Heat pump size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Heat loss of the house at the design temperature | 6,000 W |
| Flow temperature | 45 °C |
| Return temperature | 40 °C |
| Radiator exponent n | 1.3 |
| Existing radiators | Living room: room heat loss 1,000 W, room temperature 21 °C, radiator output at δt50 2,000 W |
Heat pump: at least 6.0 kW at your design conditions
| Result | Value |
|---|---|
| Heat pump output needed at design conditions | 6 kW (20,473 BTU/h) |
| For heating | 6 kW |
| For hot water | 0 kW |
| Radiators to upgrade | 1 |
| Room | Heat loss | Radiator gives | |
|---|---|---|---|
| Living room | 1,000 W | 668 W | Upgrade |
Show the working
- Heating: 6,000 W ÷ 1,000 = 6 kW
- Required at design conditions: 6 kW (no margin added)
- Living room: ΔT 21.5 K, 2,000 W × 0.334 = 668 W vs 1,000 W heat loss
How it works
- Required output (kW) = (heat loss at the design outdoor temperature + hot water allowance) ÷ 1000, with no extra margin added
- Compare it with the maker's output at your design outdoor and flow temperatures, not the headline rating
- Each radiator gives its ΔT50 output × (ΔT ÷ 50)^n, where ΔT = (flow + return) ÷ 2 − room temperature
An MCS installer sizes the heat pump from their own room-by-room heat loss (MCS MIS 3005-D). This is a planning guide.
Heat pump output falls as the outdoor temperature drops: use the maker's figure at your design outdoor and flow temperatures.
A heat pump needs its own circuit: check your supply's maximum demand with an electrician.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Radiator exponent n | 1.3 | Our assumption Not yet checked against the source | Our choice: change it to suit your job Radiator exponent n, default 1.3 as stated in brief v2 H2 ("typical from makers"). Stored as an assumption until the Researcher cites a maker's data sheet (e.g. Stelrad, Zehnder) and switches it to typical. |
Regulations and standards
- MCS MIS 3005-D (heat pump design and sizing; Researcher to confirm the clause)
- BS EN 12831-1 (heat loss)
Explained: MCS MIS 3005-D (heat pump design), BS EN 12831 and CIBSE DHDG (heat loss).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.