BTU SizerHeat Pumps in Cold Weather
A heat pump’s nameplate is a single number measured under one condition. The machine’s actual output changes with the weather, and it changes in the wrong direction: capacity falls as the house’s demand rises.
The rating point is 47 °F
Air-source heat pumps are rated for heating at 47 °F outdoors. That is a mild autumn day, not a January night, and it is the number on the box.
As it gets colder, there is less heat in the outdoor air to move and the compressor works against a bigger pressure difference, so output falls. Meanwhile the house’s demand is rising along a straight line. Typical figures:
| Outdoor temperature | Conventional single-stage | Cold-climate inverter |
|---|---|---|
| 47 °F | 100% | 100% |
| 35 °F | ~85% | ~100% |
| 17 °F | ~60% | ~85% |
| 5 °F | ~45% | ~75% |
| −5 °F | little or none | ~60% |
Those are typical, not universal. Every model publishes its own capacity table, and the gap between two units with the same nameplate can be large.
The balance point
Plot the falling capacity and the rising demand on the same axes and they cross. That crossing temperature is the balance point: above it, the heat pump carries the house alone; below it, backup heat makes up the difference.
- A conventional unit in a moderately insulated house often balances around 25–30 °F.
- A cold-climate inverter in a well-sealed house can balance near 5 °F or below.
The lower the balance point, the fewer hours a year run on backup — and backup matters because electric resistance backup has a COP of 1 while the heat pump is running at 2.5 to 3.5. Every hour on backup costs roughly three times as much.
Sizing around it
The temptation is to buy a much larger heat pump so it never needs backup. That creates a different problem: the same machine has to cool the house in summer, and a unit sized for January is enormously oversized for July. It will short-cycle and never dehumidify.
The usual answer is a correctly sized inverter unit — which modulates across a wide range — plus sensible backup for the coldest hours. In genuinely cold zones a dual-fuel setup pairs the heat pump with a gas furnace and switches over at the economic crossover temperature.
Defrost is not a fault
Below about 40 °F with moisture in the air, frost forms on the outdoor coil and the unit periodically reverses to melt it. During defrost it is briefly cooling rather than heating, which is why the supply air goes cool and the outdoor unit steams. It lasts a few minutes and it is the system working correctly. What is not correct is defrosting every ten minutes, which points at airflow or refrigerant problems.
What to ask for
Before signing, ask for the model’s capacity table at your design temperature — not the 47 °F rating and not a reassurance. Then check it against the load from the heat pump size calculator. If the unit delivers less than the load at your design temperature, that gap is the backup heat you will be paying for, and it is worth knowing the size of it in advance.