Short answer: a room loses heat two ways. Through the fabric: each wall, window, floor and ceiling passes heat at a rate set by its area and its U-value. Through the air: every time the room’s air is replaced with outside air, the heat in it goes too. Add those up per degree of temperature difference, multiply by the difference you are designing for, and you have the heat the radiator must replace on the coldest day.
Our guide on how many BTUs a room needs works three rooms through that arithmetic. This one explains the physics behind each term, so the breakdown the BTU Calculator shows you means something.
U-values: the fabric term
A U-value is a rate: watts of heat passing through one square metre of a building element for every degree of temperature difference across it, written W/m²K. Multiply by the area and by the temperature difference and you have watts.
Lower is better. The values the BTU Calculator uses for walls make the point:
| Wall construction | U-value |
|---|---|
| Timber frame | 0.4 |
| Insulated brick cavity | 0.92 |
| Uninsulated brick cavity | 1.5 |
| 220 mm solid brick | 2.1 |
| 105 mm solid brick | 3.0 |
And for glazing: a double-glazed unit in a wood or plastic frame at 2.9, single glazing in a wood frame at 6.4. A square metre of single glazing loses more than twice what a square metre of double glazing does, and more than six times what an insulated cavity wall does. This is why the glass share of a breakdown is so often the largest, despite windows being a small fraction of the room’s surface.
These are typical values for each construction type, which is what the simplified method uses. A full calculation would use the actual U-values of the building, from its construction details or from measurement.
The ventilation term
Air holds heat. Replace a room’s air with cold outside air and you have to heat the new air up. The term for this is:
floor area × height × 0.33 × air changes per hour
The 0.33 is not a fudge factor. A cubic metre of air weighs about 1.2 kg and takes roughly 1,005 joules to warm by one degree, so about 1,206 joules per cubic metre per degree. Spread that over an hour, 3,600 seconds, and you get 0.335 watt-hours per cubic metre per degree. Multiply by the room volume and by how many times an hour the air is replaced, and the result is the ventilation heat loss per degree of difference.
The air change rate is the room-type multiplier in the calculator: 1.0 for a living room, 0.5 for a bedroom, 1.5 for a kitchen or hallway, 2.0 for a bathroom. Bathrooms and kitchens change their air fastest, through extractor fans and opened windows, which is why their ventilation share is high and why a bathroom radiator is larger than its floor area suggests.
The temperature term
The fabric and ventilation losses are per degree. To turn them into watts you multiply by the temperature difference the radiator has to overcome on the design day: inside target minus outside design temperature.
The trade’s simplified method, which the BTU Calculator uses, takes the room’s target temperature as that multiplier: 22 °C for a living room, 19 °C for a bedroom, 17 °C for a kitchen or hallway, 23 °C for a bathroom. Implicitly that treats the outside design temperature as around freezing, which for much of the UK is a reasonable planning assumption and errs on the safe side for the milder south. A full calculation uses the published external design temperature for the location, which can be several degrees below zero in the north and inland.
The floor and ceiling terms
The ceiling term is the floor area times the U-value of what is above. A heated room above contributes nothing. An uninsulated roof space at 2.7 contributes a great deal, more per square metre than an uninsulated cavity wall; with 100 mm or more of loft insulation the figure drops to 0.35. If a breakdown shows a large ceiling share, the answer is in the loft, not the radiator.
The floor term uses coefficients rather than a single U-value, because heat loss through a ground floor depends on how much of its edge is exposed: a larger floor loses proportionally less through its perimeter, and a room with one external wall exposes less edge than one with three. That is why the calculator’s floor coefficient falls for rooms over 16 m² and for rooms with a single external wall.
Reading the breakdown
The BTU Calculator shows the share of each term. What the shares mean:
- Glass dominating. The windows are the weak point. Glazing upgrades will move the number more than anything else.
- Walls dominating on a solid-walled house. Internal or external wall insulation is the structural fix; until then, the radiator carries it.
- Ceiling high. Loft insulation, cheap and decisive.
- Ventilation high in a living room or bedroom. The room is draughty; look at the window seals, the floor and the chimney.
- Floor high. A suspended timber floor over a ventilated void; insulation between the joists.
The breakdown is a to-do list ordered by return. Fix the top item and re-run the calculation; the radiator that was needed may shrink by a size.
Where the quick method stops
The simplified method is the right tool for choosing a radiator for an existing room on a boiler system: it is quick, it uses information a homeowner has, and its assumptions err generous. It is not a substitute for a full heat loss calculation to BS EN 12831, which uses the building’s actual U-values, the local external design temperature, thermal bridging at junctions, and a measured or properly assumed ventilation rate.
Three situations call for the full method:
- A heat pump. Installers working to the MCS standard carry out a room-by-room heat loss calculation, because a heat pump running at low flow temperature has no margin to waste. Our guide on sizing radiators for a heat pump explains why the radiators come out so much larger.
- Sizing a boiler or a whole new system. The sum of the room figures is a sanity check on the space-heating demand, and the BTU Calculator’s whole-house total gives it. Boiler sizing also needs hot water demand and system losses, and is an engineer’s job.
- A building that is unusual: very tall rooms, large glazed areas, a conservatory, an unheated space next door.
Use the quick method to understand the room and to check the engineer’s figure looks sane. Use the engineer for the design.
U-values, coefficients and multipliers are as implemented in BTU Calculator v1.1. The derivation of the 0.33 ventilation constant uses standard properties of air at room temperature. Plan CD is not affiliated with any heating manufacturer, MCS or any certification body.