When Does a Heat Pump Switch to Emergency Heat?

Emergency heat on a heat pump is for use when the heat pump isn’t adequately heating your home because it is broken. You might also want to select it if the outside temperature is below the safe operating range of the heat pump. All this and more is explained here including emergency heat vs auxiliary heat.

When does a heat pump switch to emergency heat?

A heat pump must be manually switched to emergency heat. You should do this if the outside unit stops working. For example, the compressor might fail, refrigerant might leak out or the coil might freeze over.

Does a heat pump automatically switch emergency heat?

No. On most heat pump systems, emergency heat mode must be manually chosen on the thermostat. Use the Mode button and scroll through the modes to find Emergency Heat or EM Heat.

Does emergency heat run constantly?

Yes – it will run constantly until you turn it off.

How long can a heat pump run on emergency heat?

24 hours a day. Continuously, in other words.

But you should only do this if the heat pump isn’t working, and the only way to prevent very low temperatures inside, including freezing pipes, is to run it.

Do all heat pumps have emergency heat?

No, not all heat pumps have emergency heat, aka EM heat. Though most do, especially in climates where temperature can drop into the 30s.

Only systems that have the backup heat strips installed in the air handler include EM heat. A few systems in warm parts of the country do not have backup heat.

How do I know if my heat pump has emergency heat?

First, the air handler will have heating coils inside – that’s the equipment that provides emergency heating.

Second, the thermostat will have an Emergency Heat mode – that’s not a foolproof clue, because a heat pump thermostat will almost always have the mode. But it must have the mode for EM heat to be used.

How much more expensive is emergency heat?

Running electric heat costs 3 to 6 times more than standard heat pump heating. Electric resistance heating, which is what the heating strips provide, is the most expensive type of home heating. In standard Heat mode, electricity is only used to run the system, not produce heat.

Are auxiliary heat and emergency heat the same?

No. They are different modes of the heat pump. Here are the differences between them.

Aux heat works with the outdoor unit to provide enough heat.

Auxiliary heat comes on automatically when the temperature in the house is 3 degrees or more cooler than the thermostat setting. It assists the heat pump in getting your home up to the temperature you want. This might happen when:

  • Outside temperature drops sharply – and the heat pump needs time to catch up
  • You turn up the thermostat heat setting by 3+ degrees

For these reasons, auxiliary heat is also called supplemental heat or a secondary heat source.

Finally, aux heat also comes on when the outdoor unit is in Defrost mode – getting built-up ice off the coil. You might see steam coming off the outdoor unit to indicate it is defrosting. Or the Aux might appear on your thermostat.

  • Emergency heat works instead of the outdoor unit to provide enough heat to keep your home from getting very cold.
  • Emergency heat must be manually turned on and off and is used when the heat pump outside is providing little or no heat. EM heat won’t turn off by itself.

Also select EM Heat on the thermostat when the outdoor temperature is well below freezing – say about 25F or lower. Compressors running in temperatures that cold can be damaged, so to prevent mechanical trouble, turn off the outside unit and use emergency heating.

Should I run emergency heat in an ice storm?

Yes, turn on emergency heat if the outside temperature is below freezing. When EM heat is on, the condensing unit outside will not operate, and will be protected from potential damage that could occur if it ran during a storm with freezing rain.

If the outside temperature is above freezing, you can let the heat pump run. EM heat isn’t necessary.

What is the drawback of emergency heat?

The drawback of emergency heat is that it is expensive to operate. Plus, in extreme cold, it probably won’t be able to keep your home as you’d like it to be.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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