Roomtherm

Radiator size for your room and flow temperature

A 4.9 × 2.7 m bedroom in an older house needs about 1,425 W of heat. If your radiators run at ΔT30, look for one rated 2,769 W at ΔT50, the figure in the catalogue: at ΔT30 a radiator gives 51 % of its rating.

Updated

Measure your room — these are examples.

What are you sizing?
Units

Heat to look for

1,425 W 4,862 BTU/h

Radiator rating to look for at ΔT50: 2,769 W (9,448 BTU/h)

At ΔT30 a radiator gives 51 % of its ΔT50 rating.

Roomtherm sizes heat output only. It does not cover wiring, circuits, gas or other fuel-burning heaters; those follow the heater's instructions and local code.

How well is it insulated?
Sizing radiators?

ΔT is the average of the flow and return temperatures minus the room temperature. Radiators are rated at ΔT50; a system that runs cooler gets less heat from the same radiator. Work your ΔT out from the temperatures:

Show the math
  1. 4.9 m × 2.7 m = 13.23 m² = 142.4 sq ft
  2. 142.4 sq ft × 10 W per sq ft = 1,425 W (rounded up)
  3. 1,425 W ÷ 0.29307 = 4,862 BTU/h
  4. (30 ÷ 50)^1.3 = 0.51475 → 1,425 ÷ 0.51475 = 2,769 W at ΔT50 (rounded up)

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Radiator rating to look for, by room

Each row is a room's heat from King Electric's rates (6 W per sq ft for a newer, insulated home; 10 W for an older one), then the ΔT50 rating that gives that heat at your system's ΔT. The room sizes are examples; measure yours and use the calculator above.

Heat needed and ΔT50 rating to look for, in watts
RoomInsulationHeat needed Rating if run at ΔT30Rating if run at ΔT40Rating if run at ΔT50
Box room2 × 2.5 m, 5.0 m² older 539 W 1,048 W721 W539 W
Box room2 × 2.5 m, 5.0 m² newer 323 W 628 W432 W323 W
Single bedroom2.7 × 3 m, 8.1 m² older 872 W 1,695 W1,166 W872 W
Single bedroom2.7 × 3 m, 8.1 m² newer 524 W 1,018 W701 W524 W
MCS 021 bedroom4.9 × 2.7 m, 13.2 m² older 1,425 W 2,769 W1,905 W1,425 W
MCS 021 bedroom4.9 × 2.7 m, 13.2 m² newer 855 W 1,661 W1,143 W855 W
Double bedroom3.5 × 4 m, 14.0 m² older 1,507 W 2,928 W2,015 W1,507 W
Double bedroom3.5 × 4 m, 14.0 m² newer 905 W 1,759 W1,210 W905 W
Living room4 × 5 m, 20.0 m² older 2,153 W 4,183 W2,878 W2,153 W
Living room4 × 5 m, 20.0 m² newer 1,292 W 2,510 W1,727 W1,292 W
Kitchen-diner5 × 6 m, 30.0 m² older 3,230 W 6,275 W4,318 W3,230 W
Kitchen-diner5 × 6 m, 30.0 m² newer 1,938 W 3,765 W2,591 W1,938 W

Why the rating has to be bigger

Radiators are tested and sold at one temperature difference. Zehnder puts it plainly: In accordance with EN442 we publish our radiator outputs at ∆T=50ºC. Run the same radiator cooler and it gives less. The usual correction is output = rating × (ΔT ÷ 50)^1.3; Zehnder's calculator assumes an n coefficient of 1.3, and Stelrad's book says Average coefficient of 130 is used.

At ΔT40 the factor is 0.748, so a radiator gives 75 % of its rating; at ΔT30 it is 0.515, about half. To get the room's heat at a lower ΔT, divide by the factor: 1,425 W ÷ 0.51475 = 2,769 W, rounded up.

Manufacturers print the factor rounded. Stelrad's 2020 book gives Output at ∆t30 = 2000 x 0.515 = 1030 Btu/hr, and its correction-factor page uses 0.513 for the same step. The formula gives 0.51475, so the 2,000 Btu/hr radiator gives 1,030 Btu/hr. Roomtherm computes the factor rather than reading it from a table.

Working out your ΔT

ΔT is not the boiler setting. Zehnder's definition: (∆T is calculated as (Flow temp + return temp)/2 – room temp). With water leaving at 55 °C, coming back at 45 °C and a 20 °C room, the mean water temperature is 50 °C and ΔT is 30. Type your three temperatures into «Sizing radiators?» in the calculator and it fills ΔT for you.

How the room's heat is estimated

The heat figure comes from King Electric: approximately 6 watts per square foot for newer homes and 10 watts per square foot for older homes, 2 W more per sq ft in an extreme cold climate, and 50 % more for rooms with large windows. Those rates were written for US houses; using them for UK and European rooms is our assumption.

A room-by-room heat-loss calculation can differ. MCS 021, a heat emitter guide for heat pumps, works through a poorly insulated 4.9 × 2.7 m room and gets Room heat loss: 1671W; the older-home rate gives 1,425 W for the same floor. If you have a heat-loss figure from an installer, use it in place of ours: the rating step is the same.

MCS 021 also converts ratings with its own factors (2349 x 0.825 = 1938W), a different method from the exponent above, so its radiator sizes and ours will not match to the watt.