HVAC Air Conditioning 5 — Questions and Answers
Question 1: Which type of air distribution system delivers conditioned air through a single large duct and uses a mixing box at each zone to control temperature?
- Variable air volume (VAV) system
- Multi-zone system
- Dual-duct system (Correct answer)
- Fan-coil unit system
Correct answer: Dual-duct system
A dual-duct system supplies both cold and hot air through separate ducts to mixing boxes at each zone, where they blend to achieve the desired temperature.
Question 2: A compressor that is 'pumping down' correctly should result in what suction pressure reading?
- Equal to outdoor ambient saturation pressure
- Near 0 psig or slightly into a vacuum (Correct answer)
- Equal to the normal operating suction pressure
- Significantly above normal suction pressure
Correct answer: Near 0 psig or slightly into a vacuum
When pumping down, the compressor evacuates refrigerant from the low side until the suction pressure reaches approximately 0 psig or a slight vacuum, then the low-pressure control shuts it off.
Question 3: What is the primary cause of 'liquid slugging' in an air conditioning compressor?
- Refrigerant vapor entering the compressor at high superheat
- Liquid refrigerant or oil entering the compressor suction (Correct answer)
- High discharge pressure due to condenser fouling
- Voltage imbalance on three-phase compressor
Correct answer: Liquid refrigerant or oil entering the compressor suction
Liquid slugging occurs when liquid refrigerant or oil enters the compressor suction, and since liquids are incompressible, this can bend connecting rods or break valve reeds.
Question 4: What does a high superheat reading at the evaporator outlet combined with low suction pressure most likely indicate?
- Refrigerant overcharge
- Restricted metering device or low refrigerant charge (Correct answer)
- Condenser fan failure
- High ambient temperature outdoors
Correct answer: Restricted metering device or low refrigerant charge
High superheat with low suction pressure indicates the evaporator is starved of refrigerant, which is caused by a restricted metering device or insufficient refrigerant charge.
Question 5: Which component in a residential split-system air conditioner is responsible for moving air across the indoor coil?
- Condenser fan motor
- Induced draft blower
- Air handler blower motor (Correct answer)
- Evaporator fan relay
Correct answer: Air handler blower motor
The air handler blower motor (typically a multi-speed or ECM motor) circulates indoor air across the evaporator coil for heat exchange.
Question 6: What is the significance of the 'critical charge' in a fixed-orifice AC system versus a TXV system?
- Both system types require the same precise charge regardless of conditions
- Fixed-orifice systems are far more sensitive to charge amount than TXV systems (Correct answer)
- TXV systems require a more precise charge than fixed-orifice systems
- Charge amount only affects efficiency, not operation, in both systems
Correct answer: Fixed-orifice systems are far more sensitive to charge amount than TXV systems
Fixed-orifice (piston/capillary) systems are critically sensitive to refrigerant charge because the metering device cannot compensate for variations, while a TXV actively adjusts flow to maintain proper superheat.
Question 7: When measuring the outdoor ambient temperature for AC system performance diagnosis, where should the measurement be taken?
- In direct sunlight near the condenser unit
- At the condenser air inlet in the shade, away from the unit's discharge air (Correct answer)
- Inside the return air duct
- At the liquid line service port
Correct answer: At the condenser air inlet in the shade, away from the unit's discharge air
Outdoor ambient temperature should be measured in the shade at the condenser air inlet, away from the hot discharge airstream, to get an accurate reference for system performance charts.
HVAC Air Conditioning 5
Which type of air distribution system delivers conditioned air through a single large duct and uses a mixing box at each zone to control temperature?