What emergency heat does and when it turns on
Emergency heat is a backup heating system built into most heat pumps that switches on automatically when the outdoor unit cannot pull enough warmth from the air. On your thermostat, you'll see it labeled as "Emergency Heat," "Aux Heat," or "E-Heat." When outdoor temperatures drop below a certain point—usually between 30°F and 40°F, depending on your system—the heat pump loses efficiency. Rather than let your home get cold while the heat pump struggles, the emergency heat kicks in to keep you warm.
Emergency heat is not something you turn on manually under normal circumstances. Your system decides when to use it based on how hard the heat pump is working and what the outdoor temperature is. You might see the emergency heat indicator light up on your thermostat during a cold snap, or you might never notice it running at all. The switchover happens behind the scenes.
Understanding how this works matters because emergency heat costs more to run than the heat pump alone, and knowing when it's active helps you spot problems early. If emergency heat is running constantly on mild winter days, something is wrong with your heat pump.
Key Takeaways
- Emergency heat is a backup electric heating element that turns on automatically when outdoor temperatures drop too low for the heat pump to work efficiently.
- Most heat pumps switch to emergency heat between 30°F and 40°F outdoors, though your installer may have set a different temperature.
- Emergency heat costs roughly two to three times more per hour to run than the heat pump alone because it uses electric resistance heating instead of moving heat from outside.
- If emergency heat runs on days warmer than 35°F, or if it stays on for more than a few hours, your heat pump may need service.
- You should never manually switch to emergency heat mode unless a technician tells you to do so for troubleshooting.
How the backup heating element actually works
Inside your indoor unit sits an electric resistance heating element—essentially a coil that heats up like the coil in a toaster. When the thermostat signals that emergency heat is needed, electricity flows through this coil, and it produces heat directly. This is different from how the heat pump works: the heat pump moves heat from outside air into your home, which is efficient. The electric element creates heat from scratch, which is not efficient but is reliable and works even when it's very cold outside.
Your heat pump has a sensor that measures how much heat the outdoor unit is producing. When the heat pump cannot keep up with the temperature you've set on the thermostat, the system calculates how much additional heat is needed. If that gap is large enough, emergency heat turns on. The two systems run together—the heat pump keeps working, and the electric element adds extra warmth until your home reaches the set temperature.
Once your home warms up, the thermostat signals the emergency heat to shut off. The heat pump continues running on its own. This cycle can repeat many times on a very cold day, with emergency heat turning on and off as needed.
Why emergency heat costs more to operate
A heat pump in winter mode moves heat from outside air into your home, a process that uses less electricity than creating heat from scratch. Emergency heat creates heat directly through electric resistance, which requires much more power. Running emergency heat for one hour can cost roughly two to three times what running the heat pump alone costs for the same hour, though the exact difference depends on your local electricity rates and your system's efficiency rating.
This is why you want emergency heat to run as little as possible. If your heat pump is working correctly, emergency heat should only be necessary on the coldest days of the year—typically below 30°F. If you see it running regularly on mild winter days, your heat pump is not performing as it should, and you may have a refrigerant leak, a dirty outdoor coil, or a failing compressor. Any of these problems will cost more to fix the longer you wait.
Signs that emergency heat is running when it shouldn't
Most thermostats display which heating source is active. Check your thermostat display or look for a light labeled "Aux" or "E-Heat." If that light is on, emergency heat is running. Write down the outdoor temperature when you see it. If emergency heat is running on days warmer than 35°F, or if it stays on for more than a few hours at a time on cold days, your heat pump needs attention.
Another sign is a sudden jump in your heating bill without a change in how much you use your home. If you heat to the same temperature as last winter but your bill is 30 or 40 percent higher, emergency heat may be running too often. Call a heat pump technician to inspect the outdoor unit for ice buildup, a clogged air filter, or low refrigerant—all common reasons emergency heat activates too frequently.
Do not ignore this. The longer a faulty heat pump runs, the more it costs you, and the more wear it puts on the compressor, which is the most expensive part to replace.
When you might manually switch to emergency heat mode
Your thermostat may have a switch that lets you select "Emergency Heat" mode manually. Under normal circumstances, you should never use this switch. Leave it on "Auto" or "Heat," and let the system decide when emergency heat is needed. Switching to emergency heat mode manually disables the heat pump entirely and runs only the electric element, which wastes money and defeats the purpose of owning a heat pump.
The only time you should manually switch to emergency heat is if a technician tells you to do so while they are troubleshooting a problem. For example, if the heat pump is not working but you need heat when ready while waiting for a service call, a technician might tell you to switch to emergency heat temporarily. Once the repair is done, switch back to auto or heat mode right away.
How to reduce emergency heat use during winter
Keep your heat pump running efficiently by maintaining the outdoor unit. Clear away leaves, snow, and ice from around the outdoor condenser. If ice builds up on the coil, the system has to work harder and emergency heat kicks in sooner. Most heat pumps have a defrost cycle that melts ice automatically, but heavy snow or ice dams can overwhelm it.
Set your thermostat to a steady temperature rather than raising it suddenly. If you set the temperature 5 degrees higher than usual, the system has to produce a lot of heat quickly, and emergency heat is more likely to turn on. Raising the temperature by 1 or 2 degrees at a time gives the heat pump time to catch up. Also, make sure your home is well-sealed: air leaks around doors, windows, and ducts force your heating system to work harder.
Have your heat pump serviced before winter. A technician will check the refrigerant level, clean the coils, and make sure the defrost cycle is working. A well-maintained heat pump uses emergency heat far less often than a neglected one.
Frequently Asked Questions
Is it normal for emergency heat to run on very cold days?
Yes. On days below 30°F, especially if it is below 20°F, emergency heat may run for an hour or two while the system keeps up with heating demand. This is normal and expected. If it runs all day or on days warmer than 35°F, call a technician.
Should I turn off my heat pump and use only emergency heat in winter?
No. The heat pump is always more efficient than emergency heat alone, even on cold days. Running only emergency heat will cost you significantly more. Keep your system on auto or heat mode and let it manage both sources automatically.
Why does my emergency heat run even when my house is warm enough?
This usually means the heat pump is not keeping up with demand, often because of low refrigerant, a dirty outdoor coil, or a faulty sensor. Have a technician inspect the outdoor unit and check the refrigerant charge. Do not wait—this problem gets worse and more expensive.
Can I prevent emergency heat from running at all?
You cannot prevent it entirely on very cold days, but you can minimize it by keeping your heat pump well-maintained, sealing air leaks in your home, and avoiding sudden temperature changes on the thermostat. Regular service before winter is the best way to keep emergency heat use low.
What is the difference between emergency heat and auxiliary heat?
They are the same thing. "Auxiliary" and "emergency" both refer to the electric resistance heating element that backs up the heat pump. Your thermostat may use one term or the other, but they describe the same system.