Auxiliary heat
Supplementary heating that runs when a heat pump alone cannot meet demand: electric resistance elements in North American ducted systems, or an immersion heater and boiler in United Kingdom hydronic systems. It costs more to run than the heat pump itself, so a change in how much it operates is worth investigating.
Also called aux heat, emergency heat, backup heat
What is auxiliary heat?
Auxiliary heat is supplementary heating that runs when a heat pump cannot meet demand on its own. It is electric resistance in a ducted system, which costs more per unit of heat than the heat pump.
Auxiliary heat: how it works
Below the balance point, or during recovery from a setback, the heat pump cannot deliver everything the building is losing. Auxiliary heat makes up the shortfall. Because resistance heating delivers one unit of heat per unit of electricity while the heat pump delivers several, the same comfort costs several times as much while auxiliary is running, which is why the control logic that brings it in matters.
Where you meet the term
- On a thermostat display, where an auxiliary or emergency heat indicator shows the backup is running.
- In a bill investigation, where a mild winter with high costs points at auxiliary running more than it needs to.
- In a control configuration, where a lockout temperature decides how cold it has to be before auxiliary is allowed.
How to check it
- Watch when the auxiliary indicator comes on. Auxiliary during a deep cold snap is expected. Auxiliary at mild outdoor temperatures points at configuration rather than at weather. Owner check
- Avoid large thermostat setbacks on a heat pump. Recovering several degrees quickly is exactly what brings auxiliary heat in, which can cost more than the setback saved. Owner check
- Check the lockout and staging configuration. Manufacturers provide an outdoor lockout temperature and staging delays. Defaults left in place after a thermostat swap are a recognised cause of excess auxiliary running. Technician
Auxiliary heat: what it is not
- Emergency heat is not the same as auxiliary heat. Emergency heat runs the backup alone with the heat pump locked out, and it is for a broken heat pump rather than for cold weather.
- Auxiliary heat running does not mean the heat pump has failed. It means the demand exceeded what the heat pump was delivering at that moment.
Auxiliary heat questions
What is the difference between auxiliary and emergency heat?
Auxiliary supplements the heat pump automatically when demand exceeds its output. Emergency heat is a setting that locks the heat pump out and runs the backup alone, intended for when the heat pump itself is not working.
Why does my aux heat run so often?
Common configuration reasons are a lockout temperature set too high, aggressive staging after a thermostat replacement, and large thermostat setbacks that force fast recovery. Genuinely cold weather below the balance point is the legitimate reason.
Does the United Kingdom use auxiliary heat the same way?
The idea is the same but the hardware differs. Hydronic heat pumps use an immersion heater in the cylinder, or a boiler in a hybrid arrangement, rather than duct-mounted resistance elements.
Where this definition comes from
Written from government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Heat Pump SystemsConsumer-facing explanation of air source, ductless, and geothermal heat pump operation and efficiency metrics.