Radiator size calculator
A sizing estimate. Radiator outputs are quoted at a specific flow temperature — a heat pump running at 45 °C gets far less from the same radiator than a boiler at 75 °C.
Radiator sizing starts from room volume rather than floor area. A standard-construction living room needs about 40 watts per cubic metre; a 4 × 3.5 m room with 2.4 m ceilings therefore needs roughly 1,350 W, or 4,600 BTU per hour.
How to size a radiator
The trap in radiator sizing is the flow temperature the output was measured at. Manufacturers quote at Delta T 50, meaning the radiator is 50 degrees hotter than the room — a boiler flow around 75 °C. A heat pump running at 45 °C gives Delta T 25, and the same radiator then puts out barely half its rated figure. That is why converting a house to a heat pump usually means upsizing radiators, and why a sizing figure without a stated Delta T is not a figure at all.
Questions
Sizing works better on volume: about 40 W per cubic metre for standard construction, 30 for a modern insulated build and up to 70 for a poorly insulated one.