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How do I work out what size radiator I need?
The size of radiator a room needs is the heat the room loses on a cold day, in watts. The calculator above works it out as Stelrad's own calculator does. It adds up the heat through each outside wall and its windows, the floor and the ceiling, and the heat the air carries off.
- The room's temperature: 21°C, and 22°C in a bathroom, the figures Stelrad's calculator uses.
- London's cold day: -3°C outside, the design temperature CIBSE gives for Heathrow, which Stelrad uses too.
- The answer: the output to look for on a radiator's data sheet, in watts and BTU. Choose the next size up. In a big room, the figure can be shared between two radiators.
A 4 by 3.5m living room, for example, needs about 950W (3,230 BTU). It's on a ground floor, with one solid brick outside wall, older double glazing and a room above. With a system running at 55°C, it needs about 1,850W (6,300 BTU).
Sources: Stelrad's basic heat loss calculator and heat loss programmes, MCS's design conditions, and The Radiator Company.
Worth knowing
The room's length and width, its outside walls and its windows are what the figure is worked out from. Ring us on 020 8064 0067 and we'll ask about the room.
What is BTU, and how many watts is it?
BTU, the British thermal unit, is a second way of giving a radiator's heat output. One watt is about 3.4 BTU an hour, or 3.412 by NIST's conversion table. A 1,000W radiator gives about 3,412 BTU. Radiators are sold in both, and the calculator gives both.
What does ΔT50 mean on a radiator?
ΔT50 on a radiator means its output was measured with the water 50°C warmer than the room. In a 20°C room, that's water at about 70°C. The ΔT50 rating is what radiators are sold by, under the standard BS EN 442, and Stelrad's calculator works at ΔT50 too.
A cooler-running system gets less heat from the same radiator. At ΔT30, with the water at about 55°C, a radiator gives about half its ΔT50 output, so the room needs one about twice the size. Stelrad's correction factor puts it at 0.513 of the ΔT50 output. Makers now give each radiator's ΔT30 output as well.
A new or fully replaced heating system in England should be designed to run at 55°C or lower. How a boiler's flow temperature works.
What makes a room need a bigger radiator?
A room needs a bigger radiator the faster it loses heat. The walls, the glass, the roof above and an open chimney make the biggest difference.
Solid walls
A solid brick wall loses about four times the heat of an insulated cavity wall. Stelrad gives 2.10 watts a square metre for each degree for solid brick, and 0.50 for an insulated cavity.
Single glazing
Single glazing loses about three times the heat of newer double glazing, 5.0 against 1.7 in Stelrad's figures.
A loft with little insulation
A top-floor room under a loft with little insulation loses about three times the heat through its ceiling of one under an insulated loft. Stelrad's figures are 2.30 and 0.71.
An open fireplace
An open fireplace or chimney lets warm air out all the time. SAP, the government's method for a home's energy rating, counts 80 cubic metres an hour for one. For the living room in the example above, that adds about two thirds.
High ceilings
A high ceiling adds wall and air to heat, so a room of the same floor area needs more.
More outside walls
Each wall that faces outside adds to the heat lost, while a wall to a heated room loses next to nothing. A corner or end room needs more than a middle one.
Which way the room faces isn't counted. Stelrad's calculator and MCS's figures add nothing for a north-facing room.
Sources: Stelrad's basic heat loss calculator and its guide to a house's heating needs, SAP 10.2, and MCS's design conditions.
Can you fit a bigger radiator for me?
Yes, we fit new radiators, and swap an old radiator for a bigger one, all over Greater London. Ring us on 020 8064 0067 and we'll ask about the room, the radiator there now and how warm it gets. Central heating repair: radiators, pumps, valves and controls.
A radiator that stays cold at the bottom or in the middle may not be too small. Sludge can block it, and a power flush clears it. Radiators not heating up, and what it means.
Do you cover my part of London?
Yes: we fit radiators and repair heating all over Greater London, in every borough north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
Questions about radiator sizes
How many BTU do I need for a room?
The BTU a room needs depends on how fast it loses heat, so two rooms of the same size can need very different radiators. A 4 by 3.5m living room needs about 3,230 BTU (950W), with a solid brick outside wall, older double glazing and a room above. A 3 by 3m bedroom under an insulated loft needs about 2,180 BTU (640W). The calculator at the top of this page works out your room's figure.
Do I need bigger radiators for a heat pump?
A heat pump usually needs bigger radiators than a boiler, because it heats the water less, to about 55°C or lower. At ΔT30, a radiator gives about half its ΔT50 output, Stelrad's correction factor shows. A heat pump is designed from a room-by-room heat loss calculation, which MCS's own calculator works out.
Where do I find a radiator's output?
A radiator's output is on its data sheet or its maker's page, in watts and BTU, at ΔT50, the rating radiators are sold by. Makers must give each radiator's output at ΔT30 too, in its declaration of performance, MARC says. A radiator for a cooler system can then be checked against that figure.
Does a towel rail heat a bathroom as well as a radiator?
A towel rail usually gives less heat than a radiator of the same size. A chrome one gives out about a third less than a white painted one, Myson says. Check the rail's output against the figure the calculator gives for the bathroom.
Is one big radiator better than two smaller ones?
Two smaller radiators can heat a room as well as one big one. The room's figure can be shared between them, Stelrad and The Radiator Company say. Add their outputs together to check they meet it.
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