what's the difference between auxiliary heat and emergency heat
Auxiliary heat and emergency heat both use the same backup electric heating elements in a heat‑pump system, but they’re triggered and used in very different ways.
Quick Scoop
- Auxiliary (aux) heat turns on automatically when your heat pump needs a little extra help to keep up with the thermostat setting.
- Emergency heat is a manual override you switch to only when the heat pump itself has failed or isn’t producing heat.
What auxiliary heat does
Auxiliary heat is essentially a “second stage” of heating that kicks in when:
- Outdoor temperatures drop and the heat pump can’t pull enough warmth from the air.
- The indoor temperature falls more than a few degrees below your set point (often around 2–3°F), so the system adds electric heat strips to catch up faster.
Once the house warms close to the set temperature, auxiliary heat shuts off automatically and the heat pump resumes doing most of the work.
What emergency heat does
Emergency heat bypasses the heat pump entirely and runs only the backup electric heat strips as the primary source.
You should use it only when:
- The heat pump is not working (no warm air, strange noises, or error codes).
- You need temporary heat while waiting for a technician.
Because electric resistance heat is expensive to run, emergency heat can dramatically raise your electric bill if left on for long.
Side‑by‑side comparison
| Feature | Auxiliary Heat | Emergency Heat |
|---|---|---|
| How it’s activated | Automatic (thermostat triggers it) | Manual (you flip the switch) |
| When it runs | Alongside the heat pump during cold snaps or big temperature swings | Instead of the heat pump, when it fails |
| Energy cost | Moderate spike when it runs | High cost if used long term |
| Is it “normal”? | Yes; expected behavior in cold weather | No; indicates a problem or emergency |
What you should do day‑to‑day
- If your thermostat shows “AUX” or “Aux Heat On” in very cold weather, that’s usually normal —just keep an eye on how often it runs.
- Only flip to “Emergency Heat” if your system isn’t producing heat and you need warmth while arranging service; switch back to “Heat” once the heat pump is fixed.
If either mode is running constantly in mild weather, it can signal a refrigerant issue, dirty coils, or a failing heat pump, and it’s worth having an HVAC tech check it.
Information gathered from public forums or data available on the internet and portrayed here.