When to use emergency heat on a heat pump
When to use emergency heat on a heat pump?
Use emergency heat only when the heat pump itself isn’t working: the outdoor unit is damaged, the compressor has failed, or the unit is iced over and won’t clear. Cold weather alone is not a reason. When it’s cold, leave the thermostat on regular Heat. Your system brings in auxiliary heat automatically when the heat pump needs help.
Selecting EM Heat shuts the heat pump’s compressor off and runs only the backup heat source. That’s usually electric heat strips, or a gas furnace on a dual-fuel system. It stays on until you switch it off yourself, and it costs more to run.
Emergency Heat vs. Auxiliary Heat
The two are easy to confuse, and mixing them up is how people end up with a big electric bill.
| Auxiliary heat (AUX) | Emergency heat (EM) | |
| Who turns it on | The system, automatically | You, manually |
| Heat pump running? | Yes, backup helps it | No, compressor is off |
| When it runs | Cold snaps, big gap between room temp and setpoint, defrost cycles | Only when you select it |
| Turns itself off? | Yes | No, you must switch it back |
| Thermostat shows | “AUX” or “Stage 2” | “EM Heat” |
If you see AUX lit, nothing is wrong. That’s the system doing its job.
When to Use Emergency Heat
| Situation | Use EM heat? |
| Outdoor unit stopped running, or the compressor failed | Yes, until it’s repaired |
| Storm or impact damaged the outdoor unit | Yes, until it’s repaired |
| Outdoor unit is buried in ice that isn’t clearing (rare, often after freezing rain) | Yes, and call for service |
| Cold snap, heat pump is running, house is holding temperature | No. Stay on Heat |
| AUX light is on | No. That’s normal |
| Steam or fog rising from the outdoor unit | No. That’s a defrost cycle |
| House won’t stay warm even at mild outdoor temperatures | Call a technician. Something is wrong with the heat pump |
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You’ll see a rule that says “switch to emergency heat below 30°F.” Most manufacturers and utilities don’t recommend choosing EM heat by outdoor temperature alone. Heat pumps are built to bring in auxiliary heat as it gets colder. A few contractors suggest switching below roughly 25°F, but that has to be weighed against the cost of running resistance heat the whole time.
Check These First (5 Minutes)
Before you switch to emergency heat, rule out the cheap causes:
- Confirm the thermostat is set to Heat, with a target above room temperature, and has fresh batteries if it needs them.
- Check the breakers for both the outdoor unit and the indoor air handler.
- Replace the air filter if you can’t see light through it.
- Look at the outdoor unit. Clear snow, leaves, and debris from around it. If the outdoor fan isn’t spinning, see outdoor fan not spinning.
If none of that gets the heat pump running, switch to EM heat and call a technician.
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How to Turn It On, and Back Off
- On your thermostat, go to the mode or system setting and select EM Heat (labels vary: “Emerg,” “Emergency Heat,” or under equipment settings on some smart thermostats).
- Expect the outdoor unit to go quiet. That’s normal, because the compressor is off. Warm air keeps coming from the vents.
- To switch back, set the mode to Heat (or Auto). Emergency heat will not turn itself off. Set a phone reminder, because forgetting it is the most common way people run up a high bill.
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What It Costs
Resistance heat turns each unit of electricity into one unit of heat. A heat pump typically delivers about two to four units of heat per unit of electricity in moderate weather. Running EM heat for a few days will show up on your bill. Running it for a whole season would be very expensive. That’s why it’s a stopgap, and why needing it repeatedly means the heat pump has a problem worth fixing.
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Common Questions
Can emergency heat damage my heat pump?
Short-term use shouldn’t. Emergency mode bypasses the heat pump, so appropriate temporary use doesn’t harm it. The real risk is relying on it long-term while the underlying fault goes unfixed.
Do I need emergency heat in a mild climate?
Rarely. In mild climates the heat pump seldom struggles, so EM heat matters mostly as a fallback if the unit fails.
I use emergency heat and the house still isn’t warm. Now what?
Call a technician. The backup source itself may have a problem, such as a tripped breaker for the heat strips or a failed element.
If Your Heat Pump Has Stopped Working
Switch to EM heat to keep the house from getting dangerously cold, then call a licensed technician. Tell them what you saw: whether the outdoor unit is silent, iced over, or making noise, and what the thermostat displays. If the diagnosis is a compressor failure or a major refrigerant leak, ask for a repair-versus-replace quote. How long heat pumps typically last can help you weigh it. For a broader look at heating problems, see heater not working: troubleshoot and fix.
If you’re in the Philadelphia area and your heat pump has stopped heating, Cool Aid Co. can diagnose it and get you back on regular heat.