Will a heat pump cope with winter in Loughton?
A heat pump loses capacity as it gets colder, exactly when the house needs more. Where those two lines cross is the balance point, and below it something else makes up the difference. In Loughton the design temperature is −5.1 °C — here is how much of the year sits below each threshold, measured over 28 years at Shobdon Airfield.
How cold it actually gets here
Days a year with a daily mean at or below each temperature. This is the measured record, not a model.
| At or below | Days a year | Share of the year |
|---|---|---|
| 15 °C | 312.7 | 85.7% |
| 10 °C | 194.5 | 53.3% |
| 7 °C | 118.7 | 32.5% |
| 5 °C | 76.4 | 20.9% |
| 2 °C | 29.3 | 8.0% |
| 0 °C | 12.0 | 3.3% |
| −5 °C | 0.5 | 0.1% |
| −7 °C | 0.2 | 0.1% |
| −10 °C | 0.1 | 0.0% |
Where the two lines cross
Enter the two capacity figures from your heat pump’s datasheet. Every unit sold in Europe publishes them at +7 °C and −7 °C outdoor.
Why the balance point is the number that matters
Rated capacity is quoted at +7 °C, which is not a cold night. What decides whether a house stays warm in February is capacity at the temperature February actually reaches here, and how many days a year that is. A pump with no balance point above the design temperature never needs backup; one that balances at −2 °C needs it often.
What backup costs
Backup is almost always a resistance heater running at a coefficient of performance of 1, where the heat pump runs at 3 or more. Every kilowatt-hour it supplies costs about three times what the pump would have charged for it, which is why a few extra kilowatts of pump can be cheaper than the backup it avoids.
- Measured: the design temperature, the degree days and the monthly means. All of it from one station, Shobdon Airfield, over 28 complete years.
- Both capacity figures come from your own datasheet. Nothing here assumes a make or model, because the spread between a mid-range unit and a cold-climate one at −15 °C is more than double.
- Capacity between and beyond the two rating points is taken as a straight line. Manufacturer curves bend downward below −7 °C, so a balance point far below that point is flattering the machine slightly.
- The day counts use daily mean temperatures. A house has thermal mass, so an hour below the balance point before dawn is not a day of backup heating.
- This is a calculation, not a survey. Before spending money, have the building assessed.