You have probably noticed that on very cold days the AUX HEAT light goes on at your thermostat, or the words appear on the screen if your have a digital display. This means that the system has switched over from the heat pump compressor to an electric resistance heat strip in the air handler to heat the house. It occurs automatically when the outdoor air approaches freezing temperature. Because a heat pump moves heat instead of generating it, when there is not enough heat in the outside air for it to grab, the system efficiency plummets and auxiliary heat kicks on. 

    Auxiliary heat can also activate when the indoor temperature is more than about 3º F below the temperature setting on the thermostat, such as when you come home on a cold day and move the thermostat up from 60º to 70º F. Both the heat pump and heat strip will operate in tandem to quickly get the house up to your desired temperature. This can happen even if the outdoor temperature is not that cold. Once at the desired temperature, only the heat pump will function to maintain it.

    But the heat strip is less than half as energy efficient as a heat pump; so, ideally, the heat strip should only be used when absolutely necessary. This is where the "electric heat lock out" comes in. Newer heat pump systems have a “smart” thermostat connected to an outdoor temperature sensor. The sensor enables the thermostat to set an outside temperature (typically around 35º F) above which it will not allow the AUX HEAT heat strip to turn on. This saves both energy and dollars on your electric bill, which is always a good thing.

    The electric heat lock out will not affect the EMER HEAT (emergency heat) option on the thermostat, which is a way to manually turn on the heat strip when necessary. But EMER HEAT should only be used when the heat pump is not functional. 

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  To learn more about heating and air conditioning systems, see these other blog posts: 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.