What does EM heat mean on a heat pump thermostat?
EM heat and aux heat sound alike, but only one of them is a setting you choose. Here is when each one runs.
EM heat means emergency heat. It is a thermostat setting that shuts the heat pump's outdoor unit off and heats your home with backup heat only. You choose it yourself, and it is meant for the rare day when the heat pump itself is not working.
Aux heat is different. It is a helper that switches on by itself when the heat pump needs a hand. Mixing the two up is the most common mistake on a heat pump thermostat.
EM heat vs aux heat: the difference
| Feature | EM heat | Aux heat |
|---|---|---|
| Who turns it on | You, at the thermostat | The system, automatically |
| Heat pump outdoor unit | Off | Still running |
| Backup heat source | Runs alone | Runs alongside the heat pump |
| Typical use | Heat pump has failed or is iced over | Cold snaps and fast recovery |
When aux heat comes on by itself
Aux heat kicks in when the heat pump cannot keep up. That happens in very cold weather, during a defrost cycle, or when you raise the setpoint by several degrees at once. The thermostat usually shows an AUX light while it runs. Short bursts are normal.
When to use EM heat
Use it only when the outdoor unit is clearly not working. Examples are a unit that will not start, one that is buried in ice, or one that has stopped making its usual sound. Set the thermostat to EM, then call a licensed HVAC technician the same day. Do not leave it on as a workaround.
Why you should not run EM heat all winter
EM heat uses the backup source alone, and that source is far less efficient than the heat pump. Running it for days can push your energy bill well past normal. If your thermostat seems stuck on EM, the setting may simply have been bumped, so check it first.
What to check if aux heat runs too often
Look at the filter, the outdoor unit and the thermostat settings. A clogged filter or a blocked outdoor coil makes the heat pump work harder, and aux heat fills the gap. Clear loose leaves and snow by hand only. If the AUX light stays on during mild weather, have a technician look at it.
What not to do
- Do not open the outdoor unit or the indoor cabinet. Electrical parts inside can hold a dangerous charge.
- Do not add or check refrigerant. It requires federal EPA Section 608 certification.
- If you smell gas or a carbon monoxide alarm sounds, leave the house and call the gas company or 911.
Frequently asked questions
Is EM heat the same as aux heat?
No. EM heat is a setting you choose that turns the outdoor unit off. Aux heat comes on automatically and works alongside the heat pump.
Is it bad to leave my thermostat on EM heat?
It is not dangerous, but it is inefficient. The backup heat runs alone, so it should be a short-term fix until a technician checks the heat pump.
Why does my thermostat say AUX heat?
The heat pump needs help. This is normal in a cold snap or during defrost. If it shows in mild weather, call a licensed HVAC technician.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Air-Source Heat Pumps
- ENERGY STAR. Energy Savings at Home: Heating & Cooling