What is AUX HEAT and EM HEAT on my thermostat?

Monday, October 1, 2018

AUX HEAT means auxiliary heat and EM HEAT is emergency heat; sometimes also abbreviated as EMERG HEAT, as in the older thermostat shown above. Both settings turn on an electric resistance heat strip in a heat pump air handler, which is similar to the heating element in a toaster oven, but they activate it in two different ways.

   To understand why an electric resistance heat strip is needed, it’s necessary to review how a heat pump works. It does not actually create heat but, instead, absorbs heat from one location and moves it to another. During the summer it absorbs heat inside your home and moves it outside, with a reverse in the direction of heat transfer happening in the winter and heat moving from the outside to the inside. When the outdoor temperature gets close to freezing, it becomes difficult for a heat pump to efficiently absorb heat outside to transfer into the home.

     If the heat pump needs assistance to maintain the indoor temperature—either because the thermostat has been been set more than a few degrees above the current room temperature, or because it is unable to maintain the thermostat setting with the heat pump alone and is falling behind—it automatically switches on the electric resistance heat strip to produce additional heat. Typically, a small light turns on next to the words AUX HEAT on an older thermostat like the one above, or AUX HEAT appears on the screen of a new digital thermostat.

   If you want to manually choose the electric resistance heat strip on an older thermostat, you slide a switch to EM HEAT or EMER HEAT. You tap the selection on the screen of a newer thermostat. The end result is the same as auxiliary heat except that it is manually chosen by you, and the electric heat strip becomes the only heat source, because the heat pump is turned off in emergency heat mode.

    Also, see our blog posts Why does the "AUX HEAT" (auxiliary heat) light keep coming on at my thermostat, even when it's not that cold outside? and When should I switch my thermostat to “EMERGENCY HEAT” for my heat pump air conditioner?

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

How can I find out the SEER of my air conditioner? 

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

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

How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

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. 

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