What Auxiliary Heat Means on Your Thermostat

Your thermostat says AUX or EM HEAT and you want to know if that is bad.

Short answer. AUX in single digits, fine. AUX in the thirties, something is wrong. EM heat, you turned that on yourself and you should turn it back off.

What the backup heat actually is

A heat pump moves heat from outside to inside. The colder it gets out there, the less heat there is to move, so capacity drops as the temperature falls. At some point the heat pump alone cannot keep up with what the house is losing.

That is where auxiliary heat comes in. On most systems in this area it is electric resistance heat, a set of coils in the air handler, same idea as a toaster. On a hybrid system it might be your existing furnace or boiler instead.

Electric resistance heat is one hundred percent efficient, which sounds great and is actually the problem. A heat pump running normally delivers two to three units of heat for every unit of electricity, because it is moving heat rather than making it. Resistance heat delivers exactly one. So every hour on aux costs you roughly two to three times what the same hour on the heat pump would.

It is not broken. It is a backup, and backups are supposed to be expensive and rarely used.

When AUX is normal

• Deep cold. Below the balance point of your system, which depends on the equipment and how tight your house is. Single digits and below, expect it.
• Recovery from a setback. If you drop the thermostat eight degrees overnight, morning recovery will pull in aux to catch up fast. This is why deep setbacks do not save money on a heat pump the way they do on a furnace.
• Defrost. When the outdoor unit runs a defrost cycle it is briefly taking heat out of the house. Aux kicks in for those few minutes so you do not feel a cold draft. Completely normal.

When AUX is a problem

It runs in mild weather. Thirties and forties should be easy for a heat pump. If aux is engaging there, something is off.

It runs constantly all winter. Your bill will tell you before your thermostat does. A jump of hundreds of dollars over the prior winter with the same weather is the signal.

The house never gets to setpoint anyway. Aux running and the room still cold means you have a capacity problem on top of it.

Common causes, roughly in the order I find them:

• Filter is filthy and airflow is choked
• Outdoor unit is iced, buried in snow, or blocked
• Refrigerant charge is low, which means a leak
• Thermostat is not set up right for a heat pump, which happens a lot after somebody swaps in a smart thermostat
• Outdoor sensor failed, so the control logic is guessing
• System was undersized for the house, or the house lost insulation performance

That thermostat one is worth calling out. A heat pump needs a thermostat that understands heat pumps, wired to the right terminals, with the right lockout settings. Plenty of smart thermostats get installed with aux configured to engage far too eagerly. I have fixed a few thousand dollar winters with fifteen minutes of setup.

Emergency heat is different

EM HEAT is a mode you select. It shuts the heat pump off completely and runs on backup only.

It is there for when the heat pump has actually failed, the outdoor unit is damaged, or something is making a noise that should not be. It is not a boost button for a cold day. Running the whole winter on EM heat is the most expensive way to heat a house in New Hampshire, and I have walked into houses doing it because somebody flipped it in December and forgot.

If you are in EM heat and you did not put it there deliberately for a reason, switch it back to HEAT.

What to do

Check your filter, clear the outdoor unit, and look at what temperature aux starts engaging at. If it is coming on well above the teens, or your bill jumped without a matching cold winter, call us. Usually it is airflow or a control setting, and both are cheap fixes compared to a season of resistance heat.