A refresher for people already in the trade, not a training course. Always follow the equipment manufacturer’s instructions and current codes.
Heat Pump Operation Basics
How a Heat Pump Moves Heat
A heat pump is simply an air conditioner that can run its refrigeration cycle in reverse to move heat indoors during cold weather.
- Cooling mode: operates just like a standard AC system — indoor coil is the evaporator, outdoor coil is the condenser.
- Heating mode: refrigerant flow reverses — the indoor coil becomes the condenser (releasing heat indoors) and the outdoor coil becomes the evaporator (absorbing heat from outside air, even in cold weather).
Check yourself: In heating mode, which coil acts as the condenser?
- Outdoor coil
- Indoor coil
- Neither
- Both simultaneously
Answer: B. Indoor coil
The Reversing Valve
Switching Between Heating and Cooling
- The reversing valve is a four-way valve controlled by a small solenoid that redirects refrigerant flow direction.
- A stuck or leaking reversing valve is a common cause of a heat pump that can only cool, only heat, or runs both coils at once.
- You can often hear a "whoosh" and feel a thump through the line set when the valve shifts position.
Check yourself: What component controls the direction of refrigerant flow in a heat pump?
- TXV
- Reversing valve
- Filter drier
- Accumulator
Answer: B. Reversing valve
Defrost Cycles
Why Heat Pumps Need to Defrost
- In heating mode, the outdoor coil runs cold enough to frost over in humid, cold conditions.
- A defrost control periodically reverses the cycle briefly (running in cooling mode) to melt frost off the outdoor coil, while auxiliary heat keeps the indoor air warm.
- Defrost is initiated by a timer, temperature sensor, or pressure differential, depending on the control board.
Check yourself: How does a heat pump typically remove frost from the outdoor coil?
- By increasing indoor fan speed
- By briefly reversing into cooling mode
- By shutting the system off completely
- By opening the reversing valve permanently
Answer: B. By briefly reversing into cooling mode
Auxiliary / Emergency Heat
Backup Heat Strips
- Auxiliary heat supplements the heat pump automatically during defrost or when outdoor temperatures drop below the heat pump's efficient operating range.
- Emergency heat is a manual thermostat setting that runs electric heat strips only, bypassing the heat pump entirely — used when the outdoor unit is down for repair.
- Heat strips are typically staged (multiple stages of electric resistance heat) to avoid tripping breakers.
Check yourself: When would a homeowner use the "Emergency Heat" setting?
- Any time it is cold outside
- When the outdoor heat pump unit is not working
- Only during defrost cycles
- Never — it is not a real setting
Answer: B. When the outdoor heat pump unit is not working
Heat Pump Troubleshooting
Common Heat Pump Complaints
| Complaint | Likely Cause |
|---|---|
| Runs constantly on mild days | Undersized unit or low charge |
| Ice buildup that won't clear | Failed defrost control or sensor |
| Blows cold air in heating mode | Reversing valve stuck, low charge, or normal defrost cycle |
| High electric bills | Aux heat locked on due to a stuck relay or bad thermostat setting |
Check yourself: Ice buildup on the outdoor coil that never clears usually points to what?
- A working defrost cycle
- A failed defrost control or sensor
- Overcharged system
- Correct operation in cold weather
Answer: B. A failed defrost control or sensor
Dual-Fuel Systems
Pairing a Heat Pump with a Gas Furnace
- A dual-fuel (hybrid) system pairs a heat pump with a gas furnace, switching to gas heat once outdoor temperatures drop below a set "changeover" point.
- This gives the efficiency of a heat pump in mild weather and the stronger output of gas heat in extreme cold.
- The changeover temperature is a programmable thermostat setting, not a fixed rule — it should be tuned to local utility rates and climate.
Check yourself: What triggers the changeover from heat pump to gas furnace in a dual-fuel system?
- A random timer
- A programmable outdoor temperature setpoint
- The refrigerant charge level
- The size of the return duct
Answer: B. A programmable outdoor temperature setpoint
Ground-Source & Water-Source Heat Pumps
Geothermal and Water-Source Systems
- Ground-source (geothermal) heat pumps exchange heat with the earth through buried loops instead of outdoor air, giving much more stable performance year-round.
- Water-source heat pumps use a shared water loop (common in commercial buildings) to reject or absorb heat, letting different zones heat and cool simultaneously.
- Because ground and water temperatures stay far more stable than outdoor air, these systems avoid the defrost cycle problems air-source heat pumps face in cold climates.
- Loop fluid (often a water/antifreeze mix) needs periodic testing for freeze protection and corrosion inhibitors.
Check yourself: Why do ground-source heat pumps avoid the defrost problems that air-source heat pumps face?
- They do not use refrigerant
- Ground and water temperatures stay far more stable than outdoor air
- They only operate in cooling mode
- They use a larger compressor
Answer: B. Ground and water temperatures stay far more stable than outdoor air
Heat Pump Efficiency Ratings (HSPF & SEER2)
Reading Heat Pump Efficiency Labels
- SEER2: rates cooling efficiency under the newer, stricter M1 test standard - higher is more efficient.
- HSPF2: rates heating efficiency the same way - it reflects seasonal heating performance, not just a single test point.
- A heat pump's real-world efficiency drops as outdoor temperature drops, which is exactly why auxiliary/emergency heat and dual-fuel setups exist for cold climates.
- Always compare SEER2/HSPF2 numbers, not older SEER/HSPF ratings, when evaluating equipment sold under current standards.
Check yourself: What does HSPF2 measure?
- Cooling efficiency only
- Seasonal heating efficiency under the current test standard
- Refrigerant charge accuracy
- Sound output in decibels
Answer: B. Seasonal heating efficiency under the current test standard
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