EcoGuide · Heat Pumps · Published 2026-09-06 · Last Updated 2026-09-06 · 1,100 words
Emergency Heat vs Auxiliary Heat: What “EM Heat” Actually Does
The short version
If yours is stuck on rather than just confusing, start here: why auxiliary heat will not turn off.
They are not two settings for the same thing. Auxiliary heat runs alongside your heat pump when the pump alone cannot keep up. Emergency heat switches the heat pump off and heats the house with the backup element only.
Aux heat is the system helping itself, automatically, without you doing anything. EM heat is you telling the system to stop using the efficient part. That is why the expensive mistake runs in one direction: people flip to EM heat during the first cold snap, assume it means “more heat”, and leave it there for months.
Why the mix-up costs real money
A heat pump does not create heat, it moves it, which is why it can deliver several units of heat for each unit of electricity. Electric resistance strips — the backup element behind both aux and EM heat — convert electricity to heat at roughly one to one. They are not defective; they are simply the most expensive heat in the house.
The Department of Energy puts the gap at around 3,000 kWh a year, about $459, comparing an air-source heat pump with electric resistance heating. Running EM heat all season is, roughly, choosing to give that back.
The reason this goes unnoticed is that the house stays warm. Nothing feels broken. The thermostat hits its setpoint, the rooms are comfortable, and the only symptom is a bill that arrives weeks later.
What each setting actually does
| Auxiliary heat | Emergency heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, deliberately |
| Heat pump | Still running | Switched off |
| Heat source | Heat pump + strips together | Strips only |
| When it is right | Cold snaps, big recovery from a setback | The heat pump is actually broken |
| Running cost | Higher than normal | Highest possible |
When you should use emergency heat
There is a legitimate use, and it is narrower than most people think. Emergency heat exists for when the heat pump cannot run safely or at all:
- The outdoor unit is iced over well beyond a normal defrost cycle, or is buried in snow.
- The outdoor fan is not turning, or the unit is making a noise it does not normally make.
- A technician has told you to run on backup until a part arrives.
Notice what is not on that list: it being cold outside. Cold weather is the condition a heat pump is designed to work in, and your thermostat already brings in auxiliary heat by itself when it needs to. If the house is warm and the heat pump is running, EM heat has nothing to add.
Worth checking tonight: if you switched to EM heat during a cold spell at any point, look at your thermostat now. On many units the only indication is a small “EM HEAT” label that is easy to stop noticing, and there is nothing else to tell you it never got switched back.
If what prompted this was air that simply feels cool from the vents, that is usually normal — see why a heat pump blows cool air before changing any setting.
The more common problem: aux heat running when it should not
Most people who think they have an emergency heat problem actually have an auxiliary heat problem. Aux heat coming on at 20°F is expected. Aux heat coming on at 45°F is not, and it usually traces to configuration rather than a fault:
- Balance point set too high. This is the outdoor temperature below which the strips are allowed to help. Set conservatively by an installer, it can hand work to the strips long before the heat pump is actually struggling.
- Aggressive setbacks. Dropping the thermostat 8°F overnight means a large morning recovery, and many thermostats meet that demand with the strips. On a heat pump, deep setbacks can cost more than they save.
- No aux lockout configured. Some thermostats can prevent the strips from engaging above a set outdoor temperature. Where the option exists, it is the single most useful setting on the device.
Our guide to aux heat causing a winter bill spike walks through diagnosing this, and Nest aux heat running too much covers the settings on that specific thermostat.
Long run times on their own are not evidence of a problem either — see why a heat pump runs constantly.
How to tell which one is running right now
- Check the display. “AUX HEAT” or “AUX” means the system chose it. “EM HEAT” or “EMERGENCY HEAT” means someone selected it.
- Go outside. If the outdoor unit is silent and still while the house is calling for heat, the heat pump is not running — that is emergency heat, or a fault.
- Check the runtime history. Thermostats that log aux runtime make this obvious; hours of aux in mild weather is the signal.
- Watch the meter. Resistance strips draw heavily and continuously. A whole-home monitor makes the difference impossible to miss.
What actually helps
Two things address this, in order of cost. The first is free: confirm you are not sitting in EM heat, and find out whether your thermostat exposes a balance point or aux lockout setting. Many do, and it costs nothing to set it properly.
The second is visibility. Aux and emergency heat are invisible failures — the house is warm, so nothing prompts you to look. A thermostat that reports aux runtime, or a monitor that shows the draw, turns a bill you find out about in February into something you notice in November.
If you are choosing a thermostat with this in mind, the setting that matters is user-accessible aux lockout — see how the main heat pump thermostats compare on exactly that.
EcoHome Intelligence Team
AI-assisted research · human-reviewed
We compare published manufacturer specifications against U.S. Department of Energy and ENERGY STAR data. Research and drafting are AI-assisted; every product selection is made and reviewed by a person before publication. We do not test products in-house — see how we choose.