Aux Heat on Your Thermostat: Why It Comes On

Match it to a reason

It is cold outside. Google says heat pumps are very effective, but take longer to heat your home the colder it gets outside. Below the lockout temperature set in the thermostat, aux heat comes on. Google’s example: a lockout of 40°F means aux heat below 40°F outside. How far each kind of heat pump goes on its own: how cold is too cold for a heat pump.

The heat pump is defrosting. Google says aux heat can turn on when the heat pump needs to defrost: why a heat pump ices up in winter.

The room is well below the setting. In the advanced settings of the Nest Learning Thermostat (4th gen), Google calls it droop: the degrees below your target the room must fall before aux heat is used. Honeywell’s T6 Pro has the same setting as backup heat droop.

The heat pump has been trying for a while. Google’s name for this setting is the aux upstage timer: the longest the system tries to reach the set temperature before aux heat is allowed. Honeywell’s T6 Pro calls it the upstage timer for backup heat.

Check the settings

On a Nest: Heat Pump Balance. Google says Max Comfort is the default. It gives a higher aux lockout temperature, so aux heat comes on sooner and more often. Max Savings sets a lower lockout, so aux heat comes on later and less often. Balanced sits between the two.

Nest with the balance off. Google says the thermostat then uses aux heat based on a lockout temperature you set in the Equipment menu.

Your system still decides. Google says some systems need aux heat to keep heating even on Max Savings, and others rarely need it at all.

On a Honeywell T6 Pro. Its installer settings include the backup heat droop, the upstage timer, the compressor lockout or balance point, and an outdoor lockout for backup heat. These sit in the installer menu, so change them with care or leave them to your installer: the T6 Pro’s wiring and setup.

Know which one you are seeing

Aux. The thermostat brings the backup heat in on its own to help the heat pump, and turns it off again.

Em heat. Google says it is for when the primary heating is not working or not keeping your home warm enough, typically only in very cold weather, and it stays on until you turn it off.

How to tell if you have it. Google lists a switch or button labeled Emergency on your old thermostat, or a wire in a terminal labeled E.

Left on by mistake. If the display says Em Heat and you did not mean it, switch back to Heat: why an energy bill jumps.

Look at the heat pump

Aux in mild weather, for hours. The droop and timer settings bring aux heat in when the heat pump cannot keep up. If it is running on a mild day, look at the outdoor unit: is it running, and is it clear of snow, ice and leaves?

The outdoor unit is not running at all. Then the backup heat is doing all the work: why the outside unit is not running.

It never stops running. A heat pump running constantly in deep cold can be normal: when constant running is and is not a fault.