Your thermostat says “auxiliary heat” because your heat pump has decided it needs backup heat to help reach or maintain the temperature you set.

What “Auxiliary Heat” Actually Means

When you see “Aux Heat” or “Auxiliary Heat” on the thermostat, it usually means:

  • You have a heat pump system, not just a regular furnace.
  • The heat pump alone isn’t keeping up, so a secondary heat source (often electric resistance heat strips or a gas backup) has turned on automatically.
  • This mode is built in and is normal in cold weather , especially around or below roughly 35–40°F.

Think of it as your heat pump’s “extra muscle” for really cold or high‑demand moments.

Common Reasons It Pops On

Your thermostat may show auxiliary heat in a few typical situations:

  1. Very cold outside
    • When outdoor temperatures drop near or below freezing, your heat pump struggles to pull enough heat from the air.
 * The thermostat calls for aux heat so your home doesn’t cool down.
  1. Big jump in thermostat setting
    • If you bump the thermostat up several degrees at once (for example, from 65°F to 72°F), many systems bring on aux heat to speed things up.
  1. Defrost cycle on the outdoor unit
    • In icy conditions, the outdoor unit sometimes needs to defrost.
    • During defrost, aux heat may run so your vents don’t blow cold air indoors.
  1. System issues (if aux stays on a long time)
    • If your thermostat shows aux heat for hours even on mild days, it can be a warning sign: low refrigerant, outdoor fan problems, or heat pump not working efficiently.
 * Constant aux heat is also much more expensive to run than the normal heat pump mode.

Is Auxiliary Heat Bad?

Not inherently.

  • Normal/expected : Aux heat cycling on during cold snaps, big temp changes, or brief defrost cycles is exactly how the system is designed.
  • Potential problem : If you see “Aux Heat” on most of the time, your bills jump, or the house still feels cold, then something may be wrong with the heat pump or thermostat settings.

What You Can Do About It

Here are practical steps you can take:

  1. Give it smaller temperature changes
    • Avoid turning the thermostat up by 5–10 degrees at once.
    • Use a smaller bump (1–2 degrees at a time) or a smart/programmed schedule, which reduces how often aux heat kicks on.
  1. Improve home efficiency
    • Seal drafts, close windows fully, and use proper insulation and weatherstripping so the heat pump isn’t constantly “fighting” cold air leaks.
  1. Check thermostat settings
    • Some thermostats let you adjust how aggressively they call for aux heat (like how many degrees “behind” they allow before switching).
 * Make sure you’re not accidentally using _emergency_ heat, which is a different, all‑backup mode and even less efficient.
  1. Schedule maintenance if it’s always on
    • If aux heat stays on for long stretches, especially when it’s not extremely cold outside, an HVAC tech should inspect the heat pump (refrigerant level, outdoor fan, defrost cycle, and heat strips).

Quick Example Scenario

You wake up on a 25°F morning and nudge the thermostat up from 66°F to 72°F.
The house is several degrees below the new set point and the outside air is very cold, so the heat pump alone can’t catch up fast enough.
Your thermostat shows “Aux Heat” while the backup heater runs to help warm the house, then returns to normal once you’re close to the set temperature.

Bottom line: your thermostat says “auxiliary heat” because your heat pump is calling in backup heat to keep you comfortable when conditions are too tough for it alone. It’s normal in short bursts, but expensive and possibly a sign of trouble if it runs constantly.

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