Radiator BTU calculator
How much heat does this room need?
Built the way a heat loss survey is, on the official EPC default U-values and MCS design temperatures.
Heat this room needs on a cold day
4,361 BTU/h
1,278 watts, to keep it at 21C when it's -5.1C outside (the MCS design temperature for the place you chose).
Radiator size to look for (catalogue output at Delta T50)
- Gas or oil boiler: 1,312 W (4,477 BTU)
- Heat pump at 55C: 2,339 W (7,981 BTU)
- Heat pump at 45C: 3,850 W (13,136 BTU)
Radiators are sold by their output at Delta T50. Lower flow temperatures need a bigger radiator for the same heat, on Stelrad's correction factors, worked out for this room's temperature.
Our estimate, using default values: rooms next door are assumed to be heated to the same temperature, and windows are sized from the house's age. A room over a garage needs more than this shows, and conservatories and rooms with sloping ceilings aren't modelled. An installer's room-by-room survey measures these instead.
The Short Answer
If you're replacing radiators anyway, we'd size them for a heat pump at 45C. Put in the room's size, its outside walls and what the house is built from, and the calculator works out the heat the room loses on a cold day, in watts and BTU an hour. That's the heat the radiator has to give at your flow temperature (the catalogue rating you buy is different, below).
On our model, a 1930s semi living room, 4.5m x 3.5m in Birmingham with an unfilled cavity needs about 4,361 BTU (1,278 watts). Fill the cavity and it drops to about 3,215.
Radiators are sold by their output at "Delta T50", the catalogue standard. For a heat pump running cooler, you need a bigger radiator for the same room, and the calculator shows that too.
How This Calculator Works
It works the way a heat loss survey does, with default values where a surveyor would measure:
- Temperatures: the room temperatures in the MCS heat pump standard MIS 3005-D (21C for living and dining rooms, 18C for bedrooms, halls and kitchens), and its design outdoor temperature for the nearest of eight places, from -1.5C in South West England (Plymouth) to -5.9C in West Scotland (Glasgow). There's no adjustment for height above sea level, so a hilltop home needs a little more.
- Walls, windows, roof and floor: the default U-values the official EPC method (RdSAP 10) uses for a house of that age and construction.
- Air leakage: the EPC method's whole-house infiltration rates applied to the room, never below 0.5 air changes an hour (the MCS minimum), with extra for an open chimney.
One watt is 3.412 BTU an hour, so the calculator shows both.
Typical Rooms
On our model, for a house in Birmingham (the Midlands, -5.1C design temperature). The living rooms are on the ground floor with a heated room above; the bedroom has the loft above and a heated room below:
| Room | Heat needed | Radiator for a boiler (Delta T50) | Radiator for a heat pump at 45C |
|---|---|---|---|
| 1930s semi living room, 4.5m x 3.5m, two outside walls, unfilled cavity | 4,361 BTU (1,278 W) | 1,312 W | 3,850 W |
| The same room with the cavity filled | 3,215 BTU (942 W) | 967 W | 2,839 W |
| Back bedroom under a 100mm loft, 3m x 3m, one outside wall | 1,585 BTU (464 W) | 441 W | 1,180 W |
| Victorian front room, 4.5m x 3.5m, 2.8m high, two outside walls, solid walls, single glazing, open fire | 8,340 BTU (2,444 W) | 2,509 W | 7,362 W |
| 2015 new-build living room, 4.5m x 3.5m, two outside walls | 1,670 BTU (490 W) | 503 W | 1,474 W |
Turning BTU Into a Radiator
Look up radiators by their output at Delta T50 (sometimes written 75/65), in watts or BTU, and pick one at or above the figure. Delta T50 means the water in the radiator averages 50C above the room: water in at 75C and out at 65C, in a 20C room. If a room needs more than one radiator can give, split it between two. The calculator uses the room's own temperature, so a 21C living room needs a radiator rated slightly above its heat loss at Delta T50, and an 18C bedroom slightly below. For the heat pump sizes it also assumes a 5C drop across the radiator (our assumption). A condensing boiler turned down to a lower flow temperature also needs bigger radiators than the boiler figure. If you're replacing radiators anyway, we'd size them for 45C: the radiators are bigger and cost more, but they let a heat pump run at a low, efficient flow temperature.
Radiators for a Heat Pump
A heat pump runs the water cooler, which is what makes it efficient, so the same radiator gives out less. On Stelrad's correction factors, a radiator at a 55C flow gives a bit over half its Delta T50 output, and at 45C about a third. That's why the heat pump sizes are bigger. Our guide to whether you need new radiators for a heat pump goes further, and the heat pump size calculator does the whole house.
What It Can't Tell You
- It assumes the rooms next door are heated to the same temperature, so it ignores heat lost through internal walls and floors. A 21C living room next to an 18C hall loses a little more than shown, and a room next to or over a garage, passage or unheated extension needs noticeably more.
- Bathrooms and towel rails: choose "another temperature" and we'd set it a degree or two above your living room, since bathrooms are usually kept warmer. Towel rails have their own ratings, so check the maker's figure.
- It isn't built for conservatories or rooms with sloping ceilings, where the glass or the roof dominates.
- Windows are sized from the house's age, as a share of floor area. A room with patio doors, a bay or big windows loses more. The floor uses a typical house's figure, as the EPC method works on the whole building.
- It isn't a survey. For a heat pump, ask the installer for their room-by-room heat loss figures and compare; for a boiler, it's a reasonable starting point for choosing a radiator.