger from overheating
To equalize gas pressure across all burners

Correct answer: To ensure each conditioned space receives the correct volume of air to meet its load

Proper airflow balancing ensures each zone or room receives its design CFM, preventing over-conditioned and under-conditioned spaces that waste energy and reduce comfort.

Question 28: Ground-source heat pump hybrid systems paired with a gas furnace save energy by:

Using the heat pump efficiently at moderate temperatures and switching to gas only when temperatures drop below a balance point (Correct answer)
Using ground heat to preheat combustion air for the furnace
Running both systems simultaneously to reduce wear on each
Using the heat pump for all heating regardless of outdoor temperature

Correct answer: Using the heat pump efficiently at moderate temperatures and switching to gas only when temperatures drop below a balance point

In a hybrid system, a dual-fuel controller switches from the heat pump to the gas furnace when outdoor temperatures fall below the economic balance point where gas heating becomes cheaper.

Question 29: What is the purpose of calculating 'sensible heat ratio' (SHR) in HVAC equipment sizing?

To balance the proportion of sensible versus latent cooling capacity (Correct answer)
To size the gas supply line diameter
To determine the correct furnace input BTU rating
To calculate the flue gas temperature rise

Correct answer: To balance the proportion of sensible versus latent cooling capacity

Sensible Heat Ratio (SHR) describes what fraction of a unit's total cooling capacity handles sensible (temperature) versus latent (moisture) loads, ensuring proper dehumidification.

Question 30: The Manual D procedure is used in HVAC for which purpose?

Designing and sizing duct systems for proper airflow (Correct answer)
Selecting equipment capacity
Calculating heating and cooling loads
Testing duct leakage

Correct answer: Designing and sizing duct systems for proper airflow

ACCA Manual D is the industry standard procedure for residential duct system design, sizing trunks and branches for proper airflow distribution.

Question 31: What happens if a furnace is installed with the supply air filter missing during initial startup?

The gas valve will not open without filter differential pressure confirmation
The thermostat will display a filter alert within 24 hours
The furnace will run more efficiently due to unrestricted airflow
Dust and debris will accumulate on the heat exchanger and blower, reducing efficiency and possibly causing overheating (Correct answer)

Correct answer: Dust and debris will accumulate on the heat exchanger and blower, reducing efficiency and possibly causing overheating

Without a filter, dirt rapidly accumulates on the blower wheel and heat exchanger surfaces, reducing airflow, lowering efficiency, and potentially causing the heat exchanger to overheat.

Question 32: What is the typical manifold gas pressure for a natural gas furnace?

0.5 inches W.C.
11 inches W.C.
7 inches W.C.
3.5 inches W.C. (Correct answer)

Correct answer: 3.5 inches W.C.

Standard natural gas manifold pressure is 3.5 inches water column as specified by most furnace manufacturers.

Question 33: A cracked heat exchanger is most likely to allow which dangerous gas to enter the living space?

Carbon monoxide (Correct answer)
Nitrogen dioxide
Propane
Sulfur dioxide

Correct answer: Carbon monoxide

A cracked heat exchanger allows combustion byproducts including deadly carbon monoxide to leak into the circulating air.

Question 34: What device on a gas furnace controls the flow of gas to the main burner?

Manifold
Gas valve (Correct answer)
Orifice spud
Pressure regulator

Correct answer: Gas valve

The gas valve opens and closes to control gas flow to the main burner based on thermostat demand.

Question 35: A homeowner says the furnace is running but the house is not heating up. A technician checks and finds the supply air temperature is only 85°F instead of the expected 120–140°F. What should be inspected first?

Gas valve, igniter, and whether burners are actually firing (Correct answer)
Blower belt tension (if belt-drive unit)
Flue pipe termination cap outdoors
Thermostat heat anticipator setting

Correct answer: Gas valve, igniter, and whether burners are actually firing

Supply air at 85°F suggests the blower is running but the burners may not be firing, so verifying ignition components and gas valve operation is the first priority.

Question 36: What is the maximum permissible CO concentration in parts per million (ppm) in the flue gas of a properly operating gas furnace, as referenced by most safety standards?

400 ppm air-free (Correct answer)
50 ppm air-free
800 ppm air-free
200 ppm air-free

Correct answer: 400 ppm air-free

Most U.S. standards, including ANSI Z21.47, allow a maximum of 400 ppm CO air-free in furnace flue gases under normal operating conditions.

Question 37: What should be done to prevent carbon monoxide leaks after furnace installation?

Ensure the venting system is properly sealed and inspected (Correct answer)
Install a larger gas line.
Install more fuel-efficient burners.
Increase air flow through the furnace.

Correct answer: Ensure the venting system is properly sealed and inspected

To prevent dangerous carbon monoxide leaks after furnace installation, it is paramount to ensure the venting system is meticulously sealed and thoroughly inspected. A properly sealed and unobstructed vent pipe safely expels combustion byproducts, including carbon monoxide, outside the home, protecting occupants from this odorless and deadly gas.

Question 38: What type of duct leakage test measures the total leakage of a duct system at a reference pressure of 25 Pa?

Anemometer test
Blower door test
Pitot tube traverse
Duct blaster (duct pressurization) test (Correct answer)

Correct answer: Duct blaster (duct pressurization) test

A duct blaster test pressurizes the duct system to 25 Pa and measures airflow required to maintain pressure, quantifying total duct leakage.

Question 39: What is the stoichiometric air-to-fuel ratio for natural gas combustion?

15.4:1 (Correct answer)
20:1
12:1
10:1

Correct answer: 15.4:1

Natural gas requires approximately 15.4 parts air to 1 part fuel for complete stoichiometric combustion.

Question 40: Which component in a forced-air furnace is responsible for preventing the blower from running when the heat exchanger is too cold on startup?

Rollout switch
Fan-limit control (Correct answer)
Pressure switch
High-limit switch

Correct answer: Fan-limit control

The fan-limit control delays blower operation until the heat exchanger reaches an adequate temperature, preventing cold air circulation at startup.

Question 41: What tool is essential for verifying proper furnace installation?

Manometer (Correct answer)
Level gauge.
Thermometer.
Multimeter.

Correct answer: Manometer

A manometer is an essential tool for verifying proper furnace installation, particularly for gas furnaces. It is used to accurately measure gas pressure, which is critical for ensuring the furnace receives the correct amount of fuel for safe and efficient combustion, preventing issues like incomplete burning or excessive gas consumption.

Question 42: What is the purpose of an economizer in a commercial HVAC system with a furnace?

To recover heat from exhaust gases and preheat incoming combustion air
To store excess heat in a thermal mass for later use
To modulate gas valve position based on outdoor temperature
To use cool outdoor air for free cooling when conditions allow, reducing mechanical heating/cooling load (Correct answer)

Correct answer: To use cool outdoor air for free cooling when conditions allow, reducing mechanical heating/cooling load

An economizer uses outdoor air for ventilation and cooling when temperature and humidity conditions are favorable, reducing the need for mechanical conditioning.

Question 43: When a furnace is replaced with a higher-efficiency condensing model in a home that previously had a Category I atmospheric unit, what must be done to the existing chimney?

The chimney must be lined with an appropriate liner or the new furnace must be direct-vented with PVC, since the chimney is no longer used for furnace exhaust (Correct answer)
No changes are required; the chimney can continue to be used as-is
The chimney must be sealed at the bottom only
The chimney must be enlarged to handle condensate

Correct answer: The chimney must be lined with an appropriate liner or the new furnace must be direct-vented with PVC, since the chimney is no longer used for furnace exhaust

A high-efficiency furnace vents with PVC pipe, so the existing chimney is abandoned for furnace use and must be properly decommissioned or re-lined if other appliances still use it.

Question 44: What is AFUE and what does a rating of 96% mean for a furnace?

Annual Fuel Utilization Efficiency; 96% of fuel energy is converted to usable heat (Correct answer)
Air Flow Utilization Efficiency; 96% of airflow reaches the registers
Average Furnace Unit Efficiency; the furnace is 96% reliable
Actual Flue Usage Efficiency; 4% of flue gas is recycled

Correct answer: Annual Fuel Utilization Efficiency; 96% of fuel energy is converted to usable heat

AFUE (Annual Fuel Utilization Efficiency) measures seasonal heating efficiency; 96% AFUE means 96 cents of every dollar of fuel is converted to heat.

Question 45: What is the minimum clearance typically required between a gas furnace and combustible materials?

0 inches if listed as zero-clearance
6 inches on all sides
12 inches on all sides
1 inch on the sides, 6 inches on the front (Correct answer)

Correct answer: 1 inch on the sides, 6 inches on the front

Most furnaces require at least 1 inch side clearance and 6 inches front clearance unless listed as zero-clearance by the manufacturer.

Question 46: What is the primary function of the heat exchanger in a gas furnace?

To transfer heat from combustion gases to the circulating air without allowing gases to mix (Correct answer)
To regulate gas pressure
To filter combustion gases
To store heat between cycles

Correct answer: To transfer heat from combustion gases to the circulating air without allowing gases to mix

The heat exchanger is a sealed metal chamber that transfers heat from hot combustion gases to the circulating supply air while keeping them completely separated.

Question 47: What is the typical supply gas pressure (upstream of the gas valve) for a natural gas furnace?

5–7 inches W.C. (Correct answer)
1–2 inches W.C.
3.5–5 inches W.C.
10–14 inches W.C.

Correct answer: 5–7 inches W.C.

Natural gas utility supply pressure is typically 5–7 inches W.C. at the meter and furnace inlet.

Question 48: How is total system airflow (CFM) typically measured on a residential furnace during a commissioning check?

By counting the number of supply registers
By checking gas pressure only
By measuring duct velocity with a pitot tube at every register
Using the temperature rise method (measuring supply minus return air temperature and applying the furnace BTU output formula) (Correct answer)

Correct answer: Using the temperature rise method (measuring supply minus return air temperature and applying the furnace BTU output formula)

The temperature rise method calculates CFM by dividing the furnace BTU output by the measured temperature rise and a constant (1.08), a quick field verification.

Question 49: What is the function of a redundant gas valve?

It regulates pressure twice for accuracy
It serves as a backup if the primary valve fails open
It contains two independently operated valve seats for safety (Correct answer)
It controls both pilot and main gas with one solenoid

Correct answer: It contains two independently operated valve seats for safety

A redundant gas valve has two valve seats in series so that if one fails open, the other still prevents uncontrolled gas flow.

Question 50: What can cause a heating system to blow cold air instead of warm air?

The thermostat is set too high.
The furnace is malfunctioning or lacks sufficient fuel (Correct answer)
The air filter is clogged.
The furnace is too old.

Correct answer: The furnace is malfunctioning or lacks sufficient fuel

A heating system blowing cold air instead of warm air typically indicates that the furnace is failing to produce heat. This can be due to a malfunctioning furnace component, such as a faulty igniter or flame sensor, or a lack of sufficient fuel supply (e.g., a closed gas valve or empty oil tank). These issues prevent the system from generating and distributing warm air.

Question 51: What is the recommended maximum air velocity in residential supply duct main trunks to minimize noise?

1,500 FPM
900 FPM
400 FPM
700–900 FPM (Correct answer)

Correct answer: 700–900 FPM

Residential supply trunk velocities of 700–900 FPM are recommended to balance duct size with acceptable noise levels.

Question 52: What does a CO2 reading of 9% in a natural gas furnace flue gas indicate?

Incomplete combustion
Too much excess air
Proper combustion with slight excess air (Correct answer)
Excellent combustion efficiency

Correct answer: Proper combustion with slight excess air

A CO2 reading of 9% for natural gas indicates proper combustion with a slight amount of excess air for safe operation.

Question 53: Which type of ignition system uses a resistance element heated to approximately 1,800°F to ignite the gas?

Hot surface igniter (Correct answer)
Spark igniter
Standing pilot
Intermittent pilot

Correct answer: Hot surface igniter

A hot surface igniter (HSI) uses silicon carbide or silicon nitride that glows red-hot to directly ignite the gas-air mixture.

Question 54: Which combustion efficiency measurement tool directly measures the efficiency of the burner and heat exchanger as a system?

Manometer
Anemometer
Psychrometer
Combustion analyzer (flue gas analyzer) (Correct answer)

Correct answer: Combustion analyzer (flue gas analyzer)

A combustion analyzer measures flue gas O2, CO2, CO, and temperature to calculate combustion efficiency, identifying burner and heat exchanger performance issues.

Question 55: Which color of a burner flame indicates incomplete combustion?

Yellow or orange (Correct answer)
White
Blue
Blue with yellow tips

Correct answer: Yellow or orange

A yellow or orange flame indicates incomplete combustion due to insufficient air or a clogged burner.

Question 56: Zoning systems improve energy efficiency in a multi-story home by:

Allowing each zone to have its own fuel source
Increasing total furnace capacity to heat all zones simultaneously
Delivering conditioned air only to occupied zones, reducing overall runtime (Correct answer)
Eliminating the need for return air ducts

Correct answer: Delivering conditioned air only to occupied zones, reducing overall runtime

Zoning with motorized dampers directs heating only to occupied areas, preventing energy waste in unused rooms and reducing total system runtime.

Question 57: What is the purpose of a vent termination screen and what maintenance does it require?

It prevents birds and debris from entering; must be inspected and cleaned regularly (Correct answer)
It increases draft; no maintenance needed
It measures exhaust temperature; no maintenance
It reduces CO emissions; replaced annually

Correct answer: It prevents birds and debris from entering; must be inspected and cleaned regularly

Termination screens block birds, insects, and debris from entering the vent but must be inspected regularly because they can clog with ice or debris and cause lockouts.

Question 58: What does the term 'static pressure' refer to in a furnace installation context?

The draft pressure in the flue vent
The resistance to airflow in the duct system measured in inches of water column (Correct answer)
The gas inlet pressure at the appliance
The pressure in the combustion chamber during firing

Correct answer: The resistance to airflow in the duct system measured in inches of water column

Static pressure is the resistance the blower must overcome to move air through the duct system, and is measured in inches of water column (in. W.C.).

Question 59: What is the purpose of a static pressure test on a forced-air furnace system?

To check gas pressure
To test the heat exchanger for leaks
To measure airflow resistance in the duct system and verify proper system operation (Correct answer)
To measure combustion air volume

Correct answer: To measure airflow resistance in the duct system and verify proper system operation

Static pressure testing reveals duct restrictions, undersized ductwork, or dirty filters that reduce airflow and system efficiency.

Question 60: How often should the condensate trap and drain line on a high-efficiency condensing furnace be inspected or flushed?

Only when blockage symptoms appear
At least once per heating season (Correct answer)
Every 5 years
Every 10 years or at manufacturer intervals

Correct answer: At least once per heating season

The condensate trap and drain line should be inspected and flushed at least annually to prevent blockages that can trigger pressure switch lockouts.

Question 61: When performing a load calculation for a home addition, what must the technician verify about the existing furnace?

Whether the furnace uses a two-stage gas valve
Whether the furnace has a variable-speed blower
Whether the existing furnace has sufficient capacity to handle the additional load (Correct answer)
Whether the furnace was installed by a licensed contractor

Correct answer: Whether the existing furnace has sufficient capacity to handle the additional load

Before extending an existing heating system to a new addition, the technician must verify that the existing furnace has enough remaining capacity to handle the additional heating load.

Question 62: What is the purpose of a fresh air intake (combustion air duct) brought to the furnace room?

To cool the heat exchanger
To ensure adequate combustion air supply so negative pressure doesn't cause backdrafting (Correct answer)
To supply air for occupant comfort
To dilute gas leaks

Correct answer: To ensure adequate combustion air supply so negative pressure doesn't cause backdrafting

A dedicated combustion air duct prevents depressurization of the mechanical room, which could cause flue gases to backdraft into the building.

Question 63: A home has a calculated design heating load of 60,000 BTU/h. Which furnace output capacity is the most appropriate selection?

120,000 BTU/h output
40,000 BTU/h output
80,000 BTU/h output (Correct answer)
20,000 BTU/h output

Correct answer: 80,000 BTU/h output

An 80,000 BTU/h output furnace represents a 33% overage over the 60,000 BTU/h load, which falls within the ACCA Manual S acceptable range for gas furnaces.

Question 64: A furnace control board flashes a fault code 3 times (pause) 1 time. How should a technician interpret this?

The code is 31 and corresponds to a specific fault in the manufacturer's chart (Correct answer)
The LED is malfunctioning
The board is counting 3 minutes to reset
Three faults occurred, then one more

Correct answer: The code is 31 and corresponds to a specific fault in the manufacturer's chart

Furnace control boards use flash codes where the number of long flashes followed by short flashes encodes a specific fault code (e.g., 3-1 = code 31).

Question 65: Gas supply piping to a furnace must be tested for leaks using which method?

Pressure drop test only
CO detector
Soapy water solution or listed leak detection fluid (Correct answer)
Open flame such as a match

Correct answer: Soapy water solution or listed leak detection fluid

Gas connections must be tested with soapy water or a listed leak detector fluid; open flames are never permitted for leak detection.

Question 66: What is the minimum clearance required between a furnace and combustible materials on the sides, unless the manufacturer specifies otherwise?

12 inches
1 inch (Correct answer)
18 inches
6 inches

Correct answer: 1 inch

Most furnace manufacturers require at least 1 inch of clearance from combustible materials on the sides, but always defer to the installation manual for model-specific requirements.

Question 67: Under the IRC Section G2447, what is the maximum length allowed for an appliance connector used to connect a furnace to the gas supply piping?

10 feet
3 feet
5 feet
6 feet (Correct answer)

Correct answer: 6 feet

The IRC limits appliance connectors to a maximum length of 6 feet to reduce the risk of damage or gas leakage from flexible connectors.

Question 68: Flex duct installed with excessive sag will cause which problem?

Increased airflow restriction due to liner compression and tight bends (Correct answer)
Increased duct leakage
Overheating of the furnace
Condensation in the duct

Correct answer: Increased airflow restriction due to liner compression and tight bends

Sagging flex duct compresses the inner liner, dramatically increasing airflow resistance and reducing delivered CFM to registers.

Question 69: Why is it necessary to check the furnace's thermostat settings?

To avoid the use of too much electricity.
To verify that it is calibrated to maintain the correct room temperature (Correct answer)
To ensure the system is properly connected to the power grid.
To check if it is compatible with the furnace model.

Correct answer: To verify that it is calibrated to maintain the correct room temperature

It is necessary to check the furnace's thermostat settings to verify that it is accurately calibrated and functioning correctly to maintain the desired room temperature. An improperly calibrated thermostat can lead to inefficient heating, discomfort, and increased energy consumption, undermining the furnace's performance and leading to higher utility bills.

Question 70: Which inspection technique is commonly used by technicians to visually check for heat exchanger cracks during operation?

Measure flue gas temperature with a thermocouple
Shine a bright flashlight inside the heat exchanger only
Use a borescope camera through the flue outlet
Observe the burner flame for movement or disturbance when the blower turns on (Correct answer)

Correct answer: Observe the burner flame for movement or disturbance when the blower turns on

When the blower starts, air pressure changes can cause a cracked heat exchanger to deflect and disturb the burner flames, visible as flame rollout or flickering.

Question 71: What is the stainless steel secondary heat exchanger in a condensing furnace susceptible to if the condensate drain is not maintained?

Acid corrosion from pooled condensate causing pinhole leaks (Correct answer)
Biological fouling
Scale buildup from hard water only
Thermal fatigue cracking

Correct answer: Acid corrosion from pooled condensate causing pinhole leaks

Condensate from combustion has a pH of 3–5; if it pools rather than drains properly it will corrode even stainless steel secondary heat exchanger coils.

Question 72: What is the purpose of balancing dampers in a forced-air distribution system?

To prevent backdrafting in the furnace
To reduce duct static pressure overall
To adjust airflow to individual zones or rooms to achieve comfort balance (Correct answer)
To prevent duct condensation

Correct answer: To adjust airflow to individual zones or rooms to achieve comfort balance

Balancing dampers allow technicians to restrict airflow to over-supplied areas and redirect it to under-supplied rooms.

Question 73: A technician notices the furnace flue pipe has a visible sag between two support hangers. Why is this a maintenance concern?

Condensate can pool in the sag, corroding the pipe and blocking proper venting (Correct answer)
It causes excessive vibration in the inducer motor
It increases flue gas velocity, reducing heat transfer
It reduces combustion air pressure to the burners

Correct answer: Condensate can pool in the sag, corroding the pipe and blocking proper venting

A sagging flue section traps condensate, leading to corrosion and eventual perforation that can allow combustion gases to enter the building.

Question 74: What instrument is used to measure gas manifold pressure on a furnace?

Flue gas analyzer
Manometer (Correct answer)
Ammeter
Anemometer

Correct answer: Manometer

A manometer (or magnahelic gauge) measures the low pressure in inches of water column at the furnace manifold.

Question 75: What is the purpose of the concentric vent system on a direct-vent condensing furnace?

To increase combustion efficiency with preheated air only
To save duct space by combining supply and return
To use one pipe for exhaust and a surrounding outer pipe for combustion air intake (Correct answer)
To allow longer vent runs

Correct answer: To use one pipe for exhaust and a surrounding outer pipe for combustion air intake

Concentric venting combines the exhaust flue (inner pipe) and combustion air intake (outer annulus) in one penetration through the wall or roof.

Question 76: What safety equipment should be used when working with gas furnaces?

No equipment is necessary.
Safety glasses only.
Only gloves.
Gloves, safety glasses, and a carbon monoxide detector (Correct answer)

Correct answer: Gloves, safety glasses, and a carbon monoxide detector

When working with gas furnaces, essential safety equipment includes gloves to protect hands, safety glasses to shield eyes from debris or chemicals, and a carbon monoxide detector to monitor for dangerous gas leaks. Given the risks associated with gas and combustion, these items are critical for personal protection and ensuring a safe working environment.

Question 77: What is the purpose of the pressure switch on an induced-draft furnace?

To detect duct static pressure
To confirm the inducer motor is creating proper draft before allowing ignition (Correct answer)
To verify gas pressure is adequate
To sense indoor air pressure for comfort control

Correct answer: To confirm the inducer motor is creating proper draft before allowing ignition

The pressure switch verifies that the draft inducer has established proper negative pressure before the control board allows the ignition sequence to proceed.

Question 78: When converting a furnace from natural gas to propane (LP), what must be replaced?

The orifice spud and pressure regulator spring (Correct answer)
The blower motor
The heat exchanger
The thermocouple

Correct answer: The orifice spud and pressure regulator spring

Propane has higher BTU content and different pressure requirements, so orifices must be downsized and the regulator spring changed.

Question 79: Thermal envelope improvements such as adding insulation and sealing air leaks affect the furnace system by:

Increasing humidity levels that require more dehumidification
Requiring a larger furnace to handle the increased heat load
Causing the existing furnace to short-cycle more frequently without adjustment
Reducing heating load, potentially requiring a smaller furnace and fewer annual run hours (Correct answer)

Correct answer: Reducing heating load, potentially requiring a smaller furnace and fewer annual run hours

Reducing heat loss through improved insulation and air sealing lowers the design heating load, meaning a properly sized furnace will run fewer hours and consume less fuel annually.

Question 80: Which type of duct fitting creates the least resistance to airflow?

Long-radius swept 90-degree elbow (Correct answer)
Sharp 90-degree elbow
Square elbow with turning vanes
Tee fitting

Correct answer: Long-radius swept 90-degree elbow

A long-radius swept elbow allows air to change direction gradually, creating far less turbulence and pressure drop than sharp elbows.

Question 81: Why is using a simple 'rules of thumb' (e.g., BTU per square foot) discouraged as the sole method for sizing furnace equipment?

Rules of thumb are only valid for commercial buildings, not residential
Rules of thumb are not recognized by ASHRAE or ACCA
Rules of thumb always result in undersized equipment
Rules of thumb ignore critical variables like insulation, windows, and climate, leading to inaccurate sizing (Correct answer)

Correct answer: Rules of thumb ignore critical variables like insulation, windows, and climate, leading to inaccurate sizing

Rules of thumb ignore critical building-specific factors such as insulation levels, window area, orientation, infiltration, and local climate, which can lead to significantly over- or under-sized equipment.

Question 82: When performing a combustion analysis on a high-efficiency condensing furnace, a technician finds excess air is very high. This indicates:

The heat exchanger is cracked and should be replaced immediately
Air is infiltrating the combustion system, diluting flue gases and reducing efficiency (Correct answer)
The secondary heat exchanger is operating correctly
The gas pressure is set too high at the manifold

Correct answer: Air is infiltrating the combustion system, diluting flue gases and reducing efficiency

Abnormally high excess air (high O2, low CO2) in a sealed combustion furnace indicates an air infiltration problem, which dilutes combustion gases and reduces heat transfer efficiency.

Question 83: Static pressure in the duct system is important for efficiency because excessively high static pressure causes:

Condensation to form in the flue pipe
The heat exchanger to cool down faster
The gas valve to open wider, increasing fuel consumption
The blower motor to work harder, consuming more electricity and reducing airflow (Correct answer)

Correct answer: The blower motor to work harder, consuming more electricity and reducing airflow

High static pressure forces the blower motor to draw more amperage and can cause it to overheat, significantly increasing electrical energy consumption while reducing airflow.

Question 84: What does a rollout switch do when it trips?

Resets the thermostat
Shuts off the gas valve because flames have rolled out of the combustion chamber (Correct answer)
Opens the fresh air damper
Shuts off the blower motor

Correct answer: Shuts off the gas valve because flames have rolled out of the combustion chamber

A rollout switch detects flame rollout from the combustion area and interrupts the gas valve circuit to prevent a fire hazard.

Question 85: When lubricating a furnace blower motor with oil ports, what type of oil should be used?

Grease from a grease gun
WD-40 penetrating lubricant
Automotive motor oil (SAE 10W-30)
Non-detergent electric motor oil (SAE 10 or 20) (Correct answer)

Correct answer: Non-detergent electric motor oil (SAE 10 or 20)

Non-detergent electric motor oil is specified for furnace blower motors because detergent additives can damage the motor windings and oil ports.

Question 86: What problem does an unbalanced duct system (too much supply, not enough return) cause in a home?

Positive pressure in the home causing infiltration to push outward, and reduced blower efficiency (Correct answer)
The thermostat reads incorrectly
The furnace runs longer cycles
The heat exchanger overheats

Correct answer: Positive pressure in the home causing infiltration to push outward, and reduced blower efficiency

Excess supply without matching return creates positive pressure that forces conditioned air out through leaks and reduces blower performance.

Question 87: What is the primary purpose of an induced-draft (power vent) blower on a mid-efficiency gas furnace?

To pull flue gases through the heat exchanger and expel them outdoors (Correct answer)
To pressurize the combustion chamber for better mixing
To recirculate exhaust gases for higher efficiency
To increase combustion air temperature before mixing with gas

Correct answer: To pull flue gases through the heat exchanger and expel them outdoors

The induced-draft blower creates negative pressure in the flue, drawing combustion gases through the heat exchanger and venting them safely outdoors.

Question 88: What is the function of the blower delay-off control at the end of a heating cycle?

To purge the flue of remaining combustion gases
To prevent short cycling of the thermostat
To keep the blower running after the burner shuts off to recover residual heat from the heat exchanger (Correct answer)
To cool the combustion chamber rapidly

Correct answer: To keep the blower running after the burner shuts off to recover residual heat from the heat exchanger

The blower continues running for 1–3 minutes after burner shutdown to extract residual heat stored in the heat exchanger and deliver it to the living space.

Question 89: Which duct system characteristic most significantly reduces overall HVAC system efficiency?

Ducts made of sheet metal instead of flex duct
Ducts with 90-degree elbows instead of sweeping turns
Leaky ducts located in unconditioned spaces like attics or crawlspaces (Correct answer)
Round ducts instead of rectangular ducts

Correct answer: Leaky ducts located in unconditioned spaces like attics or crawlspaces

Duct leakage in unconditioned spaces can reduce system efficiency by 20–30% because conditioned air is lost to areas outside the thermal envelope.

Question 90: During routine maintenance, a technician finds the furnace burners coated with rust and scale. After cleaning, what additional check is MOST important before returning the system to service?

Test blower motor capacitor with a multimeter
Inspect the condensate neutralizer cartridge
Check manifold gas pressure and flame appearance to confirm proper combustion (Correct answer)
Verify thermostat calibration against a calibrated thermometer

Correct answer: Check manifold gas pressure and flame appearance to confirm proper combustion

After cleaning burners, confirming correct manifold pressure and a steady blue flame ensures safe and efficient combustion before returning the unit to service.

Question 91: Why is checking the blower motor essential during maintenance?

To improve system performance (Correct answer)
To increase fuel consumption.
To reduce the system’s noise.
To make the system last longer.

Correct answer: To improve system performance

The blower motor is responsible for circulating heated air throughout the building, making it a vital component for system performance. During maintenance, checking the blower motor ensures it's clean, lubricated, and operating efficiently. A well-maintained blower motor guarantees proper air distribution and contributes to the overall effectiveness of the heating system.

Question 92: On a two-stage gas furnace, when should the system operate in second-stage (high fire)?

Only during heat pump defrost cycles
Immediately on every call for heat
When outdoor temperature drops below 50°F regardless of indoor setpoint
Only after the first stage has run for a set period and the setpoint has not been reached (Correct answer)

Correct answer: Only after the first stage has run for a set period and the setpoint has not been reached

Two-stage furnaces start in low fire for efficiency and comfort, escalating to high fire only if the indoor temperature setpoint is not reached within a defined time period.

Question 93: When sealing duct joints and seams, which material is approved by most building codes and SMACNA standards?

Standard cloth duct tape
Foam weatherstripping
UL 181-rated mastic or foil tape (Correct answer)
Silicone caulk

Correct answer: UL 181-rated mastic or foil tape

UL 181-rated mastic sealant or pressure-sensitive foil tape are code-approved for sealing rigid and flex duct connections.

Question 94: What is the minimum required slope for a horizontal Category IV condensate drain line exiting a high-efficiency furnace?

1/4 inch per foot toward the drain (Correct answer)
Level is acceptable if trap is provided
1/2 inch per foot toward the drain
1/8 inch per foot toward the drain

Correct answer: 1/4 inch per foot toward the drain

Condensate drain lines should slope a minimum of 1/4 inch per foot toward the drain to ensure proper condensate removal and prevent blockage.

Question 95: What is the purpose of an LP gas regulator at the tank?

To prevent LP from vaporizing
To convert LP to natural gas
To measure LP consumption
To reduce high tank pressure to a safe working pressure (Correct answer)

Correct answer: To reduce high tank pressure to a safe working pressure

The LP regulator reduces tank pressure (up to 250 psi) down to 11 inches W.C. working pressure for appliances.

Question 96: What does the W terminal on a thermostat signal to the furnace?

Fan-only demand
Emergency heat
Cooling demand
Heating demand (Correct answer)

Correct answer: Heating demand

The W terminal carries the 24V heating call signal from the thermostat to the furnace control board to initiate the heating sequence.

Question 97: What is the maximum total external static pressure (TESP) typically allowed for a residential furnace?

0.5 inches W.C. (Correct answer)
1.0 inches W.C.
2.0 inches W.C.
0.1 inches W.C.

Correct answer: 0.5 inches W.C.

Most residential furnaces are rated for a maximum TESP of 0.5 inches W.C.; exceeding this restricts airflow and causes overheating.

Question 98: Return air duct sizing should be based on which primary factor?

System airflow (CFM) requirements of the equipment (Correct answer)
Number of supply registers
Duct material type
Furnace BTU input

Correct answer: System airflow (CFM) requirements of the equipment

Return ducts must be sized to carry the full system CFM with acceptable velocity and pressure drop to ensure adequate airflow to the blower.

Question 99: What effect does a dirty evaporator coil (installed above the furnace in a split system) have on furnace operation?

It lowers gas pressure
It has no effect on furnace operation
It restricts airflow, causing the furnace to overheat and trip the high-limit switch (Correct answer)
It increases furnace efficiency

Correct answer: It restricts airflow, causing the furnace to overheat and trip the high-limit switch

A clogged evaporator coil acts as a severe airflow restriction, reducing CFM across the heat exchanger and causing high-limit trips.

Question 100: What is the typical time delay between thermostat call for heat and blower motor startup on a furnace with a time-delay relay?

5 minutes after thermostat call
30–90 seconds after burner ignition (Correct answer)
Only after thermostat is satisfied
Immediately at burner ignition

Correct answer: 30–90 seconds after burner ignition

The blower is intentionally delayed 30–90 seconds after burner ignition to allow the heat exchanger to warm up before circulating air.

Furnace Installation and Maintenance Certification (FC)

The FC Furnace Installation and Maintenance Certification assesses technicians' knowledge and skills in residential and commercial furnace installation, service, and maintenance, covering combustion systems, heat exchangers, ductwork, and blower systems.

100

Questions

180 min

Time Limit

70%

To Pass

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