HVAC Heat Pump Technology 4 — Questions and Answers
Question 1: In a dual-fuel heat pump system, what determines when the gas furnace takes over from the heat pump?
- Indoor humidity level exceeds the setpoint
- Outdoor temperature drops below the economic or mechanical balance point (Correct answer)
- The thermostat senses a drop in indoor temperature of more than 2°F
- Refrigerant pressure exceeds the high-pressure cutout
Correct answer: Outdoor temperature drops below the economic or mechanical balance point
In a dual-fuel system, the gas furnace activates when the outdoor temperature drops below the balance point where the heat pump becomes less economical or loses capacity.
Question 2: What is the purpose of check valves in a heat pump refrigerant circuit?
- To prevent refrigerant from migrating to the compressor crankcase during off cycles
- To ensure refrigerant flows through the correct expansion device depending on operating mode (Correct answer)
- To prevent high-pressure refrigerant from entering the suction side
- To balance refrigerant flow between multiple indoor air handlers
Correct answer: To ensure refrigerant flows through the correct expansion device depending on operating mode
Check valves allow refrigerant to bypass one expansion device while directing flow through the active device appropriate for the current heating or cooling mode.
Question 3: Which of the following BEST describes a water-source heat pump (WSHP) loop configuration in a commercial building?
- Each unit connects directly to a city water supply for condenser cooling
- All units share a common water loop maintained between 60–90°F, with a cooling tower and boiler for temperature control (Correct answer)
- Units use well water drawn and discharged to a surface water body
- Each unit has its own dedicated ground loop for heat exchange
Correct answer: All units share a common water loop maintained between 60–90°F, with a cooling tower and boiler for temperature control
Commercial WSHPs share a common water loop maintained within an efficiency band, with a cooling tower rejecting excess heat and a boiler adding heat when needed.
Question 4: A heat pump's outdoor fan motor fails during heating mode. What is the MOST likely result?
- System will continue to operate normally since fan is only needed in cooling
- Suction pressure will drop and the system may trip on low-pressure cutout (Correct answer)
- Discharge pressure will rise rapidly and trip the high-pressure cutout
- The reversing valve will automatically switch to cooling mode
Correct answer: Suction pressure will drop and the system may trip on low-pressure cutout
Without outdoor fan airflow across the evaporating coil in heating mode, heat absorption drops, causing suction pressure to fall and potentially triggering a low-pressure safety cutout.
Question 5: What is the significance of the 'lockout temperature' setting on a heat pump thermostat?
- The indoor temperature below which the heat pump will not operate
- The outdoor temperature below which the heat pump compressor is locked out and only auxiliary heat operates (Correct answer)
- The temperature at which the defrost control initiates a defrost cycle
- The discharge temperature above which the compressor locks out for protection
Correct answer: The outdoor temperature below which the heat pump compressor is locked out and only auxiliary heat operates
The lockout temperature prevents the heat pump from operating at extremely low outdoor temperatures where it would be inefficient or potentially damaging, relying solely on auxiliary heat.
Question 6: What type of expansion device is most commonly used in modern variable-speed heat pump systems?
- Fixed orifice metering device
- Thermostatic expansion valve (TXV)
- Electronic expansion valve (EEV/EXV) (Correct answer)
- Capillary tube
Correct answer: Electronic expansion valve (EEV/EXV)
Electronic expansion valves provide precise, electronically controlled refrigerant flow modulation that complements variable-speed compressor operation across a wide range of conditions.
Question 7: A technician is checking a heat pump in cooling mode and finds high suction pressure and high discharge pressure with warm air from the supply. What does this MOST likely indicate?
- Low refrigerant charge
- Refrigerant overcharge or a dirty outdoor condenser coil (Correct answer)
- Failed reversing valve stuck in heating position
- Blocked indoor air filter
Correct answer: Refrigerant overcharge or a dirty outdoor condenser coil
High pressures on both sides in cooling mode typically indicate an overcharge of refrigerant or a dirty condenser coil restricting heat rejection.
HVAC Heat Pump Technology 4
In a dual-fuel heat pump system, what determines when the gas furnace takes over from the heat pump?