Heat pump emergency heat & aux heat
"Emergency Heat" and "Aux Heat" are two of the most confusing heat pump settings. Here is what each does and when to use it.
Auxiliary heat vs emergency heat
Auxiliary (aux) heat turns on automatically to help the heat pump when it cannot keep up on a cold day — they run together. Emergency heat is a setting you switch on that shuts off the heat pump entirely and runs only the backup (usually electric resistance strips, sometimes a gas furnace). Both use the backup heat source; the difference is automatic-assist vs manual-only.
When to use each
- Aux heat: nothing to do — the thermostat manages it automatically below the balance point.
- Emergency heat: only when the heat pump itself is broken or iced and you need heat until a technician comes. It is a temporary backup, not for daily use.
Why it costs more
Backup heat is usually electric resistance, which is only ~100% efficient versus the heat pump's 250–400%. So running on emergency/aux heat can cost 2–3× more. Minimizing it — with a cold-climate unit, good insulation, and a smart thermostat that limits aux use — is key to low winter bills. See our balance point tool.
Frequently asked questions
What is emergency heat on a heat pump?
A setting that bypasses the heat pump and runs only the backup heat (electric strips or a furnace). Use it temporarily if the heat pump is broken or iced over.
What is the difference between aux heat and emergency heat?
Aux heat assists the heat pump automatically on cold days; emergency heat is a manual setting that shuts the heat pump off and runs backup heat only.
Does emergency heat cost more?
Yes — backup electric resistance heat is far less efficient than the heat pump, so it can cost 2–3× more. Use it only when needed.
Related
Sources & further reading
Educational guide, reviewed against US DOE & ENERGY STAR guidance and updated June 2026. Estimates only — not a substitute for a professional assessment or Manual J load calculation.