Emergency heat turns on when your heat pump can't keep up with the cold
Emergency heat (often labeled "EM Heat" or "Aux Heat" on your thermostat) is a backup heating system that activates when outdoor temperatures drop too low for your heat pump to work efficiently. Most heat pumps lose effectiveness below 30 to 40 degrees Fahrenheit, depending on the model. When the system detects that the heat pump alone cannot reach your set temperature, it automatically switches on electric resistance heating—essentially a built-in space heater—to make up the difference.
You should manually switch to emergency heat only in specific situations: when your heat pump stops working entirely, when outdoor temperatures fall below your system's minimum operating range, or when you need heat faster than the heat pump can deliver it. In most cases, your thermostat handles this automatically. Forcing emergency heat on during mild weather wastes energy and money because electric resistance heating costs roughly three times more to run than a working heat pump.
Key Takeaways
- Emergency heat is electric resistance heating that backs up your heat pump when outdoor temperatures drop below 30 to 40 degrees or the heat pump malfunctions.
- Your thermostat usually switches to emergency heat automatically; manual switching is only necessary if the heat pump fails or you need when ready warmth in an emergency.
- Running emergency heat continuously costs significantly more than operating a heat pump, so avoid leaving it on during normal winter operation.
- If emergency heat stays on for more than a few hours on a day above 35 degrees, your heat pump likely needs service from a technician.
- During a heat pump breakdown, emergency heat keeps your home warm while you arrange repairs, but it should not be your permanent solution.
How automatic emergency heat set up works
Most modern heat pumps have a built-in sensor that monitors both the outdoor temperature and how well the system is heating your home. When the outdoor temperature drops below the heat pump's minimum operating threshold—typically between 30 and 40 degrees depending on the model—the system automatically activates the electric resistance heating strips inside your furnace or air handler. This happens without you touching the thermostat.
The system also triggers emergency heat if it detects that the heat pump is running but not raising your indoor temperature fast enough. This can happen on very cold days when the outdoor coil becomes too cold to absorb heat efficiently, or when you raise the thermostat setting by more than 2 or 3 degrees at once. The resistance heating kicks in to close the gap while the heat pump continues running in the background.
You can see this happening by checking your thermostat display. Most models show "EM Heat" or "Aux Heat" on the screen when the backup system is active. Some thermostats also have a light that illuminates. If you have a smart thermostat, you may receive a notification or see the status in the app.
When to manually switch to emergency heat mode
Switch your thermostat to emergency heat manually only when your heat pump has stopped working and you need when ready warmth. This is a temporary measure to keep your home habitable while you wait for a technician. Do not leave it on as a permanent replacement—emergency heat will heat your home, but the electric bills will be noticeably higher than normal winter heating.
Another reason to switch manually is if you live in an area where outdoor temperatures regularly stay below your heat pump's minimum operating range for extended periods. Some older heat pump models have a minimum of 20 degrees; if your region experiences weeks of sub-20-degree weather, you may need to run emergency heat for part of the winter. Check your system's manual or call your installer to confirm the minimum operating temperature.
Do not switch to emergency heat just because you want faster heating on a cold morning. The heat pump will warm your home; it straightforward takes longer than electric resistance heating. Switching manually wastes money for a small time savings.
The cost difference between heat pump and emergency heat
Electric resistance heating costs roughly two to three times more per hour than running a heat pump, even on cold days. A heat pump moves existing heat from outdoors into your home, which requires far less electricity than generating heat from scratch. Resistance heating generates heat by running electricity through a wire element, similar to a toaster—a much more energy-intensive process.
If you run emergency heat continuously for a month during winter, expect your heating bill to increase noticeably. The exact amount depends on your local electricity rates, how cold it is, and how much you heat your home. In regions where electricity costs 15 cents per kilowatt-hour, running a 10-kilowatt emergency heat system for 8 hours a day for 30 days could add $360 to your bill. A heat pump running the same period would cost roughly $120 to $150.
This is why automatic set up is designed to use emergency heat sparingly—only when the heat pump genuinely cannot keep up. If you notice emergency heat running frequently on days above 35 degrees, your heat pump may have a refrigerant leak or a faulty sensor, and you should contact a technician.
Signs your heat pump needs service instead of emergency heat
If emergency heat activates on days when the outdoor temperature is above 35 degrees, something is wrong with your heat pump—not with the weather. The system should be able to heat your home efficiently at that temperature. Common causes include low refrigerant (from a leak), a frozen outdoor coil, a faulty reversing valve, or a broken compressor.
Another warning sign is if emergency heat runs for more than a few hours even though you have not raised your thermostat setting. This means the heat pump is running but failing to heat, which indicates a mechanical problem. Do not ignore this and assume emergency heat is now your heating system. Call a technician to diagnose the issue.
If your heat pump stops working entirely and you switch to emergency heat, plan to have it repaired within a few days. Emergency heat is safe to use temporarily, but the cost adds up quickly. A technician can usually diagnose and repair common heat pump problems—refrigerant leaks, valve failures, or sensor issues—within a single service call.
How to use your thermostat's emergency heat setting
Most thermostats have a switch or menu option labeled "Heat," "EM Heat," "Aux Heat," or "Emergency Heat." On older manual thermostats, this is a physical switch. On digital and smart thermostats, it is usually in a menu under "Mode" or "System Settings." Consult your thermostat manual if you are unsure where to find it—the brand and model number are printed on the device itself.
To switch to emergency heat, locate the mode selector and move it from "Heat" to "EM Heat" or "Emergency Heat." The thermostat will now run only the electric resistance heating and will not use the heat pump. Set your desired temperature as you normally would. The system will maintain that temperature using only the backup heating.
To return to normal operation, switch the mode back to "Heat." The thermostat will resume using the heat pump as the primary heating source and will set up emergency heat only when needed. If you have a smart thermostat, you can usually make this change from the app or by voice command if it is connected to a voice assistant.
What to do if emergency heat stays on all winter
If your heat pump is not working and you are relying on emergency heat for the entire winter, your heating costs will be substantially higher than normal. This is not sustainable long-term. Schedule a service call with a licensed HVAC technician as soon as possible. Many common heat pump failures—refrigerant leaks, compressor issues, reversing valve problems—can be repaired, and the cost of repair is usually far less than the extra electricity you will pay by running emergency heat for months.
While waiting for service, you can reduce costs slightly by lowering your thermostat setting by a few degrees and using space heaters in the rooms you occupy most. Wear layers and use blankets to stay comfortable at a lower temperature. Close off unused rooms to concentrate heat where you need it. These steps will not eliminate the cost difference, but they will help.
If repair costs are prohibitive and your heat pump is very old, you may need to replace the system. A new heat pump is a significant investment, but it will pay for itself over time through lower heating bills. Many regions offer rebates or financing programs for heat pump replacements; check with your local utility company or state energy office for current programs.
Frequently Asked Questions
Is it bad to run emergency heat for a few days?
No, running emergency heat for a few days while you wait for a repair is safe and normal. Your system is designed to handle this. Just be aware that your electric bill will be higher for that period. If the heat pump is repaired and working again, switch back to normal heat mode to save money.
Why does my thermostat show emergency heat on a 40-degree day?
This indicates a problem with your heat pump. At 40 degrees, the system should heat your home using the heat pump alone. A faulty sensor, low refrigerant, or a broken component is likely preventing the heat pump from working. Contact a technician to diagnose the issue.
Can I use emergency heat instead of the heat pump to save money?
No. Emergency heat costs two to three times more to operate than a heat pump. Using it as your primary heating system will significantly increase your electric bill. Use the heat pump for normal winter heating and emergency heat only when the heat pump is not working.
What temperature should I set emergency heat to?
Set it to the same temperature you would normally use for heating—usually between 68 and 72 degrees. The thermostat will maintain that temperature using the electric resistance heating. You can adjust it up or down as needed, just as you would with normal heat mode.
Does emergency heat work if the power goes out?
No. Emergency heat requires electricity to operate, so it will not work during a power outage. Your entire heating system depends on electricity. If you lose power during winter, you will need an alternative heat source such as a generator, wood stove, or portable propane heater.