AUX vs EM heat in one sentence each
AUX heat is backup heat that the system turns on automatically when the heat pump needs help. EM heat is a manual mode you select that turns off the heat pump and runs backup heat only.
Backup heat is usually electric resistance strips in the air handler, or a gas furnace in a dual-fuel system.
When AUX is normal and when it isn't
| Situation | AUX on? | Normal? |
|---|---|---|
| During defrost | Yes | Normal |
| Very cold weather, house below setpoint | Yes | Normal |
| Big setpoint jump (e.g. 62→70°F) | Yes | Normal, but costly |
| Mild weather, AUX on constantly | Yes | Not normal |
| Outdoor unit off while AUX runs | Yes | Not normal (heat pump failed) |
Why your bill jumps
Electric resistance heat makes 1 unit of heat per unit of electricity. A heat pump often moves 2–3 units of heat per unit of electricity in mild winter weather. When the system leans on strips, bills can rise sharply.
Avoid big setpoint jumps on a heat pump. Smart and heat-pump-aware thermostats ramp up gradually.
Common causes of constant AUX
- Outdoor unit failed or tripped its breaker
- Heat pump undersized for the home
- Low refrigerant reducing capacity
- Thermostat aux lockout set wrong or not set
- Outdoor temperature sensor faulty
- Wiring that energizes W2/AUX with every call
When to use EM heat
- The outdoor unit is damaged, iced solid or making grinding noises.
- A technician has told you not to run the heat pump.
- Switch to EM HEAT, and call for repair.
- Switch back to HEAT once the heat pump is repaired.
Backup heat safety
Electric heat strips draw a lot of current. If the strip breaker trips repeatedly, or you smell burning at the air handler, turn the system off and call. Don't keep resetting the breaker.
Aux heat FAQs
Is it bad to run emergency heat?
It's not harmful to the equipment, but it's expensive to run. Use it as a temporary bridge.
Can I turn off aux heat to save money?
Some thermostats let you set an outdoor temperature above which AUX is locked out. A technician can set this to match your heat pump's capacity.
Next steps
Ready to get it fixed? Call (888) 555-0145 to reach a licensed provider in your area. Related guides:
Busy winter markets: heat pump repair in Maine · heat pump repair in North Carolina · heat pump repair in Virginia · heat pump repair in Washington · heat pump repair in Tennessee · heat pump repair in Georgia · all states.