Why Does My Heat Pump's Aux Heat Keep Coming On?

Heat Pumps The Leonard Splaine Team December 7, 2021

Illustration of a frosted heat pump in snow with backup heat glowing

Quick answer

Auxiliary heat is your heat pump’s electric backup, and some of the time it’s supposed to turn on. It kicks in when it’s genuinely cold outside, when you bump the thermostat up several degrees at once, or when the outdoor unit runs a quick defrost cycle. That’s all normal. It’s only a problem when aux heat runs constantly in mild weather, ignores the thermostat, or spikes your electric bill, and those are the cases worth a service call.

Here’s how to tell the difference.

What “aux heat” actually is

A heat pump heats your home by pulling warmth out of the outdoor air, which is remarkably efficient right up until the air gets too cold to give up much heat. Auxiliary heat (labeled AUX on most thermostats) is a bank of electric resistance heating strips that switch on to help out when the heat pump alone can’t keep up.

The catch: those strips make heat the expensive way, roughly the same way a space heater does. When aux heat runs, you’re burning three to four times the electricity you’d use running the heat pump by itself. A little of that is fine and expected. A lot of it, for no good reason, is money leaking out of your walls.

Aux heat is not the same as “emergency heat” (EM). Aux comes on automatically to supplement the heat pump. Emergency heat is a manual setting that shuts the heat pump off entirely and runs on the strips alone, meant for when the outdoor unit is broken and you’re waiting on a technician. Don’t run EM as your everyday setting.

When aux heat is supposed to turn on

1. It’s below about 35°F outside

This is the big one, especially on a cold Northern Virginia morning. As the outdoor temperature drops toward freezing, there’s simply less heat in the air for the pump to collect, so the system leans on the electric strips to make up the gap. Below roughly 35°F, occasional aux heat is completely normal. At 50 or 60 degrees, it shouldn’t be running at all.

2. You raised the thermostat several degrees at once

Most heat pumps are set to call in aux heat when the room is about 3 degrees or more below the thermostat setting. So if you wake up to a 62°F house and crank the thermostat to 68°, that 6-degree jump triggers aux heat to close the gap faster.

Check it yourself: nudge the thermostat up just one or two degrees at a time instead of a big jump, and aux heat usually stays off. Setting back the thermostat deeply overnight and then blasting it back up in the morning is one of the most common reasons people see AUX every day.

3. The heat pump is defrosting

In cold, damp weather, frost builds up on the outdoor coil. Your heat pump periodically runs a short defrost cycle, briefly reversing itself to melt that frost, and while it does, aux heat takes over indoors so you don’t feel a blast of cold air. Signs of a normal defrost cycle:

Steam or water rising off the outdoor unit
The outdoor fan pausing for a few minutes
On some models, a blinking indicator light

A defrost cycle should last only a few minutes. If your unit seems stuck defrosting for a long stretch, that’s not normal.

Signs your aux heat is actually malfunctioning

Call a professional when you see any of these:

Your electric bill jumps for no clear reason. After a season with your system, you get a feel for a normal bill. A sudden spike often means aux heat is running far more than it should.
Aux heat runs when it’s mild out. If it’s 50°F+ and AUX is on for long stretches, something’s off, a faulty sensor, a wiring issue, or a thermostat that’s misreading the temperature.
Aux heat ignores the thermostat. Turning the setpoint down to room temperature should shut aux heat off promptly. If it keeps running anyway, suspect a stuck relay or bad thermostat.
The house is already too warm and it’s still running. If it’s 78°F inside and aux heat won’t quit, check that the thermostat wasn’t bumped, then call.
The defrost light stays on constantly. Defrost should take minutes, not hours. A unit stuck in defrost needs attention.

Any of these usually traces back to a faulty sensor, a miswired or aging thermostat, or a poor original installation, all things a technician can pin down quickly.

How to keep aux heat from running more than it should

Change your heat pump filter every one to two months, a clogged filter chokes airflow and makes the system reach for backup heat.
Avoid deep overnight setbacks; smaller thermostat adjustments keep aux heat off.
Keep leaves, snow, and debris cleared a couple of feet around the outdoor unit.
Schedule a heating tune-up each fall so weak sensors and low refrigerant get caught before the cold does.
Don’t use emergency heat for everyday heating.

Frequently asked questions

Is it bad if my aux heat comes on every morning? Not necessarily, if you set the thermostat back overnight and jump it up in the morning, aux heat is just doing its job. Try smaller adjustments. If it’s still running midday in mild weather, then it’s worth a look.

Does aux heat mean my heat pump is broken? No. On a cold Northern Virginia day, aux heat coming on is expected. It only points to a problem when it runs constantly in mild weather or won’t respond to the thermostat.

Why is my electric bill so high in winter with a heat pump? Some winter increase is normal, but a big one usually means the electric backup heat is running too often, from cold snaps, deep thermostat setbacks, a dirty filter, or a genuine fault.

Seeing aux heat more than seems right? Leonard Splaine has kept Northern Virginia homes warm for three generations. We’ll check the sensors, thermostat, and refrigerant charge and give you the honest story. Call 703-494-2855 or request heat pump service.