Definition — how a heat pump moves heat 'out of' cold air
A refrigerant boiling at –30 °C is colder than a –15 °C outdoor day. So even bitter outdoor air still contains heat energy — and the refrigerant in the outdoor coil is cold enough to absorb it.
That heat is then compressed (which raises its temperature), delivered indoors through the indoor coil, and released to your ductwork or wall head. The compressor's electricity is the only energy input; the heat you receive comes from the outdoor air.
This is why a heat pump can deliver 2–4 kWh of heat per 1 kWh of electricity consumed — a coefficient of performance (COP) of 2–4. A resistance-electric heater sits at exactly 1.0.
When cold-climate specifically matters
Older heat pumps used single-speed reciprocating compressors. As outdoor temperature dropped, so did their rated capacity — and by –10 °C they were leaning heavily on electric-strip backup, which is expensive to run.
Cold-climate models use variable-speed inverter compressors that can spin faster and add vapour-injection (a secondary refrigerant path) to boost capacity in cold weather. The result: rated heating capacity holds down to roughly –15 °C, with usable (reduced) capacity to –25 °C or below on the best models.
Failure modes — what goes wrong on a heat pump
Ice on the outdoor unit in winter is normal — that's the defrost cycle doing its job. Ice that never leaves, though, means the defrost sensor or reversing valve is failing.
'Blowing cold air' in heating mode is usually the defrost cycle in progress (the indoor blower shouldn't run during defrost, but some poorly commissioned installs let it), or a low charge. Either way, book a diagnostic — running a heat pump with a low refrigerant charge damages the compressor.
Higher-than-expected winter electric bills almost always trace to an over-sized system, an under-set balance point (backup coming on too soon), or an unsealed duct return pulling cold air from the attic.
Proof — the ratings that matter for Canadian selection
HSPF2 (Heating Seasonal Performance Factor 2) is the seasonal heating efficiency rating. In Canada, look also at the rated heating capacity at –15 °C on the AHRI certificate — that number tells you how much of the manufacturer's stated capacity you actually get in Ontario January.
ENERGY STAR Cold Climate Heat Pump certification adds specific minimum performance thresholds at low outdoor temperatures. It's a shorthand for 'suitable for Canadian climate zones' and worth filtering on.
Action — sizing a heat pump for your GTA home
Heat pump sizing is not the same as furnace sizing. You size for the outdoor design temperature (roughly –18 °C for most of the GTA) and set the balance point where the economic switchover to backup makes sense given your electricity vs gas rates.
This requires a real Manual J load calculation. Rule-of-thumb sizing on heat pumps consistently over-sizes them, which raises install cost and hurts efficiency at part-load. Ask your installer to show the load calc — Metroheat's quotes include it.