Backup heat for a heat pump: electric, gas or none
A heat pump needs backup heat when, on the coldest days, it cannot deliver the whole heat loss of the house on its own. Whether you need it depends on the model, the size of the house and the design temperature of your area, and a cold-climate unit sized well can sometimes manage without it.
When does a heat pump need backup heat?
Heat output from an air-source heat pump drops as the outdoor temperature falls, while the house loses more heat the colder it gets. At some outdoor temperature the two lines cross, and below that point the pump alone falls short. That crossing is the balance point, explained in the guide to heat pump balance point.
Backup heat fills the gap below the balance point. It also covers a second case that has nothing to do with weather: a defrost cycle, a repair, or a power-electronics fault that takes the heat pump out of service for a few days. In a cold-climate house, that second reason is often the stronger one for keeping some kind of backup.
What types of backup heat are there?
Three arrangements cover most Canadian homes.
- Electric resistance elements. Strips in the air handler or baseboards in the rooms. They are cheap to install and simple, but every unit of heat uses a full unit of electricity, which makes them the most expensive way to heat on a per-kilowatt-hour basis.
- A gas or oil furnace kept in place. The heat pump handles mild and moderately cold weather, and the furnace takes over below a set temperature or when needed. This is the dual-fuel arrangement described in our hybrid heat pump guide.
- Existing boiler or wood stove. A home with a hydronic system or a stove can use it as the second source, as long as the controls are set so that the two systems do not fight each other.
The choice usually follows what the house already has. Adding resistance elements costs little up front, while keeping a working furnace means no new equipment but continued gas service and maintenance.
Can a cold-climate heat pump work without backup?
Sometimes. Cold-climate models are rated to keep delivering useful heat at much lower outdoor temperatures than older units, and a well-insulated, airtight house needs less output on the coldest night. The cold-climate heat pump guide covers how those ratings work and where to read the output at low temperature on the spec sheet.
The decision rests on three checks. First, a room-by-room load calculation gives the heat loss at your local design temperature, which the load calculation guide walks through. Second, the manufacturer's capacity data at that temperature shows whether the unit covers the load. Third, you decide how much risk of a cold house during an outage or fault is acceptable. Many people size the pump for most of the winter and keep a cheaper source for the rare worst days. See also heat pump sizing.
How do controls and costs affect backup use?
Controls decide how often the backup runs. A thermostat that asks for a big temperature jump may switch the elements on to recover quickly, even when the heat pump could have caught up slowly. Smart settings and staging avoid that, which is why the thermostat settings guide advises against deep overnight setbacks on resistance-backed systems.
Running cost depends on how many winter hours the backup works and what energy it uses, and the efficiency of the heat pump itself matters just as much. A heat pump that does nearly all of the heating, with a backup that handles the rest, will cost much less than a system where the backup runs every cold morning. Ask the installer to show where the backup switches on, and what the lockout temperature for the elements is set to. For how fuel prices change the comparison, see natural gas versus electric heating.
What should you ask your installer?
A short list of questions keeps the quote honest. At what outdoor temperature does the heat pump lose its ability to cover the load? What backup is included, and what is its capacity? How is it controlled, and can the lockout temperature be changed? What does the electrical panel need to support the elements, if you choose them? The answers belong in the written quote, which you can compare using the heat pump quote comparison guide.
Also check the electrical side early. Resistance backup can add a large electrical power load to the panel, and older homes sometimes need a service upgrade.
Quick answers
Do heat pumps need backup heat in Canada?
Many installations include it, because the coldest days can exceed what the heat pump alone provides. A well-sized cold-climate unit in an efficient house may not need it for comfort, though some owners keep a source for outages and repairs.
Is electric backup heat expensive to run?
Resistance heat uses one unit of electricity for one unit of heat, so it costs much more per unit of heat than the heat pump itself. It is fine for a few hours a year and costly if it runs often.
Can I keep my gas furnace as backup?
Yes. This dual-fuel arrangement is common when replacing an air conditioner with a heat pump, and the controls switch to the furnace below a chosen temperature.
How do I know how much backup I need?
A load calculation shows the heat loss at your design temperature, and the manufacturer's data shows the heat pump's output at that temperature. The difference is the backup capacity to plan for.