Aux heat is backup heat that switches on when the heat pump alone can’t keep up. Emergency heat is a setting you choose when the heat pump itself has failed. On most Tucson-basin winter mornings a working heat pump carries the load alone, but you’ll still see “aux” on the thermostat now and then. Climate Pros Tucson explains when that’s normal and matches you with a licensed pro when it isn’t.

What is aux heat on a heat pump?

Many heat pumps include electric resistance heating elements, often called strip heat, that add heat when the pump alone lags. The thermostat turns them on automatically. A short run of aux heat on a cold start is not a fault.

What is emergency heat?

Emergency heat is a manual setting that tells the system to use only the backup heat and skip the heat pump. Use it if the outdoor unit has stopped working and you need warmth until a pro can look at it. Expect a higher bill while it’s on, since all the heat comes from the backup. Switch back to regular heat once the repair is done. It isn’t meant to be an everyday setting.

What triggers aux heat in Tucson?

Tucson’s basin has mild winters. At the airport, the 1991-2020 normals show fewer than 12 nights a year at or below 32F, and the City of Tucson uses a 35F winter design temperature. Aux heat shows up when you raise the setpoint sharply, on a cold morning, or during defrost. A large setback and a big jump back up can push the system into aux because the pump alone would take longer. Holding a steadier setpoint is the usual fix.

Mount Lemmon and Summerhaven are different. NOAA lists about 53 nights a year at or below 32F there, and backup heat is a question for the pro.

Does aux heat cost more on TEP?

Yes. Electric resistance heat uses more electricity than the heat pump does for the same warmth, so long aux runs add to the bill. On TEP’s Time-of-Use plan they cost the most inside the winter on-peak windows, 6 to 9 a.m. and 6 to 9 p.m. on weekdays, October to April (tep.com/time-of-use). Scheduling around those hours is its own topic: heat pump mornings on TEP’s winter peak covers pre-heating and setback limits.

How do you read the thermostat?

  • “Aux” or “aux heat” lit: the thermostat called the backup on its own. Brief and occasional is fine.
  • “Em heat” or “emergency” lit: someone chose it. If the outdoor unit works, switch back to regular heat.
  • Aux lit for hours on a mild day: that’s the fault pattern below.
  • No indicator at all: some thermostats don’t show aux. Ask a pro how yours reports it.

Some thermostats also have settings that limit when the backup is allowed to run. Whether yours does, and how it’s set, is a question for a pro; smart thermostats with a heat pump covers the setup side.

When is aux heat a sign of a problem?

If aux runs for long stretches on mild days, the outdoor unit isn’t pulling its weight. Possible causes include a failed part, low refrigerant or a stuck reversing valve. If ice builds on the outdoor unit and stays, that is a repair, not defrost; see heat pump outdoor unit frozen or steaming. For service, see heat pump service across our Tucson service area.

When the backup won’t switch off

Call when aux heat runs for long stretches on mild mornings, or when the outdoor unit won’t run and you’re on emergency heat. Call (520) 353-2520 and we’ll match you with a licensed local pro.