Emergency heat is a backup heating system that turns on when your heat pump cannot keep up with demand or stops working

Most heat pumps have a second heating source built in—usually electric resistance heating coils. This backup is called emergency heat, auxiliary heat, or emergency mode. When you switch the thermostat to this setting, your heat pump shuts off and only the electric coils run. You should use it only in specific situations, because emergency heat costs significantly more to run than a heat pump does.

The goal is to keep your house warm while you wait for a technician to repair your heat pump, or during extreme cold when your heat pump alone cannot reach your target temperature. Emergency heat is not meant to run for days or weeks—it is a temporary measure while you solve the underlying problem.

Key Takeaways

  • Emergency heat costs roughly two to three times more per hour than running your heat pump normally, so use it only when necessary.
  • Turn on emergency heat if your heat pump stops heating entirely, or if the house temperature is dropping even though the system is running.
  • Do not leave emergency heat on after your heat pump is repaired, because it will waste energy and money.
  • If outdoor temperatures are below freezing and your heat pump is running but not keeping up, call a technician before switching to emergency heat—the problem may be a clogged defrost cycle, not a failed system.
  • Emergency heat should never be your primary heating method; if you find yourself using it regularly, your heat pump needs service.

When your heat pump stops producing heat entirely

If you turn up the thermostat and feel no warm air coming from the vents after 5 to 10 minutes, your heat pump may have failed. Check that the thermostat is set to "Heat" (not "Cool" or "Off") and that the fan is set to "Auto" or "On." If warm air still does not appear, switch to emergency heat to keep the house from dropping to an unsafe temperature.

Emergency heat will warm your home, but it will also show you that the problem is real—if the house warms up on emergency heat but not on the heat pump setting, you know the heat pump itself needs repair. Call an HVAC technician the same day. Do not wait, because the longer you run emergency heat, the higher your heating bill will climb.

When outdoor temperature drops below your heat pump's effective range

Heat pumps become less efficient as outdoor temperature falls. Most air-source heat pumps lose significant heating power once the outside temperature drops below 32°F (0°C), and many stop working well below 20°F (−7°C). If it is very cold outside and your heat pump is running but your house temperature is dropping, your system may straightforward be undersized for the conditions.

Before you switch to emergency heat, check your thermostat display. Many thermostats show whether the heat pump is actively heating or whether the backup heat has already kicked in automatically. If the backup is already running and the house is still cold, emergency heat will not solve the problem—you need a technician to check whether the heat pump is operating correctly or whether you need a larger system. Switching to full emergency heat will only waste money while the real issue goes unfixed.

When your heat pump is in defrost mode and not heating

On cold, humid days, frost builds up on the outdoor coil of your heat pump. The system automatically enters defrost mode every 30 to 90 minutes, reverses the refrigerant flow to melt the frost, and pauses heating for 5 to 15 minutes. During defrost, you may feel no warm air from the vents. This is normal and temporary.

If defrost cycles are happening too often or lasting too long, frost may be clogging the coil. The system will run defrost more frequently, and your house will get colder. You might think the heat pump has failed, but the problem is usually a dirty outdoor unit, a blocked air intake, or a refrigerant leak. Switching to emergency heat will mask the problem and cost you money. Call a technician instead—they can clean the coil or check the refrigerant level in under an hour.

When you should not use emergency heat

Do not use emergency heat as a way to heat your home faster. Turning on emergency heat does not make the house warm up quicker; it only bypasses the heat pump and runs the electric coils alone. The electric coils heat at a fixed rate, so you gain nothing by switching modes except a higher electric bill.

Do not leave emergency heat on overnight or for multiple days. If your heat pump is broken, emergency heat is a temporary bridge until a technician arrives—usually within 24 to 48 hours. If you cannot reach a technician quickly, keep emergency heat on only as long as necessary to prevent pipes from freezing or the house from becoming unsafe. Once the heat pump is repaired, switch back to the normal heat setting when ready.

Do not use emergency heat if you suspect a refrigerant leak, a compressor failure, or a faulty reversing valve. These problems require professional diagnosis. Running emergency heat while the heat pump is broken does not fix anything; it only delays the repair and increases your bill.

How to switch to emergency heat and switch back

On most thermostats, emergency heat is a separate setting on the mode dial. Look for a label that says "Em Heat," "Aux Heat," "Emergency," or "E Heat." Turn the dial to that setting. The display may show a light or message confirming the change. Your heat pump will stop, and only the electric coils will run.

To switch back, turn the dial back to "Heat." The heat pump will resume operation on the next heating cycle. If you have a programmable or smart thermostat, you may need to access the settings menu to find the emergency heat option. Check your thermostat manual if you cannot locate it, or ask your HVAC technician to show you during a service call.

What emergency heat costs compared to normal heating

Electric resistance heating (emergency heat) costs roughly two to three times more per hour than running a heat pump. The exact difference depends on your local electricity rate and your heat pump's efficiency rating. If your normal heating costs $2 per hour, emergency heat might cost $4 to $6 per hour.

Over a full day, that difference adds up quickly. Running emergency heat for 24 hours instead of your heat pump could add $50 to $100 to your monthly bill, depending on your region and utility rates. This is why emergency heat is meant only for temporary use—a few hours while you wait for a repair, not a substitute for a working heat pump.

Frequently Asked Questions

Will emergency heat damage my heat pump?

No. Emergency heat and the heat pump are separate systems. When you switch to emergency heat, the heat pump shuts down completely. Switching back to normal heat will not harm the heat pump either. The only risk is if you leave emergency heat on for weeks and never repair the broken heat pump—but that wastes money, not equipment.

Why is my thermostat showing emergency heat is on when I never switched it?

Your thermostat may have switched to emergency heat automatically because the heat pump could not keep up with demand. This often happens on very cold days or when the outdoor coil is frozen. Check the outdoor unit for ice or snow blocking the coil. If the coil looks clear and the house is still cold, call a technician—the heat pump may need service.

Can I use emergency heat if my heat pump is working fine?

You can, but you should not. Emergency heat costs much more to run and provides no benefit if your heat pump is working. The only reason to use it is when the heat pump has failed or cannot keep the house warm during extreme cold while you arrange a repair.

How long can I safely run emergency heat?

A few hours to a day or two is safe and normal. If you are running emergency heat for longer than that, your heat pump needs repair. Contact an HVAC technician as soon as possible. Continuing to run emergency heat beyond a few days will result in a very high electric bill and does not solve the underlying problem.

What should I do if emergency heat is not warming the house?

First, check that the thermostat is actually set to emergency heat and that the setting has taken effect. If the house is still not warming, the electric coils may have failed, the thermostat may be broken, or there may be a wiring problem. Call an HVAC technician—this is not a problem you can fix yourself.