HVAC Air Handler Units and Coils 5 — Questions and Answers
Question 1: What is the correct procedure if an evaporator coil is found to be completely iced over during a service call?
- Immediately add refrigerant to raise system pressure
- Turn the system to fan-only mode to thaw the coil before diagnosing the cause (Correct answer)
- Replace the coil immediately as icing indicates coil failure
- Increase blower speed to maximum to melt the ice
Correct answer: Turn the system to fan-only mode to thaw the coil before diagnosing the cause
The coil must be allowed to thaw completely (using fan-only mode) before diagnosis because testing a frozen system gives inaccurate readings and won't reveal the true cause.
Question 2: What is the difference between a 'chilled water coil' and a 'direct expansion (DX) coil' in an air handler?
- Chilled water coils use refrigerant directly; DX coils use chilled water
- Chilled water coils circulate cold water through tubes; DX coils use refrigerant directly (Correct answer)
- Chilled water coils are only used in commercial buildings; DX coils are residential only
- Chilled water coils require a TXV; DX coils use a fixed orifice only
Correct answer: Chilled water coils circulate cold water through tubes; DX coils use refrigerant directly
A chilled water coil uses chilled water pumped from a chiller as its cooling medium, while a DX coil has refrigerant flowing directly through its tubes from the compressor system.
Question 3: What is the purpose of 'subcooling' in a condenser coil?
- To reduce the temperature of supply air below the thermostat setpoint
- To cool liquid refrigerant below its condensing temperature to prevent flash gas in the liquid line (Correct answer)
- To freeze condensate on the evaporator coil for better dehumidification
- To pre-cool incoming outdoor air before it enters the condenser
Correct answer: To cool liquid refrigerant below its condensing temperature to prevent flash gas in the liquid line
Subcooling cools liquid refrigerant several degrees below its condensing temperature, preventing it from flashing into vapor in the liquid line before reaching the expansion device.
Question 4: What is a 'multi-speed' blower motor and what advantage does it provide?
- A motor with multiple windings that can run at different RPMs to match varying system demands (Correct answer)
- A motor designed to run multiple blower wheels simultaneously
- A motor with a variable frequency drive for precise airflow control
- A motor that can reverse direction to prevent coil icing
Correct answer: A motor with multiple windings that can run at different RPMs to match varying system demands
A multi-speed blower motor has multiple tapped windings that allow it to operate at different fixed speeds, enabling adjustment for heating vs. cooling or different duct system conditions.
Question 5: When replacing an evaporator coil, why must the refrigerant circuit be properly evacuated before recharging?
- To remove excess oil that accumulated during the coil replacement
- To remove air and moisture that entered the system during the open coil replacement (Correct answer)
- To reset the TXV superheat setting to factory defaults
- To clear error codes from the system control board
Correct answer: To remove air and moisture that entered the system during the open coil replacement
Evacuation removes air and moisture that entered the open refrigerant system during coil replacement; moisture in the system can cause acid formation, freezing at the metering device, and compressor damage.
Question 6: What is the function of a 'coil baffle' or 'coil pan liner' in an air handler?
- To direct airflow evenly across the entire coil face
- To prevent air bypass around the sides of the coil (Correct answer)
- To support the weight of the coil within the air handler cabinet
- To insulate the coil from the surrounding sheet metal
Correct answer: To prevent air bypass around the sides of the coil
Coil baffles seal the gaps between the coil and the air handler cabinet to prevent unconditioned air from bypassing the coil, ensuring all air passes through for proper treatment.
Question 7: What is the primary advantage of a microchannel heat exchanger coil compared to a traditional round-tube plate-fin (RTPF) coil?
- Microchannel coils are less expensive to manufacture
- Microchannel coils have smaller refrigerant volume, lower weight, and better heat transfer efficiency (Correct answer)
- Microchannel coils are easier to clean with standard fin combs
- Microchannel coils can operate with any refrigerant type without modification
Correct answer: Microchannel coils have smaller refrigerant volume, lower weight, and better heat transfer efficiency
Microchannel coils use flat multi-port aluminum tubes that require less refrigerant charge, weigh less, and provide superior heat transfer due to their large surface area-to-volume ratio.
HVAC Air Handler Units and Coils 5
What is the correct procedure if an evaporator coil is found to be completely iced over during a service call?