Heat Pump Backup Heat Explained: Auxiliary, Emergency and Dual-Fuel Options

Heat pump backup heat is a second heat source, usually electric resistance coils in the air handler or an existing gas furnace, that supplements or replaces the heat pump when it cannot keep up on its own. In Ontario, almost every ducted heat pump has some form of backup, because even cold-climate models lose capacity as outdoor temperatures fall and pause briefly to defrost.

Backup heat is a safety net, not a problem. The goal is to make sure it runs only when it is genuinely needed, because electric backup is much more expensive per unit of heat than the heat pump.

Why do heat pumps need backup heat in Ontario?

Cold-climate air-source heat pumps can operate down to about −25 °C to −30 °C, but their heating output and efficiency drop as it gets colder. A heat pump that delivers its full rated capacity at 8 °C may deliver noticeably less at −20 °C, just as your home's heat loss is at its highest.

Ontario heating design temperatures (the cold extremes systems are sized for) are roughly −18 °C to −20 °C in Toronto, −25 °C in Ottawa, −28 °C in Sudbury and −33 °C in Thunder Bay. If the heat pump's capacity at those temperatures is less than the home's heat loss, backup fills the gap.

Backup also helps:

During defrost. The outdoor unit periodically reverses to melt frost. Backup can temper the air so cool air does not blow from the vents.
During a failure. If the outdoor unit stops, backup keeps the house warm until a technician arrives.
During recovery. After a long setback or power outage, backup can bring the home up to temperature faster.

Auxiliary heat vs emergency heat: what is the difference?

Term	What happens	Who turns it on	When you see it
Auxiliary heat ("aux")	Backup runs together with the heat pump	Thermostat or control board, automatically	Very cold weather, defrost, large temperature increases
Emergency heat ("em heat")	Heat pump is locked out; backup runs alone	You, manually	Outdoor unit is broken, iced in or being serviced
Dual-fuel changeover	Furnace takes over and the heat pump stops	Control, at a set outdoor temperature	Below the chosen balance or economic point

Seeing "aux" on your thermostat during a cold snap is normal. Seeing it constantly on mild days is a sign that something is set up incorrectly or the heat pump is not working properly. Emergency heat should be used only when needed, because running backup alone in normal conditions wastes energy.

Which types of backup heat are common?

Electric resistance strips. Coils in the air handler, typically 5–20 kW depending on home size. Simple, cheap to install and reliable. The downside is operating cost and electrical load; a 15 kW strip heater may need a dedicated 60–80 A circuit and can push an older 100 A service to its limit. See our panel upgrade guide.

Gas furnace (hybrid or dual-fuel). The heat pump sits on top of a furnace and the controls switch between them. In gas-heated Ontario homes this is common, because it reuses the furnace and keeps backup costs low. A changeover point somewhere around −5 °C to −10 °C is typical, depending on prices. Our hybrid and dual-fuel guide explains how to set it.

Existing baseboards or a boiler. With ductless mini-splits, many homes keep electric baseboards in some rooms as backup. They come on only when the mini-splits cannot hold the set temperature.

Wood or pellet stove. Some rural homes use these as backup. They need a WETT-certified installation and regular chimney maintenance, and they do not respond automatically.

What does heat pump backup heat cost to run?

The fairest comparison is the cost per kWh of heat delivered into the home. The table uses example prices: electricity at an all-in 14¢/kWh after the Ontario Electricity Rebate (a typical Toronto-area home pays roughly 15–19¢ before the rebate), and natural gas at 45¢/m³ all-in variable cost. Your prices will differ.

Heat source	Efficiency	Approx. cost per kWh of heat
Heat pump at about 5 °C	COP ~3.0	~4.7¢
Heat pump at about −15 °C	COP ~2.0	~7.0¢
Heat pump at about −25 °C	COP ~1.5	~9.3¢
Gas furnace, 95% AFUE	95%	~4.6¢
Electric resistance backup	COP 1.0	~14¢

Two points stand out. Electric strip heat costs roughly two to three times more per unit of heat than the heat pump on all but the coldest days. And since the federal carbon charge on natural gas was removed on 1 April 2025, gas backup in a dual-fuel system is competitive with the heat pump in cold weather, which is why many gas-heated homes switch to the furnace below a certain temperature. Use our heat pump vs furnace calculator to plug in your own prices.

In a well-designed electric-backup system in southern Ontario, strips might supply only a small fraction of annual heat, often a few percent to perhaps 10–15% depending on sizing and climate. That is an estimate; a heat pump sized well below the home's heat loss, or a home in Sudbury or Thunder Bay, will lean on backup more.

How should backup heat be sized and controlled?

Good design keeps backup available but rarely used. A competent contractor will:

Do a room-by-room load calculation to find the home's heat loss at your design temperature. Our heat pump sizing calculator gives a rough starting estimate.
Choose a heat pump using its cold-temperature capacity, not just its nominal tonnage.
Size backup to cover the shortfall plus a margin for failure, without massively oversizing it.
Stage the backup so it comes on in steps (for example, 5 kW, then 10 kW) rather than all at once.
Set an outdoor lockout so electric strips cannot run above a chosen temperature, often somewhere around −5 °C to −10 °C. For dual fuel, set the changeover point based on your gas and electricity prices.
Use a thermostat designed for heat pumps that understands staging and lockouts. See our smart thermostats guide.

Tip: With a heat pump, avoid overnight setbacks of more than 1–2 °C unless your thermostat uses "smart recovery." A deep setback can make the system call for backup heat in the morning, wiping out the savings.

Signs backup heat is running too much

Electricity bills much higher than the contractor estimated, especially in mild months.
"Aux" showing on the thermostat when it is above about 0 °C outside.
The outdoor unit is iced over, not running, or the fan is not spinning.
The thermostat is set to emergency heat by mistake.

If you notice these, check the thermostat mode first, then make sure the outdoor unit is clear of snow and ice (see our winter care guide) before calling your contractor.

Frequently asked questions

Can I install a heat pump with no backup heat?

It is possible with a cold-climate unit sized to cover your full heat loss at design temperature, particularly with ductless systems in well-insulated homes. Most installers still recommend some backup for defrost and equipment failure, especially outside southern Ontario.

Should I keep my gas furnace as backup?

If it is in good condition, keeping it as a dual-fuel backup is often a sensible, lower-cost option. If it is near the end of its life, compare an all-electric system with strips against a new furnace paired with a heat pump.

Does backup heat affect heat pump rebates?

Rebate rules focus on the heat pump, the contractor and pre-approval. According to Save on Energy, Home Renovation Savings rebates differ for gas-heated homes and homes with other primary heat. Confirm current rules on saveonenergy.ca before work begins.

Why does my heat pump blow cool air sometimes?

During defrost, or when supply air is around 30–35 °C, the air can feel cooler than furnace air even though it is still heating the home. Backup heat may temper the air during defrost; constant cool air is worth a service call.