Many homeowners get confused when the temperatures start to drop, and they see either “aux” or “em” heat on their thermostat. This happens sometimes when it’s colder than average and on the verge of extreme temperature drop like Nashville experienced this last spring. Here's the long and short of it. The Thermostat will turn on the heat strips (1) when the temp difference is more than 2-3 degrees (white wire to W2). You shouldn’t ever use EM heat unless there’s truly an emergency—like your heat pump isn’t … For those who operate such equipments just now, perhaps you should know the difference between the two rather than just operate on the device blindly. Emergency Heat is typically triggered when it is 35°F and below outside. Auxiliary heat is when the secondary heat source turns on automatically. EM heat is NOT the same thing as AUX heat: AUX heat signifies that your heat pump has automatically switched over to backup heating because temperatures are too low. Note: In some cases, your heat pump’s AUX heat could be a gas furnace, but since Phoenix typically has mild winters, some AZ homeowners will have electric resistance as their AUX heat. Auxiliary Heat vs. A lot of people don’t really understand the difference between auxiliary heat and emergency heat. The Nest turns on W2/aux when it wants auxillary heat from the heat strip. The Nest turns on Emergency heat … In the usage of a standard heat pump, users often encounter the terms heat and emergency heat. I guess the only difference is that AUX is an ‘ indicator’ on your thermostat that you are running the heat strips when the switch is set to HEAT. Both of these wires are needed for Emergency Heat. EM heat is an option on your thermostat that you can manually turn on to activate backup heating. If there's a wire in the W1 connector, make sure it says AUX heat. So, what is the difference between auxiliary heat and emergency heat? The difference is quite simple. So the signal will either come from the White Wite on W2 or the brown wire on E to turn on the heat strips. This wire is connected to the W1 terminal on the air handler and has a second wire connection which goes to the heat pump. First “Auxiliary Heat”, “Strip Heat”, “Emergency Heat”, “Radiant Heat” are all used to reference the same heating mechanism; your electric heat strips that come with a heat pump. Note: If Alt heat (not AUX heat) appears, your Nest Thermostat E is configured to work with a dual fuel system. Emergency Heat, also known as “auxiliary heat”, is the second stage of heat that your thermostat runs on when the temperature is too cold for your heat pump to extract heat from the outside. The Auxiliary heat gets automatically turned on by your HVAC system to supplement the heat pump output when your system can’t keep up. When your heat pump breaks down, or it just isn’t working the way it’s designed to, a homeowner can switch to emergency heat. 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