Emergency heat turns on when your heat pump cannot keep up with the cold
Emergency heat (also called auxiliary heat or backup heat) is a second heating system built into most heat pumps. It activates automatically when outdoor temperatures drop so far that your heat pump pulls in less warmth than your home is losing through walls, windows, and doors. At that point, the system switches to electric resistance heating — essentially a giant toaster element — to close the gap and maintain your set temperature.
You do not choose when emergency heat turns on. Your thermostat does. Most systems trigger it automatically when the outdoor temperature falls below 30 to 40 degrees Fahrenheit, though the exact threshold depends on your equipment and how your installer programmed it. You will see "Aux" or "EM Heat" light up on your thermostat display when it is running.
The reason emergency heat exists is straightforward: heat pumps work by moving heat from outside air into your home, even in cold weather. But as outdoor temperature drops, there is less heat to move. Below a certain point — called the balance point — the heat pump alone cannot meet your heating demand. Emergency heat fills that shortfall so your home stays warm.
Key Takeaways
- Emergency heat activates automatically on your thermostat when outdoor temperature falls below your system's balance point, usually between 30 and 40 degrees Fahrenheit.
- Emergency heat is electric resistance heating and costs roughly two to three times more per hour to run than your heat pump, so you want it on only when necessary.
- You should never manually switch to emergency heat mode unless your heat pump has failed or stopped heating — running it by choice wastes money.
- If emergency heat runs constantly during normal winter weather, your heat pump may need service or your thermostat may be set incorrectly.
- A properly functioning heat pump will handle temperatures down to 0 degrees Fahrenheit or lower without your input; emergency heat is the system's job, not yours.
Why emergency heat costs more and uses more power
A heat pump moves existing heat from outside air into your home using a compressor and refrigerant — the same way an air conditioner works in reverse. This process uses less electricity than generating heat from scratch because you are not creating warmth, you are relocating it.
Emergency heat is electric resistance heating. It generates warmth directly by running current through a heating element, the way a space heater or electric oven works. This method uses roughly two to three times more electricity per hour than your heat pump does to produce the same amount of warmth. Over a winter month, that difference shows up clearly on your power bill.
This is why your thermostat is programmed to use emergency heat only when the heat pump cannot keep up. Below the balance point, the heat pump would have to run constantly and still fall short. Switching to emergency heat at that moment is the more efficient choice — it meets demand faster and actually costs less than running a struggling heat pump around the clock.
When your thermostat automatically switches to emergency heat
Most thermostats are set to trigger emergency heat when outdoor temperature drops below 30 to 40 degrees Fahrenheit. Some systems use a different trigger: they monitor how long the heat pump runs without reaching your set temperature. If the heat pump has been running for 20 or 30 minutes straight and your home is still cooling down, the thermostat switches to emergency heat to speed up heating.
You will see this happen most often during a cold snap or overnight when outdoor temperature plummets. Your thermostat detects the change, realizes the heat pump alone cannot keep pace, and automatically engages the backup system. The switch happens without any action on your part.
Some newer thermostats also use outdoor temperature forecasts. If the system predicts a sharp temperature drop in the next few hours, it may pre-stage emergency heat so it is ready to engage when ready rather than waiting for the heat pump to fall behind first. This approach keeps your home more comfortable during rapid cooling.
When you should manually switch to emergency heat mode
You should almost never manually switch your thermostat to emergency heat mode. The only legitimate reason is if your heat pump has stopped working entirely and you need heat when ready while waiting for a technician. In that case, switching to emergency heat keeps your home warm until the pump is repaired.
Some older thermostats have a manual "Emergency Heat" or "EM Heat" button or switch. Resist using it during normal winter operation. Running emergency heat by choice when your heat pump is working fine is like paying three times the price for the same warmth. Your thermostat already knows when to use it; your job is to leave it alone.
If you find yourself tempted to switch to emergency heat because your home is not warming fast enough, the real problem is usually one of these: your thermostat is set too high, your heat pump needs service, or your home has an insulation or air-sealing problem. Switching to emergency heat masks the problem and costs you money without fixing anything.
Signs that emergency heat is running too often
If your thermostat shows "Aux" or "EM Heat" running during mild winter weather — say, when outdoor temperature is above 40 degrees — something is wrong. Your heat pump should handle those temperatures without backup. Constant emergency heat use during normal winter weather means either the heat pump is not working properly or the thermostat is misconfigured.
Check your thermostat settings first. Some systems have a balance point setting that you or an installer can adjust. If it is set too high — say, 50 degrees instead of 35 — emergency heat will run whenever it is colder than that, even though your heat pump could handle it. Lowering the balance point to match your equipment's actual capability will reduce emergency heat use and lower your bills.
If the balance point is set correctly and emergency heat still runs constantly, your heat pump likely needs service. A refrigerant leak, a dirty outdoor coil, or a failing compressor will all reduce heating output and force the system to rely on emergency heat more than it should. Call a technician to inspect the unit before the coldest part of winter arrives.
How to read your thermostat display during winter
Most digital thermostats show you which heating source is active. Look for labels like "Heat", "Aux Heat", "EM Heat", or "Backup Heat" on the display or in the status menu. Some thermostats light up an indicator; others show it only when you scroll through the menu.
During normal winter operation, you should see "Heat" most of the time. "Aux" or "EM Heat" should appear only during the coldest days or nights, usually when outdoor temperature is below 30 degrees. If you see emergency heat running when it is 45 degrees outside, that is a sign to check your balance point setting or call for service.
If your thermostat does not display which heating mode is active, check the manual or the manufacturer's website. Some older models do not show this information on the screen, but you can still see it in the settings menu or by calling the manufacturer's support line with your model number.
What happens if you ignore emergency heat and let your heat pump run alone
If you disable emergency heat or your system fails to engage it, your heat pump will keep running during cold weather, trying to meet demand on its own. It will run almost constantly, cycling on and off without ever reaching your set temperature. Your home will be cold, and your power bill will be higher because the heat pump is working at maximum capacity with minimal output.
In extreme cold — below 0 degrees Fahrenheit — a heat pump without emergency heat backup may not be able to maintain any meaningful warmth at all. Your home temperature could drop 10, 15, or 20 degrees below your set point. Pipes could freeze, and living conditions become uncomfortable or unsafe.
This is why emergency heat exists: it is not a luxury or an optional feature. It is a necessary part of the system. Disabling it or preventing it from engaging defeats the purpose of having a heat pump in cold climates.
Frequently Asked Questions
Is it bad for my heat pump if emergency heat runs a lot?
No, emergency heat running during genuinely cold weather does not harm your heat pump. The system is designed to use it. However, if emergency heat runs constantly during mild winter weather, it signals that your heat pump is not working properly or your thermostat is misconfigured. Have it checked so you are not paying extra for no reason.
Can I turn off emergency heat to save money?
You should not disable emergency heat entirely, because your home will not heat properly during cold weather. If emergency heat is running too often, the fix is to lower your balance point setting or have your heat pump serviced — not to turn off the backup system. Disabling it saves nothing if your home becomes too cold to live in.
Why does my emergency heat kick in at 40 degrees when it is not that cold outside?
Your balance point is probably set higher than it needs to be. Most heat pumps can handle temperatures well below 40 degrees without backup. Ask your installer or a technician to lower the balance point to 30 or 35 degrees, which will reduce emergency heat use and lower your winter bills.
Does emergency heat use a lot of electricity?
Yes. Emergency heat uses roughly two to three times more electricity per hour than your heat pump does. That is why you want it on only when outdoor temperature is genuinely cold and your heat pump cannot keep up. During mild winter weather, your heat pump should handle heating alone.
What should I do if emergency heat runs all the time?
First, check your thermostat balance point setting and lower it if it is above 35 degrees. If that does not help, call a technician to inspect your heat pump. A refrigerant leak, dirty coil, or failing compressor will all cause excessive emergency heat use. Getting the pump serviced will restore efficiency and lower your bills.