How many radiator sections a room needs

From square metres to watts, and from watts to the number of sections. At a 45 degree flow the same radiator gives about a third of what it gives at 75, so the count changes.

Fill in the room and pick the radiator. The calculation runs in two steps, first the watts the room needs and then the sections that deliver them at the flow temperature you actually run.

1. The room to heat

Floor area (m²)

Ceiling height (m)

Degrees you want indoors

Winter climate

Insulation of the building

Windows

Walls facing outside

Which way the room faces

There is a roof or a cold loft above There is a garage, a cellar or an open passage below

Heat loss already known (W)

Optional field. If you fill it in, the room data above is ignored and the calculation starts from that number, handy when you already have a survey.

2. The radiator and the water inside it

Radiator type

Output of 1 section at ΔT 50 (W)

Exponent n

System

Flow (°C)

Flow to return drop (°C)

The output and exponent above are indicative catalogue figures and vary a lot between models. The real ones are on the radiator data sheet, under heat output at ΔT 50 K to EN 442, so copy them into the fields and the calculation becomes yours.

Sections already fitted

Optional field. Write how many sections that room already has and the result tells you whether they are enough, and the lowest flow temperature at which they still manage.

Where the watts come from

The heat loss is estimated with the volumetric method: volume × W/m³ coefficient × (design ΔT ÷ 25) × corrections. The coefficients run from 42 W/m³ for an uninsulated building from before 1976 down to 11 W/m³ for a recent low energy one, and they refer to a ΔT of 25 degrees, meaning 20 indoors and -5 outside. Transmission losses grow in proportion to the temperature difference, so the coefficient is rescaled linearly on the real ΔT: the same room on a mild coast (+5 outside) asks for about 60% of the watts it asks for in a continental winter. The outdoor figures in the menu are typical design values, and if you know the one used where you live you can type it in by choosing the last item of the list. The corrections are the classic sizing ones, 7% for every outside wall beyond the second, 5% more facing north and 5% less facing south, 12% for a roof overhead, 8% for a garage under the floor.

Why the same radiator gives far less on a condensing boiler or a heat pump

On a radiator data sheet the power is declared to EN 442 at ΔT 50 K, that is water entering at 75 degrees, leaving at 65, with the room at 20: mean water 70, minus 20 for the room, gives 50. Change the water temperature and the output does not fall in proportion, it follows a curve, Q = Q₅₀ × (ΔT ÷ 50)ⁿ, with n around 1.3 (1.33 for aluminium, 1.29 for cast iron, 1.30 for tubular steel; the exact value is on the data sheet). Here are the numbers that surprise everyone: at 55/45 with the room at 20 degrees the ΔT is 30 and the output drops to 51%, at 45/40 the ΔT is 22.5 and the output collapses to 35%. In practice you need twice the sections on a low temperature condensing boiler and almost three times as many on a 45 degree heat pump. That is not a fault of the heat pump, it is that the old radiators were chosen for much hotter water. (For anyone working in imperial figures, 1 kW is 3,412 BTU/h.)

The number to copy from the data sheet

Output per section varies a lot between models, even for the same material and height: two aluminium radiators at 500 mm centres can be rated 122 W and 160 W. The values in the menu are catalogue averages, there only so you start from something sensible. The real figure is always published as heat output at ΔT 50 K to EN 442-2, and it usually comes with the exponent n. Two traps to avoid: the centres figure (350, 500, 700) is the distance between the tappings, not the overall height, which is about 8 cm more; and flat steel panel radiators are not counted in sections but in length, so for those it is better to work from the total watts of the model rather than from an output per section.

If the radiators are already there, read the calculation backwards

Fill in the sections already fitted and the tool inverts the formula, telling you the lowest flow temperature at which those radiators still cover the room. That is the question that really matters before changing the heat source, because an oversized radiator is not waste but an advantage: it lets you drop the water temperature, and a condensing boiler only truly condenses while the return stays below 55 degrees, while a heat pump gains roughly 2.5% in consumption for every degree of flow temperature saved. If it turns out that at 45 degrees you are short by a few sections in one room only, it is usually cheaper to extend that radiator than to give up on a heat pump for the whole house.

Declared limits and common mistakes

This is a sizing estimate, not a heat loss calculation to EN 12831: it knows nothing about thermal bridges, air changes, the real build-up of the walls or the flow rate your pump can push through that circuit. It has to be done room by room, and the sum of the rooms is not the size of the boiler, which also depends on hot water and on how much runs at once. The most frequent mistakes: counting a wall shared with a heated flat as an outside wall (it is not), assuming a closed thermostatic valve saves money when the room then never reaches temperature, and boxing the radiator in behind a shelf, a long curtain or a cover, which easily costs 10 to 20% of the output. For a new system, or before moving to a heat pump, have the numbers checked by a heating engineer.