Rule-of-thumb W/m² estimate for a quick sanity check. A heat pump design and correct radiator sizing require a full room-by-room heat loss calculation (MCS / BS EN 12831). Combi boilers are normally sized by hot-water flow rate, not heat loss, so they are usually larger than the space-heating figure above.
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This calculator gives a quick estimate of a property's whole-house heat loss and the boiler or heat pump size that matches it. Enter the total floor area and pick an insulation standard, and it returns the design heat loss in kilowatts along with a suggested heat pump and boiler output.
It answers the question customers ask before you have crossed the threshold: what size boiler do I need? A rough kilowatt figure early on lets you steer the conversation and quote sensibly, whether the job is a straight boiler swap or a heat pump changeover.
The estimate uses a watts-per-square-metre rule of thumb keyed to how well the property is insulated. It is a sanity check, not a design — a heat pump and correctly sized radiators still need a full room-by-room heat loss calculation.
Design heat loss (kW) = floor area (m²) × insulation factor (W/m²) ÷ 1000
The property's design heat loss is estimated by multiplying its total floor area by a watts-per-square-metre factor that reflects the insulation standard — roughly 40 W/m² for a well-insulated new build, 70 W/m² for a typical home and 100 W/m² for an older, poorly insulated one. A heat pump is sized to that heat loss; a wet boiler carries a small margin on top.
Add up the heated floor area across all storeys in square metres. Include every room the system heats; exclude unheated garages or outbuildings.
Choose the option that best matches the property: a well-insulated new build, an average home with some upgrades, or an older, poorly insulated property. This sets the watts-per-square-metre factor.
The calculator shows the estimated whole-house heat loss in kilowatts — the amount of heat the system has to replace on a cold design day.
It suggests a heat pump size matched to the heat loss, and a system or regular boiler output with a small margin. For a combi, remember hot-water demand usually drives the size upward.
This is a rule-of-thumb screen, not a heat loss calculation. A heat pump design, and radiators sized to run at a low flow temperature, need a proper room-by-room calculation to MCS / BS EN 12831 — accounting for fabric, glazing, ventilation and location. Use this figure to scope and quote, then do the real calculation before ordering equipment.
Oversizing is the classic mistake, especially with heat pumps. A heat pump sized well above the heat loss short-cycles, runs inefficiently and costs the customer more to run; matching it to the design heat loss is the whole point. Boilers tolerate some oversizing better, but a grossly oversized boiler still cycles and wastes gas.
Combi boilers are usually sized by hot-water flow rate, not space-heating load. A small flat might only lose 5 kW but still need a 30 kW combi to deliver a decent shower flow, so the boiler output for a combi is normally well above the heat-loss figure shown here. Size the space-heating output to the heat loss and the hot-water output to the demand.
Rule-of-thumb W/m² estimate for a quick sanity check. A heat pump design and correct radiator sizing require a full room-by-room heat loss calculation (MCS / BS EN 12831). Combi boilers are normally sized by hot-water flow rate, not heat loss, so they are usually larger than the space-heating figure above.
For space heating, size to the property's heat loss — often 5–15 kW for a typical home. But combi boilers are sized by hot-water demand, so most homes end up with a 24–35 kW combi to run the taps and shower, even though the heating load is far lower. Enter your floor area and insulation above for a heat-loss figure.
Properly, it is a room-by-room calculation of the heat escaping through walls, roof, floor, windows and ventilation on a cold design day, to BS EN 12831. This calculator uses a quick watts-per-square-metre shortcut instead — good enough to scope a job, but not to design a heat pump or size radiators.
A heat pump should be sized to match the design heat loss, not oversized. If the house loses 7 kW on the coldest day, you want roughly a 7 kW heat pump. Oversizing makes it short-cycle and run inefficiently, so an accurate room-by-room heat loss calculation matters far more for a heat pump than for a boiler.
Because a combi has to heat water on demand for the taps and shower, which takes far more power than heating the rooms. A home that loses 6 kW might need a 30 kW combi purely to deliver a decent hot-water flow rate. The heating output is modulated down; the headline kW reflects the hot-water duty.
Approximate heat requirement for a room from its size and use.
Compare the annual running cost of a heat pump against a gas boiler.
Required circulation flow for a heat load at a design ΔT.
Convert a radiator’s catalogue output to the output at your system’s ΔT.
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