Emergency heat kicks in when your heat pump stops heating efficiently, usually below 35°F outside
Your heat pump has a built-in backup heating system called emergency heat (or auxiliary heat) that runs on electric resistance coils instead of the heat pump itself. You switch to it manually using a button or setting on your thermostat when the heat pump is not keeping up with the cold, or when you need heat right now and cannot wait for the pump to cycle through its defrost mode.
Emergency heat is not something you should run all winter. It uses roughly three times more electricity than the heat pump does, so it costs significantly more to operate. But there are specific situations where switching to it makes sense—and one situation where your system switches automatically without you doing anything.
Key Takeaways
- Emergency heat runs on electric resistance and costs much more to operate than your heat pump, so use it only when necessary.
- Your thermostat may switch to emergency heat automatically if the heat pump fails or if outdoor temperature drops below the system's design threshold, usually 35°F to 40°F.
- Manual emergency heat is useful during extreme cold snaps, after power outages, or when the heat pump is stuck in defrost mode and your home is cooling down.
- If emergency heat runs for more than a few hours, or if your heat pump never switches back to normal heating, call a technician to check for a malfunction.
- You can reduce emergency heat use by keeping your thermostat at 68°F or lower and ensuring your home is well-sealed against drafts.
How emergency heat works on your thermostat
Most thermostats have three heat settings: Heat (the normal heat pump mode), Emergency HeatHeat Pump (which locks the system into pump-only mode). When you flip the switch to Emergency Heat, you are telling the system to ignore the heat pump and run only the electric resistance coils inside your indoor unit.
Some newer smart thermostats do not have a manual emergency heat button. Instead, they have a setting that says "Use auxiliary heat" or "Enable backup heat," which you toggle on or off in the app or menu. Check your thermostat manual or the manufacturer's website to find the exact name and location of this setting on your model.
The switch itself is straightforward, but the decision to use it should be deliberate. Once you turn emergency heat on, it stays on until you turn it off. Your system will not automatically switch back to the heat pump.
When your system switches to emergency heat automatically
Your thermostat may flip to emergency heat on its own if the heat pump fails or if outdoor temperature falls below what your system is designed to handle. Most heat pumps are rated to heat efficiently down to about 35°F to 40°F outside. Below that threshold, the pump still works, but it pulls in less heat from the cold air, and your home may cool down faster than the pump can reheat it.
When this happens, many thermostats have a built-in rule: if the indoor temperature drops more than 2°F below your set point, switch to emergency heat to catch up quickly. This automatic switchover is a safety feature to keep your home from getting dangerously cold. Once the outdoor temperature rises or the heat pump catches up, the thermostat should switch back to normal heating on its own.
If your thermostat has switched to emergency heat automatically and stays there for hours, or if it keeps switching back and forth, your heat pump may have a problem. A technician should inspect it to check for refrigerant leaks, a faulty reversing valve, or a defrost cycle that is not working correctly.
Situations where you should manually switch to emergency heat
You might manually switch to emergency heat during an extreme cold snap—a sudden drop to well below zero—when the heat pump cannot keep up even with the automatic switchover. In this case, running emergency heat for a few hours while the outdoor temperature is at its worst is better than letting your home temperature fall. Once the temperature rises, switch back to normal heat.
After a power outage, if your heat pump is slow to restart or seems stuck, you can switch to emergency heat to warm the house quickly while you wait for the pump to come back online. Again, this is a temporary measure. Once the pump is running normally, switch back.
If your heat pump is in defrost mode and your home temperature is dropping noticeably, you can switch to emergency heat to maintain comfort while the pump clears frost from its outdoor coil. Defrost cycles usually last 5 to 15 minutes, so you may not need emergency heat at all—but if your home is very cold or the cycle is taking longer than usual, it is an option.
Why emergency heat costs so much more
A heat pump moves heat from outside air into your home, which is energy-efficient even in cold weather. Emergency heat generates heat directly using electric resistance coils, similar to an electric space heater or a toaster. Generating heat this way uses roughly three times more electricity than moving it does.
If you run emergency heat for 24 hours in winter, you may see a noticeable jump in your electric bill. If you run it for a week, the cost can be substantial. This is why emergency heat is a backup, not a primary heating method. Use it only when the heat pump is not working, when outdoor temperature is dangerously low, or when you need when ready warmth for a short period.
How to reduce the need for emergency heat
The best way to avoid emergency heat is to keep your thermostat set to 68°F or lower during winter. Every degree you lower the set point reduces the load on your heat pump and gives it more time to keep up during cold weather. If you are away during the day, lower it further—even to 62°F—and raise it back up when you return home.
Seal air leaks around windows, doors, and electrical outlets. Caulk gaps where pipes or wires enter the house. Insulate your attic if it is below R-38 (the recommended minimum for most climates). These steps reduce the amount of heat your home loses, so your heat pump does not have to work as hard and is less likely to fall behind.
Keep the outdoor unit clear of snow and ice. If snow piles up around the outdoor coil, the heat pump cannot pull in enough air and will work harder or cycle into defrost mode more often. Clear away snow after storms and make sure nothing is blocking airflow to the unit.
What to do if emergency heat stays on too long
If you manually switched to emergency heat and it has been running for more than a few hours, switch it back to normal heat mode and see if the heat pump restarts. If outdoor temperature has risen above 35°F and the pump is running but your home is still cold, the pump may have a refrigerant leak or a faulty reversing valve. Call a technician.
If your thermostat automatically switched to emergency heat and has not switched back after several hours of normal outdoor temperature, the thermostat itself may be stuck or the heat pump may have failed. Do not ignore this. A technician can diagnose whether the pump needs repair or the thermostat needs replacement.
Running emergency heat continuously is expensive and usually a sign that something is wrong with the heat pump. The longer you wait to have it checked, the higher your heating bill will climb.
Frequently Asked Questions
Will my heat pump break if I leave it on during very cold weather?
No. Heat pumps are designed to operate in cold weather, and they have built-in protections to prevent damage. However, they become less efficient as temperature drops, and they may not keep up with heating demand below 35°F. That is why emergency heat exists—not to protect the pump, but to keep your home warm.
Should I switch to emergency heat at night to save money?
No. Emergency heat uses more electricity, not less, so switching to it at any time will increase your bill. If you want to save money at night, lower your thermostat set point instead. The heat pump will run less often and use less energy overall.
Can I run emergency heat and the heat pump at the same time?
No. When you switch to emergency heat mode, the thermostat disables the heat pump and runs only the electric resistance coils. The two do not run together. Some systems use auxiliary heat as a supplement during extreme cold, but that is controlled automatically by the thermostat, not by you.
What if my thermostat does not have an emergency heat button?
Newer smart thermostats and some modern systems do not have a visible emergency heat switch. Check your thermostat's app, menu settings, or manual for an option labeled "Auxiliary Heat," "Backup Heat," or "Emergency Heat." If your system truly has no way to access emergency heat, contact your HVAC installer or the thermostat manufacturer to learn whether it is available and how to enable it.
Is it normal for emergency heat to run during a power outage recovery?
Yes. After power is restored, your heat pump may take several minutes to restart and reach full capacity. During that time, your thermostat may automatically switch to emergency heat to prevent your home from cooling down. Once the pump is running normally, the thermostat should switch back. If it does not, check that the pump is actually running, or call a technician.