What a typical 3 bed semi needs, room by room
A 1930s semi with eight heated rooms, at design temperature. The first column is the home with unfilled cavity walls, older double glazing and 100 mm of loft insulation. The second is the same home after the cavity is filled, the windows replaced and the loft topped up to 270 mm. The last column is the radiator rating you would buy for each room once it is insulated and run from a heat pump at 45 degrees flow.
| Room | Size | Before upgrades | After upgrades | Radiator rating, heat pump |
|---|---|---|---|---|
| Living room | 4 by 4 m | 1,520 W | 1,015 W | 3,565 W |
| Dining room | 3.5 by 3 m | 820 W | 595 W | 2,095 W |
| Kitchen | 3 by 3 m | 945 W | 615 W | 1,790 W |
| Hall | 3 by 1.8 m | 510 W | 345 W | 1,010 W |
| Main bedroom | 4 by 3.5 m | 780 W | 480 W | 1,390 W |
| Bedroom 2 | 3.5 by 3 m | 640 W | 385 W | 1,125 W |
| Box room | 2.4 by 2.4 m | 390 W | 225 W | 650 W |
| Bathroom | 2.4 by 2 m | 640 W | 450 W | 1,700 W |
| Whole house | 6,245 W | 4,110 W | 13,325 W |
The data behind this table
Upgrades cut the heat the whole house needs from about 6.2 kW to about 4.1 kW, and that is the difference between a heat pump that runs comfortably and one that is too small. Rounded to the nearest 5 W; rooms are 2.4 metres high.
Why a heat pump needs bigger radiators
A radiator's catalogue rating is the heat it gives out when the water in it averages 50 degrees above the room. A gas boiler can deliver water that hot. A heat pump is most efficient at about 45 degrees flow, so the radiator averages less than 25 degrees above the room and gives out only a part of its rating. Here is the same living room, heated different ways.
| Heat source | Flow and return | Share of the catalogue rating | Rating to buy |
|---|---|---|---|
| Old boiler, 75 degrees flow | 75 and 65 | 97% | 1,560 W |
| Modern condensing boiler, 55 degrees flow | 55 and 45 | 49% | 3,080 W |
| Heat pump, 50 degrees flow | 50 and 40 | 39% | 3,940 W |
| Heat pump, 45 degrees flow | 45 and 35 | 28% | 5,340 W |
The living room from the intro, 4 by 4 metres, unfilled cavity walls, kept at 21 degrees. Output follows the BS EN 442 curve, with an exponent of 1.3.
The sensible order is to insulate first, then check the radiators room by room. Rooms where the existing radiator is already large for the room often need nothing. Our radiators and heat pumps guide covers when to upsize and when a fan-assisted radiator helps, and the heat pump sizing guide turns the whole house total into a unit size.
How we work it out
Fabric loss is each surface's area times its U-value times the temperature difference. The outside design temperature is 3 degrees below freezing and the ground under a solid floor is taken as 10 degrees. U-values are typical figures based on the age defaults in SAP 10, the method behind EPCs, and on today's Building Regulations limits: solid brick 2.1, unfilled cavity 1.5, filled cavity 0.55, modern wall 0.30, single glazing 4.8, older double glazing 2.8, newer double glazing 1.6, triple glazing 1.0, a loft with no insulation 2.3, 100 mm 0.40 and 270 mm 0.16, in watts per square metre per kelvin. An uninsulated solid floor is 0.70, a suspended timber floor 0.80 and an insulated floor 0.25.
Air changes take 0.33 watts per cubic metre per air change per kelvin, with typical rates of 1 change an hour in a bedroom, 1.5 in a living room, 2 in a kitchen or hall and 3 in a bathroom. The margin adds 10 per cent. One outside wall is the room's longer side, two walls meet at a corner, and three add the other short side. Everything runs in js/radiator.js, a short file anyone can read.
What it leaves out: heat from a cold room next door, a very exposed site, and thermal bridges. Those are why an MCS heat pump installer measures the home before sizing a unit. This calculator is for choosing a radiator and for checking a quote.
Frequently asked questions
What size radiator do I need for a 4 by 4 metre room?
A 4 by 4 metre living room with two outside walls, an unfilled cavity and old double glazing needs about 1,520 W (5,200 BTU) on our method, which is 95 W per square metre. The same room with modern insulation needs about 820 W (2,800 BTU). Radiator size depends on the insulation more than on the floor area, so use the calculator with your own walls and windows.
How do I convert watts to BTU?
Multiply watts by 3.412 to get BTU per hour. A radiator rated at 1,500 W gives 5,118 BTU an hour. Divide BTU by 3.412 to go the other way.
How many BTU per square metre does a room need?
It depends on the home. On our method a room in a poorly insulated older home needs roughly 90 to 100 W per square metre (about 300 to 340 BTU), and a well insulated one 50 to 60 W (170 to 200 BTU). A single rule of thumb for every home is the reason people end up with radiators that are too small or too big.
Do I need bigger radiators for a heat pump?
Usually yes, in the rooms where the radiators are small for the room. A heat pump runs the water cooler than a boiler, at about 45 degrees against 75, so each radiator gives out roughly 28 per cent of its catalogue rating instead of nearly all of it. The same living room needs a radiator rated at about 5,340 W for a heat pump at 45 degrees, against about 1,560 W for a boiler at 75 degrees. Insulating first makes the radiators you have go further.
Is this the same as a heat loss survey?
No. It is an estimate for choosing a radiator. It uses typical air change rates and U-values, and takes no account of a room next door being cold, a very exposed site or an unusual shape. An MCS heat pump installer measures your home properly before sizing a heat pump, and you should ask to see their room by room heat loss figures.
Why does the calculator ask about the floor and the ceiling?
Heat leaves through every surface. A ground floor room loses heat through the floor, and a bedroom under an uninsulated loft loses a great deal through the ceiling. A room with a heated room above and below loses almost nothing that way, so it needs a smaller radiator.
Data sources
- BRE, SAP 10: default U-values for walls, glazing, roofs and floors, the starting point for our typical figures.
- CIBSE, Guide A: Environmental design: design temperatures and air change rates by room.
- MCS, standards library: the heat pump design standard (MIS 3005-D), which sets the room by room heat loss method installers follow.
- BS EN 442: radiator output at a mean water temperature 50 kelvin above the room, and its correction for other temperatures. BS EN 12831: the design heat load method this follows in simplified form.