Envelope · Thermal
Heat loss calculator for the whole house, room by room
A house needs exactly as much heat as it leaks, and it leaks through three things: surfaces, junctions and air. Add up the area of every surface times its U-value, add an allowance for the junctions between them, add the air that leaves and has to be reheated, and multiply the lot by how cold it gets. That number — in kilowatts — is what the boiler or the heat pump has to cover on the worst day of the year. It is almost always far smaller than the one on the appliance already installed.
Heat pump: at least — kW of output at — °C outside, at the flow temperature the radiators or the floor are designed for. That is the whole design heat load: the UK heat pump design standard, MCS MIS 3005-D, asks the heat pump on its own to cover all of it, without the supplementary electric heater. A hybrid that keeps a boiler needs at least 55 % of it from the heat pump, — kW, rated at 55 °C flow.
No heating load at these temperatures: the design outside temperature is not below the inside one, so there is nothing for a heat pump to cover.
Assumed: — °C in every room; nothing added for warming up after a setback, for hot water or for pipe losses; no sun and no internal gains. Typical heat losses and the sizing rules, with their sources. Each room at its own temperature, and its radiator: room by room.
Room by room, and the radiator for each room
The same sum, one room at a time, each room at its own temperature. Added up, the rooms give the heat pump; each room on its own gives the radiator to buy, rated at ΔT50, for water 50 K above the room and for the heat pump's lower water temperature.
| Room | °C | Heat loss | W/m² | Radiator, water 50 K above | Radiator, water 45/40 °C |
|---|
The whole calculation
There is no cleverness in it. The care is all in the inputs.
The 0.33 is the heat capacity of air in convenient units: 1.2 kg/m³ × 1005 J/(kg·K) works out at 0.33 watt-hours per cubic metre per kelvin. So one air change an hour in a 250 m³ house costs 82 watts for every degree of temperature difference, whatever the walls are made of.
What the answer is for
- Sizing a heat pump. This is the number, and it is the only number. A heat pump is chosen so its output at the design outside temperature, and at the flow temperature of the emitters, covers the design heat load: MCS MIS 3005-D (clause 5.5.1) asks for at least 100 % of it without the supplementary electric heater, backed by the manufacturer's performance data. The result above says what that comes to; the guide to average heat loss sets it next to published figures.
- Deciding whether radiators will work at 45 °C. Below roughly 40 W/m² a conventional radiator system can usually be run at heat-pump temperatures without replacing everything. Above 60 W/m², it usually can't.
- Knowing what to fix first. The breakdown above is worth more than the total. People spend on walls when the answer is air.
What the number should look like
| Design heat load | What that means |
|---|---|
| ≤ 10 W/m² | Passivhaus territory. The building can be heated through the ventilation air alone. |
| 10 – 30 | Very good. A small heat pump at low flow temperature, comfortably. |
| 30 – 50 | A decently insulated house. Heat pump works, emitters may need enlarging. |
| 50 – 80 | Insulated in places. Fabric work pays back before plant does. |
| > 80 | Largely uninsulated. Sizing plant for this is treating the symptom. |
Compare that with the rule of thumb still used to sell boilers — around 100 W/m², or "one kilowatt per room" — and you can see why so much plant is two or three times too big. An oversized boiler cycles, an oversized heat pump cycles harder and costs more to buy.
Worked example you can check by hand
The house this page opens with: 100 m² floor, 250 m³, 21 °C inside, −2 °C outside.
And the part that catches people out: the ventilation is 41.3 W/K, the single largest line on the sheet — bigger than all the walls put together, and two and a half times the roof and floor combined. Halve the leakage and the house needs 2.59 kW instead of 3.07, which is a whole heat-pump size. Insulating the walls from 0.30 to 0.18 saves less than that, and costs twenty times as much.
Room by room
The room section repeats the same sum for each room, at that room's own temperature, and adds one line the whole house does not need: heat that goes through an inside wall or floor to a heated room kept at another temperature.
The radiator line is the one the radiator size calculator uses (EN 442, n = 1.3, arithmetic mean water temperature); each room links to it with its own figures. The temperature each kind of room starts at is the one in Table 1 of MCS MIS 3005-D, taken from CIBSE Guide A: living, dining and bedsitting rooms 21 °C, bathroom 22 °C, bedroom, hall and landing, kitchen and toilet 18 °C. The standard asks for inside temperatures not less than those; type the temperature the room is really kept at if it is higher.
The bathroom of the example, by hand
5 m² of floor, 2.5 m high, 22 °C inside, −2 °C outside, y 0.08, 0.5 air changes an hour, and the 6 m² wall it shares with the bedroom at 18 °C.
The bedroom counts the same wall from the other side, 6 × 1.50 × (18 − 22) = −36 W, so the two cancel in the house total: the three rooms lose 1,094.5 W to the outside and to the loft, and that is the heat pump, 1.09 kW at −2 °C outside.
What this page does not do
- No solar and no internal gains. The design load is deliberately a worst-case: dark, cold, nobody home, no appliances running. That is correct for sizing plant and wrong for predicting a fuel bill. The annual figure is less strict — degree days to a 15.5 °C base already assume roughly 2.5 K of free heat from people, cooking and sunshine, which is why the base is not 18.
- One temperature everywhere, in the whole-house figure. Real houses have a cold spare room and a hot bathroom, and the losses between rooms cancel out in the total but decide the radiator sizes. The room-by-room section gives each room its own temperature and its own neighbours. Its ground floors use the same 0.45 factor as here, not the local annual average outside temperature that MCS MIS 3005-D (clause 5.5.1 c) asks for under a solid floor.
- Ground floors are a fudge here. The 0.45 factor stands in for the whole of ISO 13370, where the real answer depends on the perimeter-to-area ratio and on the ground itself. For a compact plan it is close; for a long thin one it is optimistic.
- Steady state. No thermal mass, so no credit for a heavy building riding through a cold snap, and no penalty for intermittent heating.
- The y-value is an allowance, not a calculation. It is a reasonable stand-in until you have the junctions, and then you should work them out.
Questions people ask
- How do you calculate the heat loss of a house?
- Add up every surface's area times its U-value, add an allowance for the junctions between them, add the air that leaves and has to be reheated (0.33 × volume × air changes per hour), and multiply the total by the difference between the inside and the design outside temperature. The house this page opens with, 100 m² of floor, comes to 3.07 kW.
- What is a good heat loss per square metre?
- At or below 10 W/m² is Passivhaus territory; 10 to 30 is very good; 30 to 50 is a decently insulated house; 50 to 80 is insulated in places; above 80 is largely uninsulated. The old rule of thumb of about 100 W/m² is why so much heating plant is two or three times too big.
- What size heat pump do I need?
- One whose output at your design outside temperature, and at the flow temperature your radiators or floor need, covers the design heat load this page calculates. The UK heat pump design standard, MCS MIS 3005-D (clause 5.5.1), asks for at least 100 % of it from the heat pump alone, without the supplementary electric heater; a hybrid that keeps a boiler needs at least 55 % from the heat pump, rated at 55 °C flow. The house this page opens with needs at least 3.07 kW at −2 °C outside.
- Will my radiators work with a heat pump?
- Below roughly 40 W/m² of design load, a conventional radiator system can usually run at heat-pump temperatures without replacing everything. Above 60 W/m² it usually cannot.
- Is this a room-by-room heat loss survey?
- Not a survey, but it does the sum room by room. The calculator at the top gives the design heat load of the whole house, the number a heat pump is sized on; the room-by-room section below repeats the sum for each room at its own temperature and turns each room's watts into the radiator to buy, rated at ΔT50, for boiler and for heat pump water temperatures. A survey measures the rooms and tests the air leakage instead of assuming it.