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Using Auxiliary Heat with a Nest Thermostat  

If you have a central HVAC (heating / ventilation / air conditioning) system called a "heat pump", and live in an area where the temperature in the winter goes below about 50°F (or 10°C) then you probably have something called "emergency heat" or "auxiliary heat". The two different names refer to the same thing: an electric heating system using resistive heating coils (like in a toaster or hair dryer) located inside the airflow path of the ventilation system.

In normal operation, a heat pump works like an air conditioner: a compressible vapor with low boiling point (like freon) is forced through a cycle that includes a compresser, a cooling heat exchanger (a bunch of radiator fins), a bottleneck valve (serving as a pressure-release point), and a warming heat exchanger. One heat exchanger is located inside the house or apartment, and the other is outside (perhaps on the roof).

In summer, the heat pump circulates in one direction, causing the refrigerant vapor to be compressed just before travelling through the outdoor heat exchanger, where heat is released; then through the bottleneck value (making the refrigerant cold) and then through the indoor heat exchanger (where the indoor air gets cooled).

In winter, solenoid-actuated valves reverse the circulation pattern, so that the compressed vapor goes immediately to the indoor heat exchanger to release heat into the indoor air, then through the bottleneck valve and the outdoor heat exchanger, where the relatively-colder refrigerant is warmed by the not-quite-so-cold outdoor air. You're "air conditioning the outdoors", cooling the air outside in order to warm the house.

When operating in this second "heating" mode, there is a limit to how well it can work when the outside temperature is very cold. Typically, if the outside temperature is below about 25°F (or -4°C), a heat pump cannot warm the inide of a house quickly enough to counteract the cooling that happens from heat going out through the walls, windows, and roof. For this reason, heat pump systems installed in climates that go below freezing need another way of providing heat, if only for a short time in the morning when your system goes from your lower "while asleep" temperature setting to your higher "while I'm awake" setting. (This process of re-heating the home every morning is called "recovery".)

The difference in name "emergency heat" vs. "auxiliary heat" results from differences in needs based on your local environment.

If you live in an area when the temperature hardly ever goes below 30 degrees, even at night during the coldest month of the year, then a heat pump should be able to keep you warm all the time. If it fails to do so, then it is probably actually broken. Thus, the only time you'd need to use the electric heating coils is when the system is broken — thus the name, "emergency heat".

On the other hand, if you live somewhere that the temperature frequently stays below freezing all day for many days of the year, then the use of the electric heat in your system will be common, even when the heat pump is working perfectly well, and in this case the term "auxiliary heat" is more appropriate.

Thermostats rarely make this distinction: it will either say "auxiliary" or "emergency", rarely both. If your HVAC contractor gave you a "warm climate" thermostat, or failed to add a jumper between the E and AUX terminals, you'll have to flip a manual switch to get the auxiliary heat to turn on.

Nest Terminology

The user interface provided by the Nest thermostat differs from the Nest website and iOS/Android apps.

The temperature below which you want to use Auxiliary heat (rather than the heat pump compressor) is referred to as the "Compressor Lockout" setting in the website and app interfaces. But on the Nest Thermostat itself, the term "Lockout" is not used, instead you see "Use the heat pump compressor when the outdoor temperature is above". This wording is ambiguous because it sounds like you are telling it that it should "always use the compressor..." above that temperature, or perhaps you are telling it that it "may sometimes use the compressor" above that temperature. In practce I have found that the second interpretation (may sometimes) is accurate.

The temperature above which you want to always use the compressor is called "Auxiliary Heat Lockout" in the website and app. Again, the actual thermostat uses the words "Use the heat pump aux. heat when the outdoor temperature is below", and again this wording is ambiguous because you don't know if you are giving it permission to sometimes use aux. heat below that temperature, or telling it to always use aux. heat below that temperature.

Setting the Nest for Emergency or Auxiliary Heat

If your heating system does not have a fan that blows air through vents, or if you get your heat from oil or gas, then these instructions do not apply to you.

It is assumed that you have already gone through the complete setup instructions, which would probably start with Nest's compatibility checker including especially the step where you identify what wires your old thermostat has. For more about identifying the functions of thermostat control wires, see Nest's identifying thermostat wires article.

Once your thermostat is set up and running, here's how to select emergency vs. auxiliary heat:

After making these settings, the Nest "home screen" (showing the current temperature and whether it's heating" will show "AUX. HEAT" or "EMERGENCY HEAT" instead of "HEATING" (as appropriate) when it is using the electric heating coils.

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This page was written in the "embarrassingly readable" markup language RHTF, and was last updated on 2018 Dec 27. s.27