Heat Pump Backup Heat Explained
Updated 2026-08-15 · 7 min read
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Almost every ducted heat pump has a backup heat source. Understanding what it is, when it runs, and how big it should be explains most heat pump complaints — high winter bills, the "AUX" light, and why the project needed an electrical service upgrade.
What is backup heat?
Two forms:
Electric resistance strip heat. Heating elements in the air handler, essentially a very large toaster. Cheap hardware, universally compatible, and the default in most installations.
A fossil-fuel furnace, in a dual-fuel setup. The heat pump handles mild weather; the furnace takes over when it's cold.
Ductless mini splits generally have no backup heat at all — one of the reasons they sidestep the electrical problems below.
Why it exists
A heat pump's output falls as it gets colder, while your home's heat loss rises. Those two curves cross at the balance point.
- Above the balance point: heat pump alone
- Below it: backup makes up the difference
Backup also covers two other situations: defrost cycles, when the outdoor coil is being cleared of ice and isn't producing heat, and equipment failure.
Auxiliary vs emergency — not the same thing
Auxiliary (AUX) runs automatically, alongside the heat pump, when the thermostat determines the heat pump alone isn't keeping up. Both run together. This is normal operation.
Emergency (EM) is a manual mode you select. It shuts the heat pump off entirely and runs backup only.
The distinction matters because emergency heat is intended for a broken heat pump, not for cold weather. Switching to it because it's cold turns off your efficient heat source and runs only the expensive one.
If your thermostat has an EM setting, leave it alone unless the outdoor unit has genuinely failed.
The cost difference
Electric resistance heat has a COP of 1.0 — one unit of energy in, one unit of heat out, always. A heat pump delivers 2 to 4 units of heat per unit of energy, even in cold weather.
| Source | COP | Relative running cost |
|---|---|---|
| Heat pump at 47°F | 3.5–4.5 | 1× (baseline) |
| Heat pump at 17°F | 2.0–2.8 | ~1.5× |
| Heat pump at 5°F | 1.7–2.3 | ~2× |
| Electric resistance | 1.0 | ~3–4× |
Every hour on strip heat costs several times what the same hour on the heat pump would. This is why a high balance point is expensive — not because backup is broken, but because it's running.
Estimate your own numbers with the electricity cost calculator.
The electrical problem
Strip heaters are enormous electrical loads:
| Strip heater | Amps at 240 V |
|---|---|
| 5 kW | ~21 A |
| 10 kW | ~42 A |
| 15 kW | ~62 A |
| 20 kW | ~83 A |
A 15 kW heater is 62 amps by itself — well over half of a 100-amp service. It's counted in the heating side of your load calculation, and it is the single most common reason a heat pump project requires a service upgrade.
The compressor is rarely the problem. The strip heater usually is.
See panel upgrade for a heat pump and run the numbers with the home electrical load calculator.
Sizing it properly
Backup should cover the gap at your design temperature — the heat pump's output there, subtracted from the home's heat loss there. That's a load-calculation output, not a default.
Common practice is to fit the largest heater the air handler accepts. It costs the contractor almost nothing, prevents comfort callbacks, and passes the cost to you twice: once in the service upgrade, and again every winter in running cost.
Three ways to reduce it:
A cold-climate heat pump. Maintains far more capacity at low temperature, which lowers the balance point and shrinks the gap. See cold climate heat pumps.
Dual fuel. Keep the existing gas furnace as backup and install no strip heat at all. The electrical load becomes just the compressor and blower.
Envelope improvements. Insulation and air sealing reduce heat loss, which lowers the balance point without buying equipment. Often the cheapest lever available.
Why yours might be running constantly
If AUX is on more than you'd expect:
- Outdoor temperature below the balance point. Expected behavior, not a fault.
- Large thermostat setbacks. Recovering 6°F quickly triggers backup because the heat pump alone recovers slowly. See heat pump thermostat settings.
- Dirty filter or restricted ducts. Reduced airflow cuts output and triggers backup. Check the filter first — it's free.
- Low refrigerant charge. A real fault; needs a technician.
- Undersized or conventional heat pump. A high balance point means backup runs often by design.
- Defrost cycles. Brief and normal. See heat pump defrost cycle.
A short AUX indication during a cold snap or a defrost is fine. Constant AUX in 40°F weather is a problem worth investigating.
Where to go next
More in our heating and cooling guides.
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