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Using Auxiliary Heat with a Nest Learning 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". This article describes how the Nest Learning Thermostat controls this feature.

Note that there are also models called "Nest Thermostat" (without the word "learning") and "Nest Thermostat E". These work differently and parts of this article will not apply.

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 this article does not apply to you.


Heat Pump Design

Auxiliary vs. Emergency

Automatic Switching

Nest "Lockout" Terminology

Setting the Nest for Emergency or Auxiliary Heat

If the Nest Thermostat Has No Internet

Heat Pump Design

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.

There is a limit to how well a heat pump 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 inside of a house quickly enough to counteract the cooling that happens from heat going out through the walls, windows, and roof.

In even colder temperatures and depending on the amount of humidity in the air outside, the outside heat exchanger can get covered in ice. Operating the heat pump for extended periods of time when it is iced up can cause permanent damage.

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 a lower "while asleep" temperature setting to your higher "while I'm awake" setting. (This process of re-heating a home after an less-heated period is called "recovery".)

"Auxiliary" vs. "Emergency"

The two different names refer to the same design being used in two different ways. In a forced-air system, at some point within the air duct there are electric heating coils (like in a toaster or hair dryer) that heat up the air an additional amount beyond what the heat pump is able to do. This is sometimes called "second stage heat" because the air has been heated once (by the heat exchanger of the heat pump) and is now being warmer further by the coils.

The difference between the terms "auxiliary" and "emergency" is in how this is used. This depends on what the thermostat maker or HVAC technician is thinking when they use the word, but in general

Auxiliary heat means using the heat pump and the electric coils at the same time.

Emergency heat usually means using the electric coils without the heat pump.

Whether your system refers to it as "emergency heat" or "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 either you are having an exceptional cold spell ("the coldest it has been in ten years") or perhaps the heat pump is actually broken. Thus, the only time you'd need to use the electric heating coils is when there is an "emergency".

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 and HVAC technicians rarely make this distinction: they 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.

Automatic Switching

Heat pump systems can use a variety of methods to decide when to use the sedond stage heat (electric heating coils):

- A thermometer to monitor the outside air temperature (probably located in the outside heat exchanger)

- A pressure gauge in the refrigerant pipes (a way to indirectly measure how much the refrigerant is being warmed or cooled by the heat exchanger coils)

- Monitoring the temperature of the air in your house to see if it is warming up quickly enough

- Information from a cloud server

In the case of the Nest Learning Thermostat, the last two methods are used — the Nest relies on an internet connection to get information from its cloud servers, telling what your outdoor temperature is; and there is a setting called "Heat Pump Balance" that you can use to control how impatient it should be when performing "recovery" heating. The Nest also has a "lockout" setting to decide when it is too cold to use the heat pump at all (in other words, whether to do "emergency" rather than "auxiliary" heat).

Nest "Lockout" 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 only the electric heat (and not 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

As mentioned at the start: 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 the "What thermostat wire labels mean" section of this help 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. (This was true in 2015 but might not be true with more recent versions of the Nest software or with different models; this article was written for the Nest "Learning" Thermostat.)

If the Nest Thermostat Has No Internet

Many Nest thermostats are being used without a wifi or internet connection. This is for multiple reasons: Faulty wifi chips were used in many Nest thermostats, which fail after only a year or two of use (the thermostat will say it cannot connect to your wifi even though you put in the correct information); still using previous residents' wifi password (common in rental situations); new residents have no wifi (also common in rental situations); unexpected interruption or cancellation of internet service; etc..

In these situations the thermostat does not know the outdoor temperature, and cannot decide if it is too cold to use the heat pump. It will try to use the heat pump whenever it wants to be heating. This can lead to permanent damage of the heat pump if the outside temperature is very cold for a long time. To prevent this, in the colder times of the year you may need to set up your equipment settings to "Emergency heat" and then turn on "EMER.HEAT" in the "Equipment Settings".

If operating without internet, it is also important to cancel some of the thermostat's "smart" or automatic features. Do some or all of these steps:

- Settings > Eco > Heat to: OFF
- Settings > Eco > Cool to: OFF

- Settings > Home/Away Assist > Stop Using

- Settings > Nest Sense > Auto-Schedule > NO
- Settings > Reset > Schedule > Reset (it will ask for confirmation twice, and then reboot).

For the "Eco" settings, each one is normally a temperature like Heat to: 60 or Cool to: 80; you want to turn the wheel as if to change this number and go all the way to the end (lowest or highest number) where it will let you choose "OFF".

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This page was written in the "embarrassingly readable" markup language RHTF, and was last updated on 2023 Jan 09. s.27