The short version
EM heat means emergency heat. On a heat pump thermostat it shuts off the outdoor heat pump and heats the house with the backup source only, usually electric resistance strips in the air handler.
It is meant for when the heat pump has failed. In normal cold weather, leave it off and let the system bring on backup heat by itself.
What happens when you switch on EM heat?
The thermostat stops calling the outdoor unit. All heat then comes from the backup source. In most ducted heat pumps that is electric resistance strips. In a dual-fuel system it may be a gas furnace.
Electric resistance heat makes one unit of heat per unit of electricity. A working heat pump moves more heat than the electricity it uses, as DOE explains. So running on EM heat usually costs much more per hour of heat.
How is EM heat different from AUX heat?
| AUX heat | EM heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually |
| Heat pump running? | Yes, with backup added | No, backup only |
| When it happens | Very cold hours, defrost, big setpoint jumps | When the heat pump has failed |
| Normal? | Yes, in moderation | Only as a stopgap |
DOE notes energy codes require controls so backup strips do not run when they are not needed.
When should you use EM heat?
No fan, no hum, or the breaker for it keeps tripping. Use EM heat and call for repair.
Some defrost is normal. A block of ice that stays for hours is not.
Sometimes during a repair visit.
The system brings on backup heat by itself when needed.
EM heat is not faster. It is just more expensive.
What should you do after using EM heat?
Book a repair. EM heat is a bridge, not a fix. Note what you saw: ice, noise, a tripped breaker, a code on the thermostat. That helps the technician. Our heat pump repair guide lists the common fault families, and heat pump troubleshooting covers safe checks.
If "AUX" shows often in mild weather, have the controls checked. DOE lists improperly wired auxiliary heat strips among common heat pump performance problems.
Why does EM heat raise the electric bill?
Backup strips turn electricity straight into heat. DOE notes an electric resistance element gives about 3,400 BTU of heat for each kWh it uses. A working heat pump moves more heat than that for the same kWh, because it gathers heat from outdoor air.
So a few days on EM heat while you wait for a repair is fine. Weeks of it will show on the bill.
If you rely on it every winter, the heat pump needs attention, or the system may be undersized for your climate.
The balance point calculator shows the outdoor temperature below which your heat pump needs help.
Safety notes
If breakers trip when EM heat runs, turn it off and call. Do not reset repeatedly.
If you smell gas, leave the house and call the gas utility or 911 from outside.
Line voltage runs to the heat strips.
Questions
Should I use EM heat when it is below freezing?
Is it bad to leave EM heat on?
Why does my thermostat say AUX heat?
Does a furnace have EM heat?
My heat pump froze over. Should I use EM heat?
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Heat pump stuck on EM heat?
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