Radiator Output at Non-Rated Temperature

Catalogue output is a promise made at one temperature. European panel radiators are rated at ΔT 50 K (75/65/20 °C), North American fin-tube at 180 °F water in 65 °F air — about 110 °F of ΔT — and neither is what your system runs at on a mild Tuesday. Output does not scale linearly, because a radiator sheds heat by convection and radiation together; the empirical exponent n lands near 1.3 for panel radiators, 1.4–1.5 for finned baseboard, and about 1.1 for radiant floors, whose enormous low-temperature surface behaves almost linearly.

This exponent is the single reason condensing boilers and heat pumps are hard to retrofit. Drop a radiator from ΔT 50 to ΔT 30 — say by running 45 °C water so the boiler can actually condense — and output falls to (30/50)^1.3 = 0.515, barely half. A 1,500 W radiator becomes 772 W, and the room goes cold unless you double the emitter surface. Run it the other way to size retrofits: if you need 4,000 BTU/hr from a baseboard rated 6,000 at ΔT 110 °F with n = 1.35, the required ΔT is 110 × (2/3)^0.74 ≈ 81 °F, meaning roughly 146 °F water — comfortably inside a heat pump's range, which is the calculation that decides whether a retrofit is possible at all.