On a thermostat, “aux heat” (auxiliary heat) means your heat pump has turned on a backup heat source to help it keep your home warm.

What “aux heat” actually means

  • Aux heat is supplemental or backup heat that works together with your heat pump when the heat pump alone can’t keep up.
  • The backup source is often electric heat strips in the air handler, but in some systems it can be gas or another secondary heater.
  • Seeing “Aux Heat” or “Aux Heat On” usually means the system is functioning as designed, not that something is broken.

Think of it as your heating system saying: “It’s really cold, I need a little help.”

When aux heat turns on

Aux heat is triggered automatically by your thermostat in a few common situations.

  • Very cold weather:
    When outdoor temperatures drop low enough that the heat pump cannot pull in enough heat efficiently (often around just above freezing or below about 40°F, depending on the system).
  • Large thermostat jumps:
    If you suddenly raise the set temperature by several degrees (often more than 3–4°F), the thermostat may bring on aux heat so the room warms up faster.
  • Defrost mode:
    When the heat pump defrosts outdoor coils (it briefly stops heating), aux heat turns on to keep indoor temperatures comfortable while that happens.

Aux heat vs. emergency heat

People often mix these up, but they are different modes.

  • Aux heat:
    • Automatic, controlled by the thermostat.
* Heat pump still runs, and the backup heat _assists_ it.

* Used in normal cold-weather operation or when demand is high.
  • Emergency heat (EM heat):
    • Manually selected on the thermostat.
* Heat pump is turned off; only the backup heat runs.

* Intended for true problems, like a broken or frozen heat pump, until repairs are done.

Using emergency heat for long periods can be very expensive because it often relies solely on high‑power electric resistance heat.

Is aux heat bad or expensive?

Aux heat is normal, but you don’t want it running constantly.

  • It uses more energy than the heat pump alone, especially if it’s electric heat strips.
  • Short, occasional aux‑heat cycles in cold snaps are expected and usually nothing to worry about.
  • If “Aux Heat” is on a lot, even when it’s not that cold, it might indicate:
    • A struggling or undersized heat pump
    • A maintenance problem (low refrigerant, dirty coils, airflow issues)
    • Thermostat settings that cause big temperature swings

Simple tips to avoid overusing aux heat

You can usually reduce aux‑heat run time with a few habits.

  1. Make smaller temperature changes
    • Increase your thermostat by 1–2°F at a time instead of big jumps.
 * Or use a schedule that slowly warms the house before you wake up.
  1. Keep the system maintained
    • Replace filters regularly, keep outdoor units clear of debris, and have periodic professional checkups.
  1. Improve home insulation
    • Better sealing and insulation means your heat pump doesn’t have to work as hard, so aux heat kicks in less often.

Mini FAQ and forum-style notes

“My thermostat says AUX HEAT and my bill is huge. Is something wrong?”
Often, it just means your system has been relying heavily on the backup heater during a cold period, which is more expensive. If it happens when weather is mild, it’s worth having a technician check the heat pump.

“Can I turn aux heat off?”
You usually cannot (and shouldn’t) disable it entirely in a normal heat‑pump setup, because it’s part of how the system is designed to protect comfort in low temperatures. A pro can fine‑tune settings if it’s overactive.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.