Auxiliary heat is a backup or secondary heating source in a heat pump system that turns on when the main heat pump can’t keep your home warm enough, usually in very cold weather. It’s normal to see “Aux” or “Aux Heat” on your thermostat during cold snaps, but it can cost more to run than your regular heat.

What auxiliary heat means

  • In most homes with a heat pump, auxiliary heat is an extra set of electric heating strips or a backup furnace that helps the main system when outdoor temperatures are very low.
  • Your thermostat will automatically switch to auxiliary heat when the heat pump struggles to reach the set temperature or goes into a defrost cycle, so you don’t feel cold air blowing inside.

When auxiliary heat turns on

  • On many systems, aux heat kicks in when the outside temperature gets near or below freezing and the heat pump alone can’t keep up with demand.
  • It may also come on if the indoor temperature falls several degrees below the thermostat setting, or while the outdoor unit is defrosting to prevent cold air from coming through the vents.

Aux heat vs. emergency heat

  • Auxiliary heat works automatically alongside the heat pump; you don’t have to manually select it.
  • Emergency heat is a manual setting you choose when the heat pump is not working properly, telling the system to bypass the heat pump and rely only on the backup heat source.

Is auxiliary heat bad?

  • Occasional aux heat use in very cold weather is expected and helps maintain comfort.
  • Constant or frequent “Aux Heat” in mild weather can signal issues such as incorrect thermostat settings, an undersized system, or a problem with the heat pump, and may noticeably increase energy bills.

Simple tips to use it wisely

  • Avoid large thermostat jumps; raising the temperature slowly (1–2 degrees at a time) reduces how often aux heat kicks in.
  • Keep filters clean and schedule regular maintenance so the heat pump runs efficiently and needs less backup help.
  • If you see “Aux Heat” constantly when it’s not very cold outside, consider having an HVAC technician check the system.

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