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:
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