Homeowner Questions · Cold weather and bills
What is auxiliary heat, and why can it raise my hydro bill?
Auxiliary heat is the backup heat built into a heat pump system, often electric resistance strips in the air handler or a furnace in a dual-fuel setup. It is more expensive to run, especially electric strips, because it makes heat one-for-one instead of moving heat the way the heat pump does.
Checked October 26, 2026 · Educational guide, not a quote.
Quick answer
Auxiliary heat is the backup heat built into a heat pump system, often electric resistance strips in the air handler or a furnace in a dual-fuel setup. It is more expensive to run, especially electric strips, because it makes heat one-for-one instead of moving heat the way the heat pump does.
The short version
- Aux heat is backup heat, not a second main system.
- Electric strips are one-for-one resistance heat.
- Large thermostat jumps can trigger aux heat.
- Frequent aux use in mild weather needs checking.
Why electric strips are expensive
Electric resistance heat turns one unit of electricity into one unit of heat. A heat pump can move two or three units of heat for the same electricity in milder winter weather. Every unnecessary strip hour narrows or erases that advantage.
Why aux heat turns on
It may stage in when the heat pump cannot keep up, during defrost, or when the thermostat is raised several degrees at once. Large morning setbacks are a common cause: the system sees a big recovery gap and calls the expensive backup.
Furnace backup works differently
In a dual-fuel system, the furnace can take over below the chosen balance point. That may be the right economics for propane or gas, but the switchover still needs to be set deliberately rather than left at a random default.
What to do next
Avoid large thermostat setbacks.
Ask what outdoor temperature stages the backup.
Watch for aux heat running in mild weather.
Read the full guide
This answer connects to Electric vs Propane Backup, where the full topic is covered with the current Ontario figures and the next checker.
Keep aux heat as backup, not the main system
The thermostat setting can matter as much as the equipment. Ask how your backup is staged before winter.
Compare backup optionsQuestions people also ask
What is auxiliary heat, and why can it raise my hydro bill?
Auxiliary heat is the backup heat built into a heat pump system, often electric resistance strips in the air handler or a furnace in a dual-fuel setup. It is more expensive to run, especially electric strips, because it makes heat one-for-one instead of moving heat the way the heat pump does.
Is any auxiliary heat use normal?
Yes. During defrost and extreme cold, brief auxiliary operation can be part of normal design.
Should I use a night setback with a heat pump?
Usually a small setback or none. Big setbacks can trigger expensive recovery heat and cost more than they save.