Mixed Deck — All Air Conditioning Repair Topics Flashcards

What is the correct procedure for adding refrigerant to a car AC system that is slightly low?

Answer: Add refrigerant as a liquid to the low-side port with the engine running and AC on

Refrigerant is added as a gas (or low-pressure liquid from an inverted can) to the low-side service port with the engine running and AC on max, never to the high side.

Which component's failure would most likely cause high superheat AND high subcooling simultaneously?

Answer: Restricted liquid line or filter drier

A restriction in the liquid line or a clogged filter drier causes refrigerant to back up (high subcooling) before the restriction and starve the evaporator (high superheat) after it.

According to technician A, a gasket or O-ring is used to seal the system at every air conditioning connection. According to Technician B, lubricating the O-ring with the appropriate refrigerant oil before replacing it would help prevent nicks and make installation easier. Who is correct?

Answer: Both Technician A and B

Technician A is correct because AC systems operate under high pressure and require hermetic seals to prevent refrigerant leaks, which are provided by gaskets and O-rings at every connection. Technician B is also correct; lubricating O-rings with the appropriate refrigerant oil before installation is standard practice to prevent damage like nicks or twisting during assembly, ensuring a proper and durable seal. Both practices are crucial for effective AC system repair and maintenance.

An HVAC technician measures 0V AC between the R and C terminals at the air handler control board with the disconnect closed. What does this indicate?

Answer: The 24V transformer has failed or is not receiving primary voltage

No voltage between R and C with power applied means the 24V transformer is not producing output, either due to a failed transformer or no primary voltage reaching it.

After recharging, a technician notices the AC vent temperature is warmer than expected despite correct refrigerant charge. What component should be inspected next?

Answer: Condenser fins for dirt and blockage

A clogged or bent condenser prevents adequate heat rejection, raising refrigerant temperature and reducing the system's ability to produce cold air even with a proper charge.

When replacing an AC compressor, why must a specific amount of oil be added to the new unit?

Answer: To compensate for oil retained in old system components

Old system components (condenser, evaporator, lines) retain a portion of compressor oil, so the replacement compressor must be pre-filled to match total system oil capacity.

What happens in a condenser in a cooling and refrigeration system?

Answer: The refrigerant releases the latent heat.

In a condenser, the high-pressure, high-temperature gaseous refrigerant releases its latent heat to the cooler ambient air, typically aided by a fan. As it releases this heat, the refrigerant changes phase from a gas back into a liquid. This process is essential for dissipating the heat absorbed from the cabin and completing the refrigeration cycle.

What does a locked rotor amperage (LRA) rating indicate on a compressor nameplate?

Answer: Current draw when the motor shaft is stationary

LRA is the inrush current drawn by the compressor motor at startup before the rotor begins to spin.

During a service call, a technician measures suction pressure lower than normal and discharge pressure lower than normal. This most likely indicates:

Answer: Low refrigerant charge

Both suction and discharge pressures below normal simultaneously is a classic indicator of an undercharged (low refrigerant) system.

A technician measures voltage across a contactor coil and reads 24V AC, but the contactor does not pull in. What is the most likely cause?

Answer: Open circuit in the coil winding

If 24V AC is present at the coil terminals but the contactor won't pull in, the coil winding itself is open and must be replaced.

A residential AC system's SEER2 rating is used to measure what?

Answer: Seasonal cooling efficiency over an entire cooling season under varied conditions

SEER2 (Seasonal Energy Efficiency Ratio 2) measures average cooling efficiency across a full season under a range of temperature conditions using updated test procedures.

What condition is indicated when the liquid line feels warm to the touch near the condenser on a properly charged system on a hot day?

Answer: This is normal — the liquid line should be warm, carrying high-pressure liquid

The liquid line carries high-pressure, warm liquid refrigerant from the condenser and will naturally feel warm to the touch under normal operation.

When charging a system by weight, the technician should zero the refrigerant scale:

Answer: Before connecting the hose, with the empty hose weight accounted for

Zeroing the scale with the hose already attached accounts for the hose weight, so the display shows only the refrigerant transferred rather than hose plus refrigerant.

What is the minimum recommended vacuum level (microns) before closing service valves and charging a residential AC system?

Answer: 500 microns

Most manufacturers and industry standards require a vacuum of 500 microns or lower to ensure all moisture and non-condensables are removed.

Which of the following electrical safety devices is designed to open the control circuit and shut down the compressor in the event of a restricted condenser coil or a failed condenser fan motor?

Answer: A high-pressure safety switch

A high-pressure safety switch monitors the refrigerant discharge pressure. Conditions like a dirty condenser coil or a failed condenser fan motor prevent proper heat rejection, causing a rapid and dangerous increase in head pressure. The high-pressure switch is designed to detect this rise and open the 24V control circuit to the compressor contactor, stopping the compressor to prevent serious damage.

What is 'triple evacuation' (also called 'three-step evacuation') used for?

Answer: Removing moisture more effectively by breaking vacuum with dry nitrogen between pulls

Triple evacuation involves pulling vacuum, breaking with dry nitrogen, then repeating twice more — each cycle draws out more moisture than a single long pull.

What is the purpose of a condensate trap installed on an AC air handler?

Answer: To prevent negative duct pressure from pulling sewer gases back through the drain line

A condensate trap creates a water seal that prevents negative static pressure in the air handler from breaking the drain flow and introducing odors or pathogens.

A potential relay used in a hard-start kit opens its contacts when which condition is met?

Answer: Back-EMF across the start winding reaches approximately 75% of line voltage

The potential relay monitors the voltage induced in the start winding; once back-EMF rises to the relay's pick-up voltage, it opens to remove the start capacitor from the circuit.

What is the purpose of trapping the suction line at the base of a vertical rise when the evaporator is located above the condensing unit?

Answer: To prevent oil from draining back and flooding the compressor during off cycles

An oil trap at the base of a suction line riser ensures oil circulates back to the compressor and does not accumulate in the low point of the line.

While diagnosing a system, a technician observes low suction pressure, low superheat, and a frosted evaporator coil only near its inlet. Further inspection reveals an abnormally high subcooling reading. What is the most probable diagnosis?

Answer: A restriction in the liquid line filter-drier.

A restriction before the metering device (like in the filter-drier) causes refrigerant to back up in the condenser, leading to high subcooling. The restriction starves the evaporator of refrigerant, causing a large pressure drop across the blockage. This results in a very low suction pressure and causes the small amount of refrigerant that passes through to flash off and freeze immediately at the evaporator inlet, leading to low superheat readings.