Heat Pump Size Calculator

How the heat pump size is worked out

The MCS heat pump standard, MIS 3005-D, sets the rule: work out the heat load room by room at the design outdoor temperature, then choose a heat pump that provides at least 100% of it at the design flow temperature, without any input from the backup heater. The calculator does the first step, then for each of four ranges lists the smallest model whose headline rating is at least the load. A headline rating does not show the output at your design conditions, so for the second step, check the manufacturer's data.

Design heat load

If the heat loss was worked out at your area's design temperature, as the heat loss calculator does, the load is that figure. If it came from an older survey at a flat -3 °C, it is scaled to your area, because heat loss rises in proportion to the temperature difference. The scaling is an approximation, because solid floors and party walls lose heat to temperatures that do not change with the weather. Moving the 1930s semi from our heat loss tests from Leeds to Birmingham, the scaled figure comes out about 0.4% above a full room-by-room recalculation; scaling to a milder place reads a similar amount low.

Estimating the heat loss without a survey

The estimate panel is for when you have no heat loss figure yet. It models a simple house from your floor area, storeys, built form and age, and applies the heat loss calculator's room-by-room method to it. The model house has a square plan, two external doors, RdSAP's default window area for its age, and the air changes and room temperatures of a typical room mix. Ceilings are 2.4 m, or 2.7 m for a house built before 1930, unless you enter a height. The loft takes RdSAP's default insulation for the age unless you choose one. If yours has been topped up the estimate reads high, and if it has less it reads low. On three test houses, a 1930s semi, a mid-terrace and a Victorian terrace, the estimate was between 4% under and 6% over the full room-by-room figure. Start with it, then measure the rooms when you need a figure you can rely on.

Choosing a size

The table shows, for each range, the smallest model whose headline rating is at least the load. Any that are under 125% of the load are marked to check. The ranges are Vaillant aroTHERM plus, Mitsubishi Ecodan R290, Daikin Altherma 3 monobloc and Samsung EHS Mono R290, from the manufacturers' own pages read in September 2026. Those ratings are the headline figures, usually measured at 7 °C outside and 35 °C water. On the design day at 45 or 50 °C flow a heat pump can give out less than that, and by how much varies by model. Treat the list as a first cut and check the output at your design conditions in the manufacturer's data. Our ventilation defaults are also cautious, so an installer's MCS figure for an older house can come out lower than ours; that is not by itself a sign of undersizing. Model numbers do not always give the output: Viessmann's Vitocal 150-A types A04 to A16 are rated from 4.0 to 9.1 kW.

Hot water

MIS 3005-D sizes hot water separately: 45 litres a day for each person, counting the greater of the bedrooms plus one or the people living there. That sets the cylinder. The heat pump reheats it between heating periods, so the hot water does not normally add to the heat pump size.

Sizing a heat pump for a 1930s semi in Leeds

The 1930s cavity-wall semi in Leeds from the heat loss calculator's test set loses 7,370 W at Leeds' design temperature of -3.3 °C.

• Design load: 7.37 kW, no rescaling needed
• Vaillant aroTHERM plus: the 7 kW is too small, so 10 kW
• Mitsubishi Ecodan R290: 8.5 kW, marked to check
• Daikin Altherma 3 monobloc: class 8, rated 7.5 kW at 7 °C outside and 35 °C water, marked to check; at -3.3 °C and 45 °C it may not reach 7.37 kW
• Samsung EHS Mono R290: 8 kW, marked to check
• Hot water for three bedrooms: 45 × 4 = 180 litres a day

Now take a heat loss of 6,800 W from an old survey done at -3 °C for a house in Birmingham, where the design temperature is -5.1 °C: 6,800 × (21 + 5.1) ÷ (21 + 3) = 7,395 W. The 2.1 °C difference adds 8.75%, taking it past a 7 kW unit.