What is em heat on a thermostat?

EM Heat stands for Emergency Heat. It’s a setting on heat pump thermostats that turns off the heat pump’s compressor and runs the backup heat source — usually electric resistance heat strips, or a gas furnace on a dual-fuel system — by itself. Some thermostats label it EMH instead; it’s the same feature.

It exists for one specific situation: your heat pump itself isn’t working. A failed outdoor unit, storm damage, or a compressor fault are the real reasons to select it. Cold weather alone isn’t one of them — your system brings in backup heat automatically when it’s cold, through a separate, automatic feature called auxiliary heat (AUX).

EM Heat vs. Auxiliary Heat — Don’t Mix Them Up

This is the most common point of confusion, and it costs people money when they get it wrong:

• Auxiliary heat (AUX) turns on automatically when the heat pump needs help — during a cold snap, or briefly during a defrost cycle. The heat pump keeps running alongside it. Normal, and nothing to worry about if you see it on your display.
• Emergency heat (EM) is turned on manually, by you, and it shuts the heat pump off entirely. It doesn’t turn off by itself — you have to switch it back.

If your display shows “AUX,” that’s your system working correctly. EM Heat should only be something you select, and only when the heat pump has actually stopped working.

When to Actually Use It

• The outdoor unit isn’t running at all
• The heat pump is damaged or you’re waiting on a repair
• A technician has told you to use it temporarily

Not a reason to use it: it’s just cold outside, or you saw frost on the outdoor unit (that’s usually a normal defrost cycle, not a malfunction).

One Thing Worth Knowing Before You Use It

Emergency heat won’t turn itself off. If you switch to it, set a reminder to switch back once the heat pump is repaired — leaving it on by accident is the most common way people end up with an unexpectedly high electric bill, since resistance heat costs more to run than a working heat pump.

Want the Full Picture?

For the complete breakdown — a side-by-side comparison with auxiliary heat, exactly how to turn EM Heat on and off, what it costs to run, and what to do if your heat pump has actually failed — see our full guide: When to Use Emergency Heat on a Heat Pump (and When Not To).