Modern cold-climate heat pumps can heat buildings through much of a Canadian winter, but their capacity and efficiency drop as the outdoor temperature falls. In the coldest regions, most installations still need backup heat for the coldest days. The key to a good result is matching the equipment to the building's heat loss and the local climate, and designing the backup and controls properly.
How a heat pump heats
A heat pump uses the same refrigeration cycle as an air conditioner, run in reverse. It absorbs heat from a source, such as outdoor air, the ground or water, and releases it inside the building. Because it moves heat rather than creating it, a heat pump can deliver more heat energy than the electrical energy it uses. That ratio, called the coefficient of performance (COP), is what makes heat pumps efficient.
In cooling mode, the cycle reverses and the heat pump works as an air conditioner.
Types of heat pumps
- Air-source heat pumps draw heat from outdoor air. They come as ducted systems, ductless mini-splits, packaged rooftop units and larger variable refrigerant flow (VRF) systems.
- Ground-source (geothermal) heat pumps use buried loops or wells. Ground temperatures are steadier than air, so performance holds up well in cold weather, but installation is more involved.
- Water-source heat pumps use a building water loop, often in larger buildings where some zones need cooling while others need heating.
- Air-to-water heat pumps heat water for hydronic systems such as radiant floors.
What happens when it gets cold
As outdoor air gets colder, an air-source heat pump has less heat available to collect and must work harder. Two things happen at once: its heating capacity drops, and the building's heat loss rises. At some outdoor temperature, called the balance point, the heat pump alone can no longer keep up.
Cold-climate models use variable-speed compressors and refrigerant circuits designed for low temperatures, and they keep useful capacity well below freezing. Manufacturers publish capacity and efficiency at several low temperatures. Compare those figures with the winter design temperature for your location, which is listed in the building code climate data, rather than relying on a single rated number.
Defrost
Below freezing, frost builds up on the outdoor coil. The heat pump periodically defrosts it, briefly reducing heat output. That is normal. The outdoor unit should be mounted above expected snow depth, with drainage for meltwater so ice does not build up beneath it, and away from roof edges where snow and ice can fall on it.
Backup heat
Most air-source heat pump installations in the colder parts of Canada include backup heat: electric resistance elements, an existing gas furnace in a dual-fuel arrangement, or a boiler. The controls decide when to switch over. Poorly set controls can bring on backup heat too early, losing much of the efficiency benefit, or too late, leaving the building cold. Review the switchover settings with your contractor and check them during the first cold spell.
Size for the building, not the old furnace. A heat pump should be sized from a proper heat loss calculation. Matching the size of an old furnace often leads to the wrong equipment, since furnaces were frequently oversized. Electrical service capacity should also be checked, especially if electric backup heat is planned.
Before you install: questions to answer
- What is the building's heat loss at the local winter design temperature?
- Can the heat pump meet that load, and at what temperature does backup take over?
- Is the electrical service large enough for the heat pump and any backup heat?
- Are existing ducts large enough? Heat pumps often deliver air at lower temperatures and may need more airflow.
- Where will the outdoor unit go, with clearance from snow, drainage and access for service?
- Would envelope improvements such as air sealing or insulation reduce the size of equipment needed?
- How will refrigerant work be handled, and by whom? Only certified technicians may handle refrigerants.
In commercial buildings
In commercial buildings, heat pumps increasingly replace gas-fired rooftop units and boilers. Larger buildings can recover heat from zones that need cooling and move it to zones that need heating. These systems depend heavily on controls and commissioning. Our guide to commissioning a new building explains why that step matters. An energy audit can help decide where a heat pump will pay off.
How ehZee can help
Our mechanical system inspections can look at existing heat pump installations, including outdoor unit placement, defrost and drainage, controls and backup heat settings. If you are planning a conversion, we can inspect the existing equipment and help you set out the questions your designer and contractor need to answer.
Common questions
Will a heat pump work at minus 30?
Some cold-climate models are rated to operate at very low temperatures, but with reduced capacity and efficiency. Check the manufacturer's data at your local design temperature, and plan for backup heat in the coldest regions.
Should I keep my gas furnace?
Many owners keep an existing furnace as backup in a dual-fuel setup. Whether that makes sense depends on the furnace's condition, energy prices and your goals for reducing emissions.
Does a heat pump need more maintenance than a furnace?
It runs in both heating and cooling seasons, so it works more hours. Keep filters clean, keep the outdoor unit clear of snow and debris, and have it serviced as the manufacturer recommends.