When “auxiliary heat” (or “AUX HEAT”) comes on, it means your heat pump is using a built‑in backup heating source to help the main system heat your home, usually because it’s very cold or the house needs to warm up quickly.

What “auxiliary heat” actually is

  • In most modern homes, it’s part of a heat pump system.
  • Auxiliary heat is a secondary or backup heat source that automatically turns on when the heat pump alone can’t keep up with your thermostat setting.
  • It often uses electric resistance strips inside the air handler, or sometimes a gas/oil furnace as backup (called “dual fuel”).

Think of it as the car’s “passing gear” for your heating system: it kicks in for extra power, then should back off once it’s not needed.

Common reasons AUX HEAT turns on

  1. Very cold outside
    • Heat pumps lose efficiency as outdoor temperatures drop; many systems bring on aux heat somewhere around 30–40°F (about −1 to 4°C), depending on design and settings.
 * The thermostat senses the heat pump isn’t raising the indoor temperature fast enough, so it calls for auxiliary heat to help.
  1. Big thermostat changes
    • If you bump the thermostat up several degrees at once, the system may turn on aux heat to reach that new temperature more quickly.
 * This is especially common if the indoor temperature is more than a couple of degrees below the setpoint.
  1. Defrost cycles
    • In cold, humid weather, the outdoor unit can frost over; the system briefly reverses to cooling mode to defrost itself.
 * During that short time, auxiliary heat can come on to keep warm air blowing inside.
  1. System issues or inefficiency
    • If AUX HEAT is on a lot even when it’s not that cold, it can hint at problems: low refrigerant, dirty filters, poorly sized system, or thermostat settings that are too aggressive.
 * A constantly lit AUX indicator over long periods can signal something’s wrong or that your backup heat is doing more work than it should (and your bills will show it).

Is auxiliary heat bad?

  • It’s normal and safe for auxiliary heat to come on in cold weather or after a big setpoint change.
  • The downside: it is usually more expensive to run than the heat pump itself, especially if it’s electric resistance heat.
  • Occasional AUX use in winter is expected; frequent or constant AUX use is a sign to check:
    • Air filter cleanliness
    • Outdoor unit (clear of ice, leaves, and debris)
    • Thermostat programming
    • Whether it’s time for a professional inspection

A simple example:
If it’s 20°F outside and you set your thermostat from 65°F up to 72°F, the heat pump alone might take a long time to get there. The thermostat senses the lag and brings on aux heat so your home warms up faster—your AUX HEAT light comes on, then goes off once you’re near 72°F.

Auxiliary heat vs. emergency heat

These two are easy to mix up, but they’re not the same:

  • Auxiliary heat (AUX)
    • Automatic, controlled by the thermostat.
* Works together with the heat pump when needed.
* Used for normal cold‑weather “help” or defrost support.
  • Emergency heat (EM HEAT)
    • Manually chosen thermostat setting.
* Tells the system to shut off the heat pump and run the backup heat **only** (electric strips or furnace).
* Intended for when the heat pump is broken or severely iced and it’s too cold to be without heat.

A quick way to remember:

AUX = “assist” the heat pump; EM HEAT = “replace” the heat pump.

Simple checks if AUX HEAT is on a lot

If you’re seeing AUX HEAT frequently and are worried about comfort or bills, you can:

  1. Make smaller thermostat changes
    • Raise the temperature 1–2 degrees at a time instead of big jumps to avoid triggering aux heat as often.
  1. Use a steady schedule
    • Programmable or smart thermostats that avoid big swings help keep aux use lower.
  1. Do basic maintenance
    • Change filters regularly, keep outdoor unit clear, and have the system checked yearly.
  1. Call a pro if it seems off
    • If AUX is on when it’s mild outside, or your home still feels cold while bills spike, an HVAC tech can evaluate whether the system is undersized, malfunctioning, or poorly configured.

In one line: When auxiliary heat is on, your system is using a built‑in backup heater to help your heat pump keep the house warm—normal in cold weather, but more expensive if it runs too often.

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