What Is Auxiliary Heat (aka “Aux Heat”) — and Why Your Thermostat Shows It

If you’ve ever noticed “Aux Heat” on your thermostat and felt a mini-panic, breathe easy. Auxiliary heat is simply the backup heating system your heat pump uses when it can’t keep up — or when it needs to defrost. It’s designed to keep your home comfortable, not to run all winter. ars.com+1

Quick summary: Aux heat helps your heat pump hit the thermostat set point faster. Emergency heat is different; it’s a manual mode used only when the heat pump itself isn’t working.

Aux Heat vs Emergency Heat

• Auxiliary heat (Aux Heat): Automatic supplemental heat that helps your heat pump reach the set temperature. It’s normal in very cold weather or during defrost cycles. ars.com

• Emergency heat (Em Heat): Manual mode that bypasses the heat pump and runs only the backup heat. Use only if the heat pump is broken — it’s more expensive and not intended for regular use. hvac.com

How Aux Heat Works

Your thermostat watches the indoor temperature. If the heat pump can’t get the house up to the setpoint (often when there’s a 2–3°F gap or during defrost), the thermostat energizes the aux heat (electric heat strips or gas backup) to boost output. When the house reaches the set temperature, the aux source turns off. ars.com+1

Common reasons Aux Heat turns on (and quick fixes)

1. Very cold outdoor temps — heat pumps are less efficient in extreme cold. (Fix: accept short aux bursts or consider a cold-climate heat pump upgrade). trane.com

2. Big thermostat setpoint jumps — raising the temp several degrees quickly calls for a boost. (Fix: increase setpoint 1–2°F at a time.) hvac.com

3. Defrost cycles — the outdoor unit occasionally reverses to melt ice; aux supplies indoor heat during that time. (Fix: normal — but long defrosts indicate service needed). ars.com

4. Thermostat wiring or programming issues — incorrect staging or a stuck relay can cause persistent aux. (Fix: have a technician check wiring/staging.)

5. Poor insulation or airflow — heat loss or blocked vents force aux to run longer. (Fix: seal drafts, change filters, check vents.)

Quick diagnostic checklist (do these now)

• Is the thermostat showing “Aux Heat”? → Note the indoor temp and outdoor temp.

• Did you raise the thermostat more than 2°F recently? → If yes, set it back 1–2°F and watch for 30 minutes.

• Are vents warm? → If vents are cold but “Aux” shows, call Paschal (possible fault).

• Is there ice on the outdoor unit? → If yes, the unit may be in defrost mode (normal brief aux).

Thermostat model quick tips (Honeywell / Nest / Ecobee)

• Honeywell: Look for an “Aux Heat” indicator; verify W1 wiring.

• Nest: Check equipment settings for the heat pump and look for “Aux” or “Balance” settings.

• Ecobee: Inspect the Aux/Emerg wiring and stage configuration.
  If you tell me the three thermostat models you see most in your service area, I’ll write the exact model checks to paste under each brand.

When Aux Heat Running Means You Should Call a Pro

Call Paschal if:

• Aux heat stays on for long stretches (hours) or every day.

• Your electric bill suddenly spikes.

• Vents stay cold while the thermostat shows “Aux.”

• Defrost cycles are frequent/long — could indicate failing components. Healthy Home Heating and Cooling

What our tech does in a 20-minute diagnostic:

• Confirm thermostat staging & wiring.

• Inspect the outdoor unit for defrosting and fan operation.

• Test backup heat strips and measure temps.

• Recommend repair or a system upgrade if appropriate.

Why choose Paschal?

We’ve been serving for over 50 years, handling heat pump tune-ups and emergency heating repair with a focus on efficiency and homeowner education. Our techs leave a checklist and show you exactly what they tested.

FAQ (short answers — put these in the FAQ block for schema)

• Is aux heat bad? No — it’s normal sometimes. It’s bad only if it runs a lot or when it shouldn’t. hvac.com

• Should I use emergency heat? Only if the heat pump is broken. Emergency heat bypasses the heat pump and is more expensive. hvac.com

• How can I reduce aux heat use? Avoid big temperature jumps, keep filters clean, seal drafts, and check thermostat settings. hvac.com

Bottom line

Aux heat is a normal safety net — but frequent or long operation signals wasted energy or a fault. Use the video and checklist above, do the quick checks, and call Paschal at 479-502-9229 if it keeps running. We’ll fix the root cause and help you save on winter bills.