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Do heat pumps work in cold weather?
Yes — and the reason surprises people. A heat pump is just an air conditioner running backwards. Even air at 0 °C is full of heat energy; it's only "cold" relative to you. If you make the refrigerant colder than the outdoor air — say −7 °C — then heat flows into it, exactly as physics demands. The heat pump then squeezes that harvested heat up to a useful temperature and releases it in your living room.
It's the same loop, pointed the other way
Nothing about the refrigeration cycle changes. There's still a compressor, a condenser, a metering device, and an evaporator, in that order. What changes is which coil is which. A component called the reversing valve redirects the hot gas leaving the compressor:
- Cooling: hot gas goes to the outdoor coil, which becomes the condenser and dumps your home's heat outside. The indoor coil is the evaporator, absorbing heat from your rooms.
- Heating: hot gas goes to the indoor coil, which becomes the condenser and releases heat into your rooms. The outdoor coil is now the evaporator, absorbing heat from the cold outside air.
That's the whole trick. The "waste heat" that a normal AC throws away outdoors is, in winter, the entire point.
What happens at each stage in heating mode
1 Compression — Compressor
Outdoor unit
It works harder in heating: the colder the outdoor air, the bigger the squeeze it has to make, so the discharge gas runs even hotter than in cooling.
2 Condensation — Condenser coil
Indoors — this is what warms your home
In heat-pump mode this coil is indoors. The 'waste heat' is now the whole point — it's what blows warm air into your rooms.
3 Expansion — Expansion valve
Feeds the outdoor coil
In heating it has to chill the refrigerant below the freezing outdoor air so it can still absorb heat — that's why heat pumps need defrost cycles.
4 Evaporation — Evaporator coil
Outdoors — it steals heat from cold air
Surprisingly, even 0 °C air has heat in it. Outdoors, this coil pulls that heat out — which is why a heat pump can warm your house using cold air.
Why a heat pump needs a defrost cycle
To pull heat out of freezing air, the outdoor coil has to be colder than that air — often around −7 °C. Moisture in the air condenses on it and freezes, and a coil wearing an ice coat can't absorb anything. So periodically the unit briefly flips itself back into cooling mode, sending hot gas out to melt the ice off its own outdoor coil. That's what's happening when you see your heat pump steaming outside in winter and the indoor air goes briefly cool — it isn't broken, it's defrosting.
Where the efficiency comes from
A resistance heater turns one unit of electricity into one unit of heat, and that's the ceiling. A heat pump doesn't make heat — it moves heat that already exists, so one unit of electricity can deliver two, three, or four units of heat into your house. You're paying for the pump, not the heat.
The catch is that the colder it gets outside, the less heat there is to harvest and the harder the compressor has to squeeze — so efficiency falls as the temperature drops. Modern cold-climate units keep working well below −15 °C, but older models lean on backup resistance heat once it gets cold enough, which is why some people remember heat pumps as expensive.
What goes wrong
A heat pump fails in the same ways an AC does — it's the same machinery. A dirty filter, a refrigerant leak, or a failed capacitor will stop it heating just as surely as they stop an AC cooling. One heat-pump-specific tell: if the outdoor coil stays encased in ice rather than clearing periodically, the defrost cycle itself has failed and needs a tech.