Sheet 06 · Heating and damp
What size radiator do I need?
Radiators are sold by the BTU, a unit of heat the Victorians would recognise, and the number on the box is for a boiler running hot. That is two ways to buy the wrong one before you have measured anything. The sum here is the same one the heater sheet uses: the room’s size, how much heat it loses, the lift from a cold night to a warm room. It comes out in BTU because that is what the box says, and in watts because that is what the radiator actually does.
Measure the room. Be honest about the walls. Then buy the radiator whose rated output is at least the number, or two that add up to it, and read the Delta T bit before you do, because that is where people get caught.
1Work it out
1,848 watts. 33.6 cubic metres of living room held at 21 °C on a cold day. Buy a radiator whose rated output (at Delta T 50, the figure on the box) is at least this, or two that add up to it.
On a heat pump, or a boiler turned down to a low flow temperature, the same radiator gives out about half its box figure. To get 1,848 W out of it at Delta T 30 you would need one rated about 3,588 W (12,236 BTU/h) at Delta T 50.
Get a type 22 double panel radiator on Amazon.co.ukor search Amazon.co.uk for “type 22 double panel radiator 600mm”
2BTU, watts, and why both
The sum is the one the radiator trade publishes for when nobody has surveyed the house: the room’s volume in cubic metres, times a figure in watts per cubic metre for what the room is (a living room wants more than a bedroom, a bathroom more again), then a bit on for poor insulation, big or single-glazed windows and north-facing or exposed walls, and a bit off for a modern, well-insulated room. A draughty Victorian room needs about a third more than the same room in a new build, which is why ‘what size heater for a 4 by 4 room’ has no single answer and this sheet asks about your walls.
The figures assume the design day the trade sizes for: 21 degrees inside a living room, 18 in a bedroom, 22 in a bathroom, and about three below freezing outside, so the heater copes on the worst night in January and not the average one. Not sure about the insulation? Knock it down a notch. A heater that is too big just clicks off sooner.
3Delta T 50, the catch
The output on the box is measured with water 50 degrees hotter than the room going through the radiator, which an old boiler at 75 degrees flow will do. A condensing boiler set up properly runs cooler, and a heat pump runs cooler still, around 45 degrees flow, and at that temperature the same radiator gives out roughly half its box figure. So if the house is on a heat pump, or you have turned the boiler flow down to save gas, size for the lower output: the sheet prints that figure under the main one. It is why a heat pump install comes with bigger radiators. Not because the pump is weak, because the water is cooler.
4Type 11, 21, 22
The type number is the panels and the fins. Type 11 is one panel and one set of fins; 21 is two panels and one set; 22 is two and two. A type 22 gives nearly twice the heat of a type 11 the same size on the wall, which matters when the wall under the window is the only wall there is. Pick the type that gets you the BTU in the width you have, and check the depth clears the sill.
5Electric radiators
An electric radiator is a radiator without the plumbing: the same panel, an element inside, plugged into the wall. It is sized the same way, and the heater sheet covers the plug-in ones. It costs more per unit of heat than gas, and it is the answer in the one room the heating never reaches.
6Where to get it
- Get a type 22 double panel radiator on Amazon.co.ukor search Amazon.co.uk for “type 22 double panel radiator 600mm”Pick the width that gives the BTU. The listing states the output at Delta T 50.
- Get a 2 kW electric radiator on Amazon.co.ukor search Amazon.co.uk for “2000W electric radiator wall mounted”For the room the pipes never reached.
Those links go to Amazon.co.uk and earn this site a commission if you buy. The price is the same either way, and nothing on this sheet changes because of it.
7Where the numbers come from
- MIS 3005-D Heat Pump Design, Issue 1.0: Table 1 internal design temperatures (from CIBSE Guide A) and Table 2 external design temperatures, MCS. Living room 21, bedroom 18, kitchen 18, hall 18, bathroom 22 °C; outside, London minus 3.0 °C (the temperature beaten only 0.4% of hours) (read 17 October 2026)
- BTU and radiator size guide: watts per cubic metre by room type, and the adjustments, BestHeating. Living room 50 to 60 W/m³, bedroom 40 to 50, bathroom 60 to 70, kitchen 50 to 60; poor insulation +15 to 25%, large windows +10 to 20%, north-facing +10%, well insulated −5 to 10%; watts × 3.41 = BTU/h. No hall figure is published, so the sheet uses the bedroom one (read 17 October 2026)
- Heating calculator, advanced adjustments, Trade Radiators. Single glazing +10%; two outside walls +5%, three +10% (read 17 October 2026)
- This sheet’s own rules of thumb, stated where they are used, whichfits.uk (read 17 October 2026)
Data on this sheet last changed 17 October 2026.