Radiator Size for a Room

Room heat loss (W) ≈ room volume × a factor of 30–60 W/m³ depending on insulation. Radiators are rated at ΔT50 (mean water 70 °C in a 20 °C room); at lower flow temperatures output falls by a factor of (ΔT ÷ 50)^1.3, so the radiator must be bigger for a heat pump or condensing boiler.