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Heat Pump Aux Heat: What It Means and When to Worry

If you have a heat pump, you've probably seen "Aux" or "Aux heat" on the thermostat on a cold morning. Most of the time it's doing its job. Sometimes it's a sign the heat pump isn't carrying its share, and that shows up on your electric bill. Here's how to tell the difference.

Updated October 14, 2026 · HVAC Doctor Heating & Air, Lexington, KY

What aux heat is

Aux (auxiliary) heat is your heat pump's backup. In most all-electric homes it's a set of electric heat strips in the indoor air handler. In a dual fuel system, the backup is a gas furnace. The thermostat turns the backup on automatically when the heat pump alone can't keep up or isn't heating for a few minutes.

Electric heat strips work like a big toaster. They make heat reliably, but they use much more electricity than the heat pump does for the same amount of heat. That's why aux heat running all day in mild weather is worth checking.

When aux heat is normal

  • During defrost. In cold, damp weather the heat pump periodically reverses for a few minutes to melt frost off the outdoor coil. The backup heat keeps the air from feeling cold while it does.
  • On the coldest days. A heat pump's output drops as the temperature outside falls. On the coldest nights, the backup fills the gap.
  • When you raise the thermostat several degrees at once. Many thermostats call for backup heat to close a big gap quickly.
  • Coming back from a setback. If the thermostat drops the temperature overnight or while you're away, the morning recovery can bring on aux heat.

Aux heat vs. emergency heat

Aux heat comes on by itself and works alongside the heat pump. Emergency heat ("Em Heat") is a setting you choose by hand. It shuts the heat pump off and heats the house with the backup alone. Use it only when the heat pump is broken, for example if the outdoor unit is iced solid, making a bad noise or not running, and call for service. Switch back once it's repaired, because running on emergency heat usually costs more.

Signs something is wrong

Call for a check if you notice:

  • Aux heat on for most of the day when it's in the 40s or 50s outside.
  • An electric bill that jumps well beyond what the weather explains.
  • The outdoor unit not running while the indoor fan blows.
  • Ice that covers the outdoor coil and stays after a defrost cycle.
  • A house that keeps losing temperature even with aux heat on.

How to keep aux heat from running more than it needs to

  • Change the temperature a degree or two at a time instead of making big jumps.
  • Use a thermostat built for heat pumps, with its smart or adaptive recovery feature turned on, so it warms the house gradually.
  • Keep the outdoor unit clear of snow, leaves and anything stacked against it.
  • Replace the filter regularly. Weak airflow makes the heat pump work harder.
  • Have the thermostat set up for a heat pump. Settings like the aux heat lockout temperature decide when the backup can run. See thermostat installation.

How HVAC Doctor can help

Call or text (859) 684-3259. One of the owners, Ahmad or Ammar, answers 24/7, and nights, weekends and holidays cost the same as a weekday. Mention our online special and the diagnostic is $39 (regularly $79), credited to the repair if you go ahead, and you get a firm price before any work starts. Most Lexington calls are same-day. We'll check the heat pump, the defrost controls, the backup heat and the thermostat setup, and tell you whether it's working the way it should.

Common questions

Is it bad if my heat pump is running on aux heat?

Not by itself. Aux heat is normal during defrost, on very cold days and after you raise the thermostat a few degrees. It's worth a check if aux heat runs most of the day in mild weather, or your electric bill jumps more than the weather explains.

What's the difference between aux heat and emergency heat?

Aux heat comes on automatically and works alongside the heat pump. Emergency heat is a setting you choose by hand that shuts the heat pump off and heats with the backup only. Use emergency heat only when the heat pump is broken, then call for service.

Why does aux heat come on when I turn up the thermostat?

Many thermostats bring on backup heat when the room is several degrees below the setting, so the house warms up faster. Raising the temperature a degree or two at a time usually keeps the heat pump doing the work by itself.

At what temperature does aux heat come on?

It depends on the system and how the thermostat is set up. Some thermostats use the outdoor temperature, some use how far the room is from the setting, and some use both. We can check your settings and adjust them for your heat pump.

Does a heat pump work in a Kentucky winter?

Yes. Central Kentucky winters spend a lot of time in the 30s and 40s, where heat pumps work well. On the coldest nights the backup heat helps out, which is what it's for.