In short
Auxiliary heat is the backup heat a heat pump system turns on by itself: electric strips or a gas furnace. AUX on the thermostat during a cold snap near -15 °C or a defrost cycle is normal. AUX at 0 °C and above, or for hours on end, points to a fault, a big thermostat setback or a wrong lockout setting, and it raises the hydro bill.
AUX on the thermostat worries people more than it should, and sometimes less than it should. What is auxiliary heat? It is the second heat source in a heat pump system, held in reserve and switched on by the controls, not by you. In an all-electric home it is a bank of resistance heat strips in the air handler; in a hybrid system it is the gas furnace.
When auxiliary heat is supposed to run
The thermostat or the system's control board brings auxiliary heat on in three situations, and all three are normal.
- Deep cold: the outdoor temperature has dropped below the system's balance point, so the heat pump is running flat out and the house is still losing ground. During a -15 °C cold snap in Burlington or Caledon, AUX cycling on and off is expected.
- Defrost: the outdoor unit briefly runs in cooling mode to melt frost off its coil, and the strips or furnace temper the air so the vents do not blow cold. This lasts a few minutes at a time.
- Recovery: the thermostat is asked to raise the temperature by several degrees at once, for example after a night setback, and calls for help to close the gap quickly.
With electric strips, AUX runs together with the heat pump. In most hybrid systems the furnace and heat pump do not run at the same time, because the furnace's heat would overload the refrigerant coil sitting above it. The controls switch from one to the other at a set outdoor temperature instead.
AUX heat at mild temperatures means something is wrong
Decision rule: if AUX shows when it is around 0 °C or warmer outside, and you have not just turned the thermostat up, treat it as a fault. These are the usual causes, in the order we find them.
- Large thermostat setbacks. Dropping the setpoint several degrees overnight makes the system call the strips every morning; hold a steady temperature or keep the setback small.
- A lockout set wrong. Thermostats have an auxiliary lockout that keeps the strips off above a chosen outdoor temperature, and it is often left unconfigured, especially when no outdoor sensor is connected.
- A heat pump that is not producing: low refrigerant charge, a failed outdoor fan motor, a stuck reversing valve or a defrost fault. The strips quietly heat the house and nobody notices until the bill arrives.
- A stuck heat-strip relay or sequencer that leaves the elements on with no call for them.
- A dirty filter or coil cutting airflow, so the heat pump delivers less than it should.
Our post on heat pump thermostat settings covers the setback and lockout side, and why AUX heat is always on goes through the equipment faults.
What auxiliary heat costs to run, and auxiliary vs emergency heat
Electric strip heat turns one unit of electricity into one unit of heat, a COP of 1. A heat pump in mild weather delivers two to three units of heat for the same electricity. So every hour the strips do work the heat pump could have done costs roughly two to three times as much. A stuck-on AUX shows up as a spike on the Burlington Hydro, Alectra or Hydro One bill before it shows up as a comfort problem.
Emergency heat is a different thing. Auxiliary heat is automatic and works alongside the heat pump; emergency heat (EM HEAT) is a manual thermostat setting that locks the heat pump out and runs the backup alone. Use it only when the outdoor unit has failed or is damaged, and switch back once it is repaired.
When to book heat pump repair for an AUX problem
Call if AUX stays lit for hours in mild weather, if the outdoor unit is not running while the house is heating, or if your bill jumps with no change in the weather. A diagnostic visit is $90–$150 and goes toward the fix. We check the refrigerant charge, defrost operation, the strip sequencer and the thermostat's lockout settings on the same call. Book heat pump repair or get in touch through the contact page.
Questions people ask about Auxiliary heat
Is it bad if my thermostat says AUX heat?
Not by itself. AUX during a cold snap, a defrost cycle or right after you raise the setpoint is the system working as designed. It becomes a problem when it appears in mild weather or stays on for long stretches, because that means the heat pump is not carrying its share and you are heating with the most expensive source in the house.
Should I switch to emergency heat when it gets very cold?
No. A working heat pump still produces heat for less electricity than resistance strips do, and the system adds auxiliary heat automatically when it needs to. Emergency heat shuts the heat pump off entirely. Reserve it for a failed or iced-over outdoor unit while you wait for a technician, then return the thermostat to its normal heat setting.
Does a hybrid heat pump with a gas furnace show AUX heat?
Many thermostats still label the furnace stage AUX. In a hybrid system the controls switch from the heat pump to the furnace below a chosen outdoor temperature instead of running both, so AUX there means the furnace has taken over. Whether that switch point suits your gas and hydro rates is a separate setting, the dual-fuel changeover temperature.