– Frank Alexander, February 03, 2020
Have you ever wondered what those two heat settings are on your thermostat? Are you confused by the difference between “Aux. Heat” and “Em. Heat” and just regular “Heat” settings on your thermostat? Or have you ever wondered why some thermostats have these and others do not?
If you came here because your emergency or auxiliary heat light is on for an extended or frequent amount of time, you probably need to pick up a phone and call us to check out why your system is running in that mode. It isn’t normal unless it’s below 10 degrees outside. For service call 919-886-4822
First let’s define the two types of air conditioning systems used in the Southern United States, Air Conditioners with Furnaces, and Heat Pumps. The former will not require the Aux or Em Heat settings. The latter, Heat Pumps, have two sources of heat for your home so their thermostat needs to be able to communicate to the equipment when to use the two different heat sources. So only heat pump systems require the Emergency heat setting on their thermostats. Furnaces do not unless they are installed in conjunction with a heat pump in which case this is called a dual fuel system, and is not very common. The thermostat you use for each type of system is unique. So when buying a thermostat, you need to make sure it is compatible with heat pumps if you own a heat pump.
OK, so these terms are really used interchangeably. 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.
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.
This is to inform you that you are using the more expensive heating method of ‘radiant’ heat. Radiant heat is much like you see in plug-in space heaters, it is a coiled wire that becomes read hot and is used to warm the air. This light comes on in normal operation from time to time, so it really isn’t something to worry about if your system is in proper operating condition. Every few hours the heat pump needs to reverse itself and defrost the outside unit. While it is defrosting, the auxiliary heat comes on to prevent you from feeling cold air in your vents. Basically, the defrost mode is when the heat pump flips back to air conditioning to warm up the outside coil. So in order to avoid frigid air through your vents while this is happening, the heat strips are energized and you will see the auxiliary light come on. Another time it may come on when the heat pump just isn’t strong enough to keep up with the outside temperature. This is normally below 40 degrees F. So the auxiliary heat comes on to assist the heat pump to maintain the set temperature. Here’s a caveat, if your heat pump is low on refrigerant, the strip heat may come more frequently and at higher outside temperatures. You may also detect low refrigerant if you happen to see your thermostat more than 2 degrees off of the set point. That is because the auxiliary heat does not get triggered until the thermostat and room temperature are about 3 degrees different. (See this article about refrigerant Leaks)