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:

Go to SETTINGS > EQUIPMENT. (This setting is about 4 or 5 from the rightmost end, or about 15 steps from the left)
You'll see "Equipment detected" and a diagram of the backplate with the wires that are connected and their labels. In the lower right the "*" connection should say either "emer. heat" or "aux. heat"; or in the upper-right on "W₂/AUX" you should see "aux. heat". If not, then these instructions probably don't apply to you.
Next screen is "Your System", which should read: "heat pump heating", ("auxiliary heating" or "emergency heat"), "heat pump cooling", "fan". (If you don't see this, these instructions don't apply to you.) Select CONTINUE.
Next you have the screen where you can actually adjust the settings. The choices are "HEAT TYPE", "HEAT PUMP", "SAFETY TEMP", "* WIRE TYPE", and "PRO SETUP".
If you had a connection to the "*" wire then select "* WIRE TYPE". If you live in an area where the outdoor temperature never goes below about 20-25°F, select "E: EMERGENCY", because you should never need to use the electric heating coils unless the main heat pump is actually broken. But if you live in a colder climate, select "AUXILIARY HEAT".
If you chose "E: EMERGENCY", then you're done, there is nothing else to set. Otherwise continue:
Now you're back at the "Equipment Settings" menu; select "DONE".

Go back into SETTINGS > EQUIPMENT; click through the "Equipment detected" screen to get to "Your System". Go to the bottom and select "TEST". Test the "AUX. HEAT" or "EMERGENCY HEAT". It should turn on the fan and after a minute or two you should feel distinctly warm air coming out of the vents. If the fan does not turn on, stop the test immediately. Electric heating coils will overheat, burn out or cause damage if the fan isn't going. (Your system might have a fail-safe for this, but it's good to know right away, it may mean your fan wire (perhaps the G connection) isn't connected.)

Go to SETTINGS > NEST SENSE (which is the 4^th setting from the left). Select "Heat Pump Balance". Choose "OFF". You must do this first, because the other settings won't be settable unless you turn off this feature (temporarily, just so you can access the options).

Once again go to SETTINGS > EQUIPMENT; click through the "Equipment detected" and "Your System" screens to get back to "Equipment Settings". Now select "HEAT PUMP".
It will ask if you have an "O wire" or a "B wire". You probably already have this set properly (if it were set improperly, your system would be heating the house when it should be cooling, and vice versa), so just leave this setting as-is.
Then you'll see "Use the heat pump compressor when the outdoor temperature is above" and the word "ALWAYS". Turn the thermostat wheel to the right, and it will let you choose a temperature, starting at about -20°F. I chose 20°F. Your setting will depend on how efficient your home is at retaining heat. If you have a lot of windows, poor insulation, no south-facing windows, etc. then you lose heat easily and will need to set this setting somewhere in the 20-30°F area. The way to find out what setting is best is by leaving this set to "ALWAYS", and wait until a day when it is so cold outside that the heat pump cannot keep up. Then you know that the outside temperature is too cold, so you'll need to set this setting higher than that temperature. My house has trouble when the outside temperature is in the low-20's or lower, so I set it to 20.
If you see "Compressor min. temperature should only be used if AUX controls an auxiliary heater.", and if your test of Aux. heat was successful (it fan the fan and blew warn air out the vents) then hit CONFIRM and proceed.
Now you'll get "Use the heat pump aux. heat when the outdoor temperature is below". Here you can select ALWAYS or a temperature from 35 to 90. This is an upper limit for aux. heat: it will not use the aux. heat above whatever number you choose (but if you pick ALWAYS, Nest is free to use Aux heat whenever it thinks it's a good idea).
Now you're back at Equipment Settings; select DONE.

Go to SETTINGS > NEST SENSE. Select "Heat Pump Balance". Choose "MAX SAVINGS", "BALANCED", or "MAX. COMFORT". I chose "BALANCED" and then after a few weeks, when things seemed to be working well, I switched to "MAX. SAVINGS". Note that if you want to change the min. and max. temperatures for aux. heat, you'll need to turn Heat Pump Balance OFF again.

Once NEST SENSE > Heat Pump Balance is enabled, if you go back into SETTINGS > EQUIPMENT > Equipment Settings > HEAT PUMP you'll have a setting for the "Use compressor when ... temperature is above" option which is ignored, followed by a message "Nest Sense Heat Pump Balance is on. Nest will use auxiliary heat as needed.". This indicates that Nest is using its "avoid aux. heat except when really needed" intelligent algorithm.

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.

This page was written in the "embarrassingly readable" markup language RHTF, and was last updated on 2019 Jan 05.

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