Problem
Aux heat running all the time?
Auxiliary heat is the backup — usually electric resistance — that helps a heat pump in very cold weather or during defrost. If the thermostat shows AUX most of the time in mild weather, or the winter bill jumps with no change in comfort, the heat pump may be underperforming or the controls may be set up to call backup heat too early.
Not dangerous, but expensive every day it continues. Book a visit.
What usually causes this
These are the common possibilities, not a diagnosis. A page cannot tell which one it is — that takes a technician testing the system.
- Thermostat configuration
- Thermostats not set up for the specific heat pump, or with aggressive recovery settings, call backup heat more than needed.
- Large temperature setbacks
- Turning the heat down a lot overnight and back up in the morning can bring on backup heat to catch up.
- Heat pump losing capacity
- A low refrigerant charge, dirty coils or an airflow problem makes the heat pump fall behind, so backup heat covers the gap.
- Outdoor temperature sensor or lockout settings
- Where fitted, these decide when backup heat is allowed. Wrong settings let it run in mild weather.
- Defrost problems
- Backup heat usually runs during defrost. Excessive defrost means excessive backup heat.
What you can safely check yourself
- Check the thermostat is in Heat, not Emergency Heat.
- Try smaller temperature setbacks — a steady setting is often more efficient with a heat pump.
- Replace the filter and clear the outdoor unit.
- Compare this winter's bills with last year's for the same months to confirm the change.
Leave these alone
- · Thermostat installer or wiring settings, unless you're confident with them.
- · The indoor unit's electric heat strips and their wiring.
Call rather than book online if:
- · You smell burning from the indoor unit when backup heat runs
What we check when we get there
- 1The technician checks the heat pump's own performance — charge, airflow and defrost — then reviews thermostat configuration and backup-heat staging so backup heat runs only when it should.
What it is likely to cost
Sometimes the fix is a configuration change during the diagnostic visit. If the heat pump itself is underperforming, the repair is priced before any work.
- Diagnostic
- $89
- Refrigerant leak / recharge
- From $499
- Smart thermostat installation
- From $299
Prices are starting points for standard work in the Savannah area. Final price depends on the system and site, and is confirmed before work begins. Permits and code-required upgrades may be additional.
Questions people ask about this
- What's the difference between AUX and EM heat?
- AUX means the thermostat has automatically brought in backup heat to help the heat pump. EM (emergency) heat means you've switched the heat pump off and are running only on backup heat — useful if the heat pump fails, but expensive.
- Is it normal to see AUX heat sometimes?
- Yes — on the coldest mornings, during defrost, and when you raise the set point a lot. Seeing it most of the time in mild weather isn't.
Where to go next
The service that fixes this
Heat Pump Service & Installation
Heating and cooling from one system — well suited to our climate, and unforgiving of a bad install.
Heat PumpsRelated symptoms
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Work out the decision
Get it looked at
We cover Savannah, the Coastal Empire and the South Carolina Lowcountry. Tell us what the system is doing — you do not need to know the cause. The diagnostic price is given to you before the visit, and the repair price before any work starts.