Emergency heat activates 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—usually an electric resistance heater or a gas furnace. You turn it on when outdoor temperatures drop so low that your heat pump works inefficiently, or when the heat pump fails and you need warmth when ready. The switch is on your thermostat, labeled "Emergency Heat," "Aux Heat," or "EM Heat." Flipping it forces your system to use only the backup heater and stops the heat pump from running.

The key word is emergency. Emergency heat costs significantly more to run than a heat pump because resistance heating and gas furnaces burn fuel or electricity directly instead of moving existing heat from outside. You should use it only when necessary—typically a few times per winter, not as your main heating method.

Key Takeaways

  • Emergency heat is a backup system on your thermostat that runs independently of your heat pump and costs more to operate.
  • Use it when your heat pump stops working, when outdoor temperature drops below your system's balance point (usually 30–40°F), or when you need heat faster than the heat pump can deliver.
  • Emergency heat should run only for hours or days, not weeks—if you are using it regularly, your heat pump needs service.
  • Leaving emergency heat on permanently wastes money and defeats the purpose of owning a heat pump.
  • Your thermostat may switch to emergency heat automatically if the heat pump fails; check your settings to understand when this happens.

When your heat pump stops working or loses refrigerant

If your heat pump breaks down—compressor failure, refrigerant leak, or electrical fault—it cannot move heat from outside to inside. In this case, emergency heat is your only source of warmth until a technician repairs the system. Switch it on when ready and call a heat pump service company. Do not wait for the system to fix itself.

A refrigerant leak is common in older systems or after physical damage. The heat pump will still run, but it will not produce enough heat, and the outdoor unit may ice over. If you notice the outdoor unit frozen solid, the indoor air is cold, and the system is running, turn on emergency heat and call for service. Running a heat pump with low refrigerant damages the compressor and makes repair more expensive.

When outdoor temperature falls below your system's balance point

Every heat pump has a balance point—the outdoor temperature below which the heat pump alone cannot meet your home's heating demand. For most systems, this is between 30°F and 40°F, though it varies by the size of your heat pump, the insulation in your home, and how cold you want it inside. Below the balance point, the heat pump runs constantly but cannot keep up, and your home slowly gets colder.

When this happens, you have two choices. You can leave the system on and let the backup heat cycle on automatically (most thermostats do this without you touching anything), or you can manually switch to emergency heat to speed up heating. Manual emergency heat is faster because it bypasses the heat pump entirely and runs the backup heater at full power. Automatic backup is more efficient because it uses the heat pump as long as it is helping, then adds backup heat only when needed.

Check your thermostat manual or settings to see whether backup heat is set to "automatic" or "off." If it is set to "off," the heat pump will run alone even below the balance point, and your home may not reach the temperature you set. In that case, you may need to switch to emergency heat manually during very cold snaps.

When you need heat faster than the heat pump can deliver

Heat pumps warm slowly compared to furnaces or resistance heaters. If you have been away for a week and your home is 55°F, or if you set the thermostat much higher than the current temperature, the heat pump will take hours to catch up. If you need the house warm quickly, emergency heat will do it in half the time, though at higher cost.

This is a judgment call. If you can wait a few hours, let the heat pump work. If you are coming home to a cold house with children or elderly people present, or if you need the space warm for a specific reason, emergency heat is reasonable for a few hours. Just remember to switch back to normal mode once the house reaches your target temperature.

How to tell if your thermostat is using backup heat automatically

Most modern thermostats have a setting called "Auxiliary Heat" or "Backup Heat" that is set to "automatic" by default. This means the thermostat monitors the heat pump's output and switches on the backup heater if the indoor temperature falls below the setpoint for a certain period (usually 30 minutes to an hour). You do not have to do anything—the system handles it.

You can usually see whether backup heat is running by looking at your thermostat display. It will show a small icon or the words "Aux Heat" or "EM Heat" when the backup system is active. If you see this icon during normal winter operation, your system is working as designed. If you see it constantly, your heat pump may need service, or your home may be losing heat faster than the system can replace it (poor insulation, air leaks, or an undersized heat pump).

If your thermostat does not have automatic backup heat enabled, you can usually turn it on in the settings menu. Consult your thermostat manual or call your HVAC technician to confirm the current setting and change it if needed.

Why leaving emergency heat on all winter costs too much

Emergency heat uses 2 to 4 times as much electricity (or gas) as a heat pump to produce the same amount of warmth. A resistance heater converts electricity directly to heat with no efficiency gain. A gas furnace burns fuel to create heat. A heat pump, by contrast, moves heat from outside air (even cold air) into your home, so it produces more warmth per unit of energy consumed.

If you leave the emergency heat switch on all winter, your heating bills will spike noticeably. The backup heater will run instead of the heat pump, and you will lose the efficiency advantage that made you choose a heat pump in the first place. Emergency heat is meant for hours or days of use, not weeks or months.

What to do if you are using emergency heat too often

If you find yourself switching to emergency heat regularly—more than a few times per winter—something is wrong. Either your heat pump is not sized correctly for your home, the system needs service, or your home is losing heat too quickly.

Call an HVAC technician to inspect the heat pump. They will check refrigerant levels, clean the outdoor coil, test the compressor, and verify that the system is operating at full capacity. If the heat pump is working correctly but still cannot keep up, the problem may be insulation, air leaks, or an undersized unit. A technician can help you understand whether upgrading the heat pump or improving home insulation is the better investment.

Frequently Asked Questions

Will emergency heat damage my heat pump?

No. Emergency heat and the heat pump are separate systems. Running emergency heat does not harm the heat pump itself. However, if you use emergency heat because the heat pump has failed (refrigerant leak, compressor damage), continuing to run the broken heat pump while emergency heat is on can make the damage worse. If the heat pump is broken, leave it off and use emergency heat only until repair.

Should I switch to emergency heat at night to save money?

No. Emergency heat costs more to run, not less. Switching to it at night or any other time will increase your heating bill. If you want to save money, lower the thermostat setpoint at night instead—the heat pump will use less energy to maintain a lower temperature than to heat faster with emergency heat.

What if my thermostat does not have an emergency heat button?

Older thermostats or straightforward models may not have a separate emergency heat switch. If your system has a backup heater, it may be controlled by the HVAC unit itself or by a separate panel. Check your thermostat manual or contact your HVAC technician to learn how backup heat is activated on your specific system.

Can I use emergency heat if my heat pump is frozen?

Yes. If the outdoor unit is iced over and the heat pump is not producing heat, switch to emergency heat when ready and call for service. A frozen heat pump usually means a refrigerant leak or a defrost cycle problem. Emergency heat will keep you warm while you wait for repair, but do not run the frozen heat pump at the same time—it can damage the compressor.

Does emergency heat turn on automatically when it gets very cold?

It depends on your thermostat settings. Most modern thermostats have automatic backup heat enabled, which means the system will switch on the auxiliary heater if the heat pump cannot keep up. However, some older or manually configured systems do not have this feature. Check your thermostat manual or ask your HVAC technician whether automatic backup is turned on.