Radiator BTU Output by Size and Type

Delta T 50 (ΔT50) is the standard test condition where the average water temperature in the radiator is 50°C above the room temperature. For example, if the room is at 20°C, the average water temperature is 70°C (flow at 80°C, return at 60°C). Most radiator manufacturers quote outputs at ΔT50. Heat pumps operate at lower temperatures (ΔT30 or ΔT20), which significantly reduces radiator output — typically to 50-60% of the ΔT50 rating.

As a rough guide for UK homes: well-insulated rooms need approximately 40-50 watts (137-170 BTU) per m², average insulation needs 60-80 watts (205-273 BTU) per m², and poorly insulated rooms need 80-100+ watts (273-341+ BTU) per m². These figures should be adjusted for room height, number of external walls, window area, and floor level. A proper heat loss calculation is always recommended.

The type number indicates panels and convector fins. Type 11 (K1) has 1 panel and 1 convector fin set. Type 21 (P+) has 2 panels and 1 convector fin set. Type 22 (K2) has 2 panels and 2 convector fin sets, giving the highest output for a given length and height. A Type 22 radiator typically produces 60-70% more heat than a Type 11 of the same dimensions.

Yes, but you will need larger radiators because heat pumps operate at lower flow temperatures (typically 35-45°C vs 60-80°C for gas boilers). At ΔT25 (heat pump conditions), a radiator produces roughly 50% of its ΔT50 output. This means you may need to double the radiator size or switch to Type 22 or column radiators to compensate. Underfloor heating is ideal for heat pumps as it works well at low flow temperatures.

1 watt = 3.412 BTU/h. To convert BTU to watts, divide by 3.412. To convert watts to BTU, multiply by 3.412. For example, a radiator rated at 5,000 BTU produces approximately 1,465 watts. UK radiator manufacturers increasingly quote both BTU and watt outputs on their specification sheets.