Heat Pump Auxiliary Heat vs. Emergency Heat: When Each Runs and Why Aux Runs Too Much
Customers see "AUX" on the thermostat and worry. Sometimes it's normal; sometimes it's the only clue that the heat pump has stopped heating and the electric strips are doing all the work.
Aux heat vs. emergency heat
| Auxiliary heat | Emergency heat | |
|---|---|---|
| Who turns it on | The thermostat or control, automatically | The user, by selecting EM HEAT |
| Heat pump | Still running | Off |
| When it's normal | Defrost, big setpoint increase, very cold weather | Only when the heat pump has failed, until it's repaired |
| Cost to run | Higher than heat pump alone | Highest; electric strips only |
When aux heat normally comes on
- During defrost, to keep air from blowing cold. See heat pump defrost.
- When the setpoint jumps a few degrees above room temperature (many thermostats bring on aux to recover faster).
- Below the balance point, the outdoor temperature where the heat pump alone can't keep up with the house's heat loss.
- When the thermostat's lockout settings allow it, based on outdoor temperature on thermostats that have an outdoor sensor.
Aux heat running all the time
Check whether the heat pump is actually heating:
- Is the outdoor unit running in heat mode? Compressor and fan both on?
- Is the supply air warm with aux heat off? Temporarily disable the strips (with the manufacturer's method or at the breaker), and measure supply air.
- Check the reversing valve: in heat mode, the large line to the indoor coil should be hot. If it's cold, the valve may be stuck in cooling or the O/B wiring may be wrong.
- Check the charge the right way for heat mode. See checking heat pump charge in heat mode.
- Check the outdoor coil: iced solid means defrost isn't working.
- Check the thermostat setup: system type, O vs. B, aux heat lockouts, and how much temperature difference triggers aux.
Aux heat never coming on
- No strips installed or a strip breaker off at the air handler
- Failed sequencer or relay, or open strip limits
- Wiring: W2/AUX not connected at the thermostat or air handler
- Thermostat lockout set so aux never runs above a certain outdoor temperature
Symptom: cold air during defrost and a house that can't keep up on the coldest days.
Common mistakes
- Telling the customer aux on during defrost is a problem. It's normal.
- Leaving a system in emergency heat after the repair.
- Missing a stuck reversing valve because the strips are keeping the house warm.
- Wrong O/B setting after a thermostat replacement, so the heat pump cools while the strips heat.
FAQ
Is it bad if my heat pump runs on aux heat?
Not if it's brief: defrost, recovery, or very cold weather. If it's on most of the time in mild weather, the heat pump may not be heating.
When should I use emergency heat?
Only when the heat pump has failed or the outdoor unit is damaged, until it's repaired. It's expensive to run.
Why does my thermostat say aux heat when it's not that cold?
A big setpoint increase, defrost, or a heat pump that isn't keeping up. If it stays on, have the heat pump checked.
Sources
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