The objection arrives in the same form every time: heat pumps are fine in mild climates, but they cannot cope with a real winter.
It was true. It was true of single-speed equipment sold in the 1990s and early 2000s, and a lot of people formed a durable impression from a unit that genuinely could not manage a cold January. The equipment changed. The impression did not.
Why the efficiency number looks impossible
A gas boiler burns fuel and converts the chemical energy to heat. At best it captures nearly all of it, so its efficiency approaches but cannot exceed 100 percent. An electric resistance heater converts electricity to heat at essentially exactly 100 percent.
A heat pump does not convert anything. It moves heat that already exists from outside to inside, and the electricity is spent on the moving, not the heat itself. So the ratio of heat delivered to electricity consumed is not bounded by 100 percent.
300-400%
Typical effective efficiency of a heat pump in mild conditions, expressed as a COP of 3 to 4
This is the same mechanism as a refrigerator, run in the direction that people find counterintuitive. A fridge takes heat out of a cold box and dumps it into a warmer kitchen. A heat pump takes heat out of cold outdoor air and dumps it into a warmer house. Neither violates anything; both need work input to move heat against the gradient.
"There is no heat in cold air" is a scale error
At minus fifteen degrees Celsius, outdoor air is about 258 kelvin. Absolute zero is 0 kelvin. The air contains a great deal of thermal energy by any absolute measure — it is simply colder than your living room, which is the only comparison our intuition makes.
The refrigerant cycle exploits that. It evaporates at a temperature well below the outdoor air, so heat flows into it from air that feels frigid to us. Compressing that vapour raises its temperature above the indoor air, and the heat flows out. The compressor's work is what makes the gradient run the way you want.
What genuinely does change
Efficiency and capacity both fall as it gets colder, because the temperature gap the compressor must bridge grows. That is a real limit and it is why sizing matters. It is not the same claim as "it stops working".
What the numbers look like across a winter
| Outdoor temperature | Typical COP | Capacity vs. rated | Notes |
|---|---|---|---|
| 10°C | 4.0-5.0 | 100% | Shoulder season |
| 0°C | 3.0-3.5 | 90-100% | Most of a temperate winter |
| -10°C | 2.2-2.8 | 70-85% | Cold-climate models hold here |
| -20°C | 1.8-2.2 | 55-70% | Still ahead of resistance heat |
| -25°C | 1.5-2.0 | 45-60% | Rated floor for many units |
Ranges are indicative and vary substantially by model and installation.
The line worth noticing is the bottom one. Even at minus twenty-five, a COP of 1.5 delivers half again as much heat as the resistance heater it might otherwise be compared against. There is no temperature at which a working heat pump becomes worse than direct electric heat.
Why some installations disappoint anyway
Field data from cold regions is consistently better than reputation, which points at installation rather than equipment. Four causes account for most of it.
- Undersizing. The unit is sized for a mild design temperature and runs out of capacity in a cold snap, falling back to resistance backup and producing an alarming bill.
- Oversizing. The opposite error. A unit far too large short-cycles, never reaching steady state, and both efficiency and compressor life suffer.
- Emitter mismatch. Radiators sized for 70-degree boiler water simply cannot deliver enough heat at 45 degrees. The building is cold and the heat pump takes the blame.
- Poor commissioning. Wrong refrigerant charge, wrong flow rate, weather compensation left disabled. A well-specified unit set up carelessly loses a large share of its rated performance.
“Almost every heat pump horror story I have looked into turns out to be an installation story. The physics is the least uncertain part of the system.”
The economics are separate from the physics
A heat pump can be three times as efficient as a gas boiler and still cost more to run, because electricity often costs more per unit of energy than gas.
The test is one division. Take your electricity price per kilowatt-hour, divide by your gas price per kilowatt-hour, and compare the result to the seasonal COP you expect.
- Ratio of 3.5 against an expected COP of 3.0 — gas is cheaper to run.
- Ratio of 2.5 against an expected COP of 3.0 — the heat pump wins.
That ratio is a policy variable as much as a market one, and it differs wildly between regions with similar climates. Which is the actual reason adoption varies, and it has nothing to do with whether the technology works in the cold.
The short version
The physics has never been in question. Cold-climate equipment closed the capacity gap. What is left is a specification-and-installation problem and a pricing problem, both solvable, and neither of them the objection people usually raise.