Resources / Heat pumps
What Size Heat Pump Do I Need?
Size comes from heat loss and design conditions — not the old boiler badge or a floor-area rule of thumb. Here is how to think about it for Devon and Cornwall homes.
Quick Answer
Heat pump size (kW) must match design heat loss on a cold day at the flow temperature your radiators or underfloor heating can use — not floor area alone, and not the badge on the old boiler.
As a planning band only for Devon and Cornwall: many renovated 3-bed homes land around 8–12 kW. Small well-insulated homes can sit lower; solid-wall or larger detached homes often need more. Final size comes from a room-by-room heat loss, not a rule of thumb.
How heat pump size is actually calculated
A designer estimates how much heat each room loses on a cold design day, adds them up, and selects a heat pump that can deliver that load at the outdoor temperature and flow temperature you will run.
Inputs that matter:
- •Room sizes, construction and insulation
- •Windows, floors, roof
- •Ventilation / air leakage
- •Indoor target temperatures
- •Outdoor design temperature for the location
- •Emitter output at the chosen flow temperature
Full explanation: what is a heat loss calculation.
Typical heat pump sizes in Devon & Cornwall
These are planning bands only — useful for sense-checking quotes, not for ordering a unit. Every home needs a room-by-room heat loss before size is locked.
| Property (illustrative) | Better fabric | Average | Poor / solid wall |
|---|---|---|---|
| 1–2 bed flat / small terrace | 4–6 kW | 5–7 kW | 7–9 kW |
| 3-bed semi / terrace | 6–8 kW | 8–11 kW | 10–14 kW |
| 3–4 bed detached (typical SW) | 8–10 kW | 9–12 kW | 12–16 kW |
| Larger / exposed / limited fabric work | 10–13 kW | 12–16 kW | 16 kW+ / multi-unit |
Many renovated three-beds in Devon and Cornwall cluster in the 8–12 kW planning range. That is a pattern, not a design result.
South West design weather: coastal figures are milder than northern UK references (Plymouth-class outdoor design temperatures around −1.5°C appear in published data). Inland, upland and exposed sites are colder — always ask which weather location the heat loss used.
Related: heat pump costs in Devon & Cornwall · heat pump vs gas running costs.
Rough W/m² checks (with caveats)
Some guides quote watts per square metre. As a very rough sense-check only:
- •Better fabric: often toward ~40–70 W/m²
- •Average existing stock: often ~70–110 W/m²
- •Poor / solid wall / high ceilings: can exceed ~110–150 W/m²
Multiply by heated floor area and you get an order-of-magnitude kW figure — then treat it with suspicion. Orientation, glazing, exposure, loft conversions and unheated spaces move the real number. This is why MCS design still uses room-by-room heat loss, not a single W/m² rule.
How much insulation changes the size
Better fabric lowers design heat loss. That can mean a smaller heat pump, lower flow temperatures and better efficiency — or the same size plant running less hard.
If insulation is planned after the heat pump, either size for the house as it is now, or lock the fabric works into the same programme. Sizing for insulation that never happens is a classic undersize path.
Older stock: heat pumps in older homes.
Underfloor heating vs radiators
Emitters do not change the room heat loss — they change how you deliver heat into the room at a given flow temperature.
- •UFH often supports lower flow temperatures when designed properly
- •Radiators need enough surface area at that same low flow temperature
- •A correctly sized outdoor unit with tiny radiators still leaves cold rooms
Plant size and emitter size are a pair. Radiator upgrades · flow temperature.
Want a planning band for your own home?
The free Planning Passport uses your property details to show an indicative size class and cost range for Devon & Cornwall — still not an MCS certificate.
Start heat pump Passport →Common sizing mistakes
| Mistake | Result |
|---|---|
| Copying the boiler kW | Often oversized historically; poor match for heat pumps |
| Floor-area rules only | Ignores fabric, glazing, volume and location |
| Undersizing to win the quote | Cold weather failure, high flow temps, unhappy clients |
| Ignoring capacity at low outdoor temp | Brochure kW at mild conditions is not design-day output |
| Assuming solar reduces required kW | Solar is annual energy; peak heat loss is still a kW problem |
When homes struggle: not heating properly.
Nameplate kW vs real output
Manufacturers publish capacity at stated conditions (outdoor air temperature and flow temperature). Output falls as it gets colder outside and as flow temperature rises.
Ask: What capacity does this unit have at our design outdoor temperature and our design flow temperature? If the answer is only the brochure headline, push for the table.
What you should do
- Use a calculator only for a planning band
- Commission a room-by-room heat loss before locking size
- Decide fabric works that are real vs “someday”
- Match emitters to the flow temperature you will run
- Check unit capacity at design conditions, not only nameplate
- Confirm noise/siting for the outdoor unit
Honest bottom line
The right heat pump size is the one that covers design heat loss at conditions you will actually see — with emitters that can deliver that heat at a sensible flow temperature.
8–12 kW is a useful planning band for many local 3-beds. It is not a design. Heat loss first, quote second.
Frequently asked questions
What size heat pump do I need for a 3-bedroom house?
Many renovated 3-bed homes in Devon and Cornwall fall in a planning band around 8–12 kW, but insulation, exposure and emitters can push that lower or higher. Treat bedroom count as a starting hint only — heat loss decides.
What heat pump sizes are available for homes?
Domestic air source units commonly span roughly 5–16 kW class (product lines vary). Larger loads sometimes use cascaded or multiple outdoor units. Nameplate kW is not the same as output at your design outdoor temperature and flow temperature.
Is bigger safer?
Not always. Oversizing can mean more cycling and poorer control. Undersizing fails on cold days. Correct for the design load is the goal.
Can I size a heat pump from floor area or my EPC?
No. Floor-area rules and EPC ratings are not a room-by-room design heat loss for plant and emitter sizing. They are at best a rough sense-check.
Does underfloor heating mean I need a smaller heat pump?
Not automatically. UFH can help you run lower flow temperatures efficiently. The heat loss of the rooms is still the heat loss of the rooms.
What if I am on oil in a rural Devon or Cornwall cottage?
Same sizing method — heat loss first. Grant support may be higher for eligible oil/LPG off-gas cases; it does not change peak load physics. Solid walls and exposure often matter more than the fuel you are leaving.
Next step
Get a realistic planning band for your home, then use the checklist when you speak to installers.