Does where you live change the heat pump you need?
Usually, yes. We took one house, changed nothing about it, and moved it around the country. For roughly three house sizes in four, it needs a different sized unit in Plymouth than it does in Glasgow.
Every figure below is produced live by the same engine behind the size and cost calculator, so this page cannot drift out of step with it. Change the underlying data and these numbers change with it.
1 One house, fourteen places
The house is held completely constant: . The only thing that changes is the weather station, which sets the outside temperature the system has to cope with on the coldest days.
| Where | Design outside | Unit needed |
|---|
2 Why that difference often changes nothing
Heat pumps are not made in every size. They come in steps, and the steps are wide: . A house whose heat loss moves from one end of the country to the other can easily stay inside a single step, and then you buy exactly the same machine either way.
This is why testing one house tells you almost nothing. Our own reference house is one of the sizes where location makes no difference at all — which, if we had stopped there, would have produced a confident and wrong answer.
3 Where it does change the answer
The same house again, this time at every floor area from 50 to 200 m², compared between the mildest and the coldest places in the table above. The rows marked in the last column are the ones where you would end up buying a different machine.
| Floor area | Milder | Colder | Same unit? |
|---|
4 What this means if you are buying one
- A calculator that never asks where you live cannot be right for everyone. If it only wants bedrooms or floor area, it is using one climate for the whole country.
- Being near a step is the risk, not being north or south. If your heat loss lands close to a boundary, the difference between a quote for one size and the next is thousands of pounds, and a survey is the only way to settle it.
- Rounding up is not free. An oversized unit short cycles, which wears it out and lowers the efficiency that decides whether it beats gas. See running cost against a gas boiler.
- This is still an estimate. It assumes air leakage from the age of the property rather than measuring it, and that is the largest single uncertainty in any figure on this page.
5 Method and sources
- Design external temperatures are the 99.6% values from CIBSE Guide A (2015), Table 2.5, for each station.
- Fabric U-values come from the BRE RdSAP 10 specification, by nation and by the age band of the property.
- Heat loss per degree is multiplied by the difference between an internal design temperature of 21 °C and the design external temperature.
- The unit is the smallest available size that meets the loss. Nothing is rounded up for comfort.
The full working for a single house, step by step, is on how sizing works.
Every number this page uses
| What | Value | Source |
|---|