Why your thermostat shows AUX when it is not that cold

What does AUX mean on the thermostat screen?

It means the thermostat has switched on the backup heat that comes with your heat pump system, usually electric heat or a furnace. The heat pump keeps running too. AUX is the thermostat deciding that the heat pump needs help reaching the setpoint right now.

That is different from emergency heat, which you choose by hand and which turns the heat pump off. If you are not sure which one you are looking at, see AUX heat vs emergency heat.

Why does AUX come on when it is not very cold?

Cold weather is the obvious trigger, but not the only one. Google lists two causes for Nest systems: AUX heat can turn on when it is too cold outside, or when your heat pump needs to defrost. Carrier and Trane add a third.

• You raised the setpoint a lot at once. Carrier says AUX helps raise the indoor temperature quickly if you increase the setting by more than 2 to 3 degrees at once. Trane gives 3 to 4 degrees.
• The schedule is recovering from a setback. If the schedule drops the temperature overnight, the morning jump back up looks to the thermostat like a big setpoint increase. Carrier advises against dropping the temperature significantly at night for exactly this reason. Honeywell Home, ecobee, Nest and Sensi thermostats can also start heating early to hit the scheduled temperature on time, which is recovery mode.
• The heat pump is defrosting. In cold, damp weather the outdoor coil can frost over, and the heat pump runs a short defrost cycle to clear it. Trane says you may see AUX on your thermostat during defrost, and that a heat pump can easily enter defrost when it is in the low 40s outside. Trane's own demand defrost cycles usually last 3 to 5 minutes.

A short burst of AUX for any of these reasons is the system working as designed.

How long is too long?

Trane gives a simple rule of thumb: if it is above 40°F and your thermostat shows AUX heat for more than an hour or two, you might want to investigate. It also says that if you lower the indoor setpoint by several degrees, the AUX indicator should disappear. That quick test tells you whether AUX is responding to the setpoint gap or running for some other reason.

If AUX stays on even after you lower the setpoint, or comes on for hours in mild weather day after day, the settings below are the first things to check, and the signs at the bottom of this page tell you when it is time for a technician.

Which settings control when AUX comes on?

Every brand lets you, or your installer, tune the balance between comfort and running cost. The names differ:

Settings that decide when auxiliary heat comes on

Brand	Setting	What it does	Range and factory value
Honeywell Home (T6 Pro Programmable)	Backup Heat Droop (installer option 340)	Degrees the room may drop before backup heat helps (the guide does not define the Comfort option)	0 (Comfort) from the factory, or 2 to 15°F
Honeywell Home (T6 Pro Programmable)	Upstage Timer for Backup Heat (option 350)	How long the heat pump tries alone before backup heat is added	Off from the factory, or 30 minutes to 16 hours
Honeywell Home (T6 Pro Programmable)	Outdoor Lockout Backup Heat (option 356)	Outdoor temperature above which backup heat is locked out	Off from the factory, or 5 to 65°F
Google Nest	Heat Pump Balance	Trades comfort against AUX use; sets the AUX lockout temperature for you	Max Comfort (default), Balanced, Max Savings or Off
ecobee	Aux Heat Max Outdoor Temperature	Prevents auxiliary heat from running above an outdoor temperature	Not stated
ecobee	Compressor to Aux Temperature Delta, or Compressor to Aux Runtime	Temperature gap, or compressor run time, before AUX is added	Not stated
Emerson Sensi	Aux Lockout	Above this outdoor temperature, only the heat pump runs	Minus 4 to 80°F
Emerson Sensi	Balance Point	Outdoor temperature that works with Aux Lockout to decide when the heat pump and AUX run	Minus 5 to 55°F; Aux Lockout can never go below it

A few details from the manufacturers are worth knowing:

• On a Nest, Google says Max Comfort, the default, generally gives a higher AUX lockout temperature, so AUX turns on sooner and more often. With Heat Pump Balance off, you set the lockout yourself; Google's example is a 40°F lockout, which means AUX comes on when it is below 40°F outside. Heat Pump Balance needs Wi-Fi and is not available on dual fuel systems.
• On an ecobee, thresholds can only be changed on the thermostat itself, under Installation Settings > Thresholds, not in the app.
• On a Sensi, the Aux Lockout and Balance Point need Wi-Fi, because they depend on outdoor weather data. Sensi's Early Start also holds back AUX during morning recovery: it will not bring on auxiliary heat unless it has three or more degrees to go with only 30 minutes left before the scheduled time.

These are installer level settings on most systems. If you want AUX to come on less, the gentle options are a savings setting such as Balanced on a Nest, or a smaller overnight setback, rather than locking AUX out entirely. Some thermostats also have a cycles per hour setting for auxiliary heat, and a compressor protection delay that can make the heat pump wait before it starts at all.

What can you do on the thermostat?

Did you, or the schedule, just raise the setpoint by several degrees?

• Yes

  That is the most common trigger. Raise the temperature in smaller steps next time, or reduce the overnight setback. AUX should switch off once the room is close to the setpoint.

• No

  Does AUX disappear within a few minutes, or when you lower the setpoint by a few degrees?

  • Yes

    It was most likely a defrost cycle or a short boost. That is normal.

  • No

    AUX is running for another reason. Check the AUX settings for your brand above, and the signs below before going further.

Do not switch to emergency heat to try to fix this. It turns the heat pump off and makes the backup heat do all the work, which is the opposite of what you want.