Heat Pump Auxiliary Heat: Why Your Backup Keeps Kicking In (and How to Use Less of It)
The thermostat says AUX, the outdoor unit is running, and the hydro bill is climbing faster than the temperature. Nothing is necessarily broken. Auxiliary heat is your heat pump's backup doing exactly what it was installed to do — the question worth answering is whether it is helping at the right moments, or quietly becoming your main heating system at resistance-heat prices. This guide explains the balance point, the difference between auxiliary and emergency heat, and the settings that keep backup use where it belongs. General information only: for gas, electrical, or refrigerant work, hire a licensed technician.
Why heat pumps need a backup at all
A heat pump moves heat rather than making it, which is why its efficiency — its coefficient of performance, or COP — can run at roughly 2 to 4 in mild weather: two to four units of heat delivered for each unit of electricity used (PickHVAC). As outdoor air gets colder there is less heat to move, so both capacity and COP fall. Cold-climate models hold their capacity far better in deep cold than standard models, but every air-source system eventually reaches a temperature where it cannot carry the house alone. Equipment shutoff points vary by model, commonly somewhere in the −12°C to −25°C range (The Weather Network). Canadian design temperatures put the problem in perspective: roughly −18°C for Toronto, −26°C for Calgary, −30°C for Edmonton (EcoSolar climate data). A mild-climate unit rated to −15°C is a reasonable match for Vancouver and a poor solo act in Edmonton.
The balance point, in plain language
Your home loses heat at a rate that rises as it gets colder outside. Your heat pump supplies heat at a rate that falls as it gets colder. The outdoor temperature where those two lines cross is the balance point. Above it, the heat pump carries the house alone. Below it, the thermostat calls for auxiliary heat to cover the gap — not to replace the heat pump, but to top it up (PickHVAC). Their worked example: a home losing 25,000 BTU/hr at about −9°C, served by a standard 2-ton heat pump delivering roughly 20,000 BTU/hr at that temperature, will run continuously and call for auxiliary heat to supply the missing 5,000 BTU/hr. A cold-climate unit delivering more at the same temperature might need no help at all in those conditions.
Auxiliary heat is not emergency heat
| Auxiliary (AUX) heat | Emergency (EM) heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually, at the thermostat |
| Heat pump's role | Keeps running; aux supplements it | Shut out; backup carries the whole house |
| Typical source | Electric resistance strips or a gas furnace in a dual-fuel system | The same backup, used alone |
| Cost | Higher than heat-pump-only running | The highest-cost mode the system has |
| Correct use | Cold snaps, defrost cycles, recovery from a setback | Heat pump broken, unsafe to run, or unable to hold temperature at all |
The comparison follows Logan Inc.'s auxiliary-versus-emergency explainer, checked 2026-10-14. The expensive mistake is living in EM heat for weeks after a minor fault, or selecting it during a cold snap "to give the heat pump a rest" — the heat pump needs no rest, and resistance backup costs roughly three times as much per unit of heat as the heat pump at a COP of 3. Canadian systems commonly pair the heat pump with an electric coil or a gas furnace as backup, with the thermostat and an outdoor sensor orchestrating the changeover (The Weather Network). Know which backup your house has before judging your bills: gas dual-fuel changeover is an economics decision, while electric-strip auxiliary use is a pure cost to minimise.
Normal aux vs too much aux
Normal: AUX appears during defrost cycles — the system briefly reverses to melt frost off the outdoor coil and uses backup to temper the air, sometimes with visible steam and sizzling sounds for a few minutes (PickHVAC). AUX in the early morning after a night setback, during single-digit cold, or while recovering several degrees is also expected. Too much: AUX running in mild weather, AUX that never switches off, a house that still loses temperature with AUX engaged, ice building on the outdoor unit beyond defrost's ability to clear it, or a sudden bill jump with no weather change to explain it. Those patterns point to low refrigerant, a failing reversing valve, a mis-set lockout, or a thermostat problem — diagnosis territory for a technician, not a settings tweak.
Settings that reduce auxiliary use
- Shrink the setback. Deep night setbacks are a furnace habit. With a heat pump, recovering 4–5°C in the morning can trigger auxiliary heat and erase the night's savings. Use modest setbacks of about 2–3°F (roughly 1–2°C), and enable adaptive (smart) recovery if your thermostat offers it, so recovery starts early and gently instead of calling for backup (PickHVAC).
- Leave the setpoint alone. Repeatedly nudging the thermostat up is a common aux trigger. Pick a temperature — guidance for winter settings commonly starts around 20°C (68°F) — and let the system hold it.
- Check the aux lockout. Many thermostats can lock out auxiliary heat above a set outdoor temperature. Published guidance puts typical aux lockouts around 30–35°F for standard systems and 20–25°F for cold-climate models — but the right value is your system's balance point, so treat those ranges as a conversation to have with your installer, not a number to copy blindly (PickHVAC).
- Use Heat mode, not Auto, in winter. Auto can toggle between heating and cooling on sunny shoulder-season days; Heat mode keeps the system on one job (DIY Home Comfort).
- Keep airflow honest. Clean or replace filters, keep registers open, and clear snow, ice, and debris from the outdoor unit. A starved system calls for backup earlier than a breathing one.
- Reduce the load itself. Every draft sealed and every degree of heat loss removed raises the temperature at which the heat pump can cope alone — the balance point moves in your favour. Insulation work is aux-reduction work.
What success looks like: over a typical winter week, AUX appears mainly on the coldest mornings and during defrost, the heat pump does the bulk of the running, and your electricity use tracks the weather — colder week, higher bill — instead of jumping independently of it.
FAQ
Should a heat pump run constantly in winter? Long runtimes are normal and usually efficient for heat pumps in cold weather; a heat pump running steadily is not the warning sign it would be for a furnace. Constant auxiliary running is the pattern to investigate.
Is it cheaper to switch to emergency heat in a cold snap? No — emergency heat is the backup alone, at the system's highest running cost. It exists for a heat pump that cannot run at all.
Steam is coming off the outdoor unit. Is it broken? Probably not: steam and sizzling during a defrost cycle are described as normal by HVAC guidance sources. Steam that never stops, or ice that keeps growing, is a service call.