Quick answer
Emergency heat (“EM Heat” or “E-Heat”) shuts off your heat pump’s outdoor unit and heats the house using backup heat only — usually electric heat strips, sometimes a gas furnace. Use it only when the heat pump itself isn’t working: it’s broken, iced over, or damaged, and you’re waiting for a repair. With electric strips, it typically costs two to three times more to run than normal heat pump heating, so don’t use it just because it’s cold outside.
Emergency heat vs auxiliary heat
These two get mixed up constantly, but they’re different:
| Auxiliary heat (“Aux”) | Emergency heat (“EM Heat”) | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually |
| What runs | Heat pump plus backup heat | Backup heat only — heat pump is off |
| When it happens | Very cold weather, defrost cycles, or big temperature jumps | When you select it on the thermostat |
| Is it normal? | Yes, in moderation | Only during a heat pump problem |
| Cost | Higher than heat pump alone | Highest — all heat comes from backup |
If you see “Aux heat” on your thermostat on a cold morning, that’s the system doing its job. You don’t need to do anything. “EM Heat” should only be on if you turned it on.
When to use emergency heat
Switch to emergency heat when the outdoor unit can’t or shouldn’t run:
- The heat pump is broken and blowing cold air, or not running at all, and you’re waiting for a technician.
- The outdoor unit is encased in ice that isn’t clearing, or buried in snow. Run EM heat while you defrost it.
- The outdoor unit is damaged — a fallen branch, flood water, a bent fan, or loud grinding or screeching noises.
- A technician tells you to while they diagnose a problem or wait on parts.
- During an ice storm, if freezing rain is coating the outdoor unit faster than it can defrost.
Once the problem is fixed, switch back to regular Heat right away.
When not to use it
- Just because it’s very cold. Modern heat pumps, especially cold-climate models, keep working well below freezing, and the thermostat brings in aux heat automatically when it’s needed.
- To warm the house faster. Emergency heat doesn’t heat faster than normal mode with aux heat; it just costs more.
- Because the heat pump is running constantly. Long run times are normal in cold weather.
- Because you see frost or steam outside. That’s a normal defrost cycle.
- As a regular nighttime or vacation setting. It’s a temporary backup, not an operating mode.
One exception: in a dual-fuel system with a gas furnace as backup, emergency heat runs the furnace. That’s not necessarily expensive, and some owners use it when gas is cheaper than electricity on very cold days — though a properly set switchover temperature does this automatically.
What emergency heat costs
Electric heat strips turn 1 unit of electricity into 1 unit of heat. A heat pump moves heat instead of making it, so it typically delivers 2–3 units of heat per unit of electricity in winter. That gap is why emergency heat is expensive.
Illustrative comparison for a typical home using about 34,000 BTU of heat per hour, at $0.17/kWh:
| Heat pump (normal Heat mode, COP ~2.5) | Emergency heat (10 kW electric strips) | |
|---|---|---|
| Power used per hour of heating | ~4 kW | ~10 kW |
| Cost per hour of heating | ~$0.68 | ~$1.70 |
| Cost per day (12 hours of heating) | ~$8 | ~$20 |
| Cost per 30-day month (12 hours a day) | ~$245 | ~$612 |
Residential heat strips range from about 5 kW to 20 kW, so your number may be lower or higher. To estimate your own cost, multiply the strip size (kW, on the air handler’s label) by hours of use and your electric rate.
Bottom line: a few days on emergency heat while you wait for a repair costs tens of dollars. Leaving it on all winter by mistake can add hundreds of dollars a month.
If your backup is a gas furnace (dual-fuel), emergency heat costs whatever your furnace costs to run. In areas with cheap gas, that can be similar to or less than heat pump operation.
How to turn emergency heat on and off
The exact steps depend on your thermostat, but the pattern is similar:
- Basic thermostats: slide or press the mode switch to EM Heat (or “E-Heat” / “Emergency”). An indicator light usually comes on. To turn it off, switch back to Heat.
- Honeywell and similar digital models: press Mode until “Em Heat” appears.
- Ecobee: open the menu, choose System or HVAC Mode, and select Emergency Heat (it may appear only when the system is set up as a heat pump).
- Nest: Emergency Heat is in the thermostat’s mode options (or the app) on heat pump setups with backup heat wired.
If you don’t see an emergency heat option, the thermostat may not be configured for a heat pump with backup heat, or your system may not have backup heat installed. Check the thermostat’s installer settings or ask a technician.
Don’t forget to switch it off. Put a sticky note on the thermostat or set a phone reminder for the day of the repair.
Troubleshooting emergency heat
| Problem | Likely cause | What to do |
|---|---|---|
| EM heat is on but air is cool | Heat strip breaker tripped, or a strip or sequencer failed | Check the air handler’s separate breaker(s); call a technician if they’re on |
| EM heat light came on by itself | Some thermostats show a light when backup heat runs automatically, or a heat pump fault was detected | Check the thermostat menu; if the heat pump stopped working, call a technician |
| Burning smell when EM heat first runs | Dust burning off unused heat strips | Normal for the first few minutes of the season; if it persists or smells electrical, shut it off |
| Breaker trips when EM heat is on | Overloaded circuit or failing heat strip | Don’t keep resetting; call a technician |
| Electric bill spiked | EM heat left on, or aux heat running in mild weather | Switch to Heat; have a technician check the aux heat lockout setting |
Frequently asked questions
At what temperature should I switch to emergency heat?
There’s no outdoor temperature that calls for it. Your thermostat brings in auxiliary heat automatically when the heat pump needs help. Use emergency heat only when the heat pump itself has a problem.
Is it bad to run a heat pump on emergency heat?
It won’t damage anything — it actually protects a broken heat pump from running. It’s just expensive. Use it for days, not weeks, and switch back once the system is repaired.
How long can I run emergency heat?
As long as you need to while waiting for a repair. The only downside is cost.
Why is my emergency heat light on?
Either someone selected EM Heat on the thermostat, or your thermostat displays the light when it detects a heat pump fault or runs backup heat. If you didn’t turn it on, check the thermostat mode and call a technician if the heat pump has stopped.
Does emergency heat use more electricity than aux heat?
Yes. Aux heat runs alongside the heat pump, which still supplies efficient heat. Emergency heat turns the heat pump off, so all heat comes from the backup.
Can I use emergency heat to defrost my heat pump?
It doesn’t defrost the outdoor unit, but switching to EM heat while you clear ice by hand keeps the house warm and stops the iced-over unit from running.
Related guides
- Heat Pump Blowing Cold Air in Heat Mode: 9 Causes and Fixes
- Heat Pump Frozen Outside: How to Defrost It Safely
- Heat Pump Running Constantly in Winter: Normal or a Problem?
- Heat Pump Backup Heat: Electric Strips vs Furnace Backup
- Best Thermostat Settings for a Heat Pump in Winter
- Heat Pump Operating Cost per Month: Calculator and Real Utility Bills
This guide is general information. Costs are illustrative 2026 estimates; your system’s heat strip size and electric rate determine your actual cost. Leave electrical and internal repairs to a licensed HVAC technician.