HVACopediaHeat pump data
Defrost, balance point, reversing valve, charging conditions and installation.
Key numbers
| Item | Value |
|---|---|
| Defrost timer intervals | 30, 60 or 90 minutes (a jumper on the board) |
| Defrost starts | when the coil sensor is below about 26 °F |
| Defrost ends | on coil temperature, or after a maximum of about 10 minutes |
| Outdoor fan during defrost | off |
| Auxiliary heat during defrost | on |
| Check the charge | cooling above 70 °F outdoors, heating below 40 °F outdoors |
| Outdoor unit on risers | 8 to 18 in above the snow line |
| Supply air temperature | cooler than a furnace; warn new owners |
Reversing valve
- The lone stub on one side is the compressor discharge. The middle stub of the three is the suction (to the accumulator or compressor). The two outer stubs go to the coils.
- Most valves heat when the solenoid is de-energized (O/B terminal: O energizes in cooling, B in heating).
- When replacing one, wrap the body in a wet rag or heat paste. Many techs cut in all new stubs.
Good to know
- Balance point: the outdoor temperature where the heat pump output equals the heat loss of the building. Below it, auxiliary heat is needed.
- The large line carries gas in both modes. The small line always carries liquid.
- A heat pump needs its own accumulator, TXV and thermostat. Parts from an air conditioner are not interchangeable.
- Never run furnace heat while the heat pump compressor is on: the hot air raises the head pressure and can burst the coil. Check the interlock on every visit.
- Steam from the outdoor unit in winter is normal defrost. Ice around the unit means the base-pan heater or drain may have failed.
- Run cooling, heating and defrost on every seasonal visit.
- Smell like dirty socks: mould on the indoor coil. Clean the coil and pan, and consider a UV lamp.
- Flush the condensate line with air and a 1 : 4 mix of bleach and water.
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.