Catalog output is not real output
Radiator manufacturers publish a heat output measured under standard conditions. In Europe, the EN 442 standard uses water entering at 75 °C, leaving at 65 °C and a room at 20 °C, which gives a temperature difference, or delta T, of 50 K. In the US, cast-iron and baseboard ratings usually assume an average water temperature of 180 °F in a 65 °F room, a delta T of 115 °F. If your system runs cooler than that, the radiator gives less heat, often much less.
The correction formula
The heat output of a radiator follows a power law:
P = P rated × (ΔT / ΔT rated)^n
- ΔT is the mean water temperature, the average of flow and return, minus the room temperature.
- n is the radiator exponent, about 1.3 for panel and column radiators and about 1.4 for convectors and fin-tube baseboard, which lose output faster at low temperatures.
Because of the exponent, halving the delta T cuts the output by more than half.
Worked example
A panel radiator is rated at 1,500 W at ΔT 50. After installing a heat pump, the water flows at 55 °C and returns at 45 °C into a 20 °C room. The mean water temperature is 50 °C, so ΔT = 30 K. The correction factor is (30 / 50)^1.3 = 0.51, and the radiator delivers about 770 W, half its rating. If the room loses 1,000 W on the coldest day, you would need a radiator rated at about 1,940 W at ΔT 50.
Why this matters for heat pumps
A heat pump’s efficiency, its coefficient of performance, falls as the water temperature rises: running at 35 °C instead of 55 °C can cut electricity use by roughly 30%. The goal of a retrofit is therefore to heat each room at the lowest possible flow temperature. Many radiators in older homes are already oversized because they were sized for poor insulation or with generous safety margins. Upgrading only the undersized ones, often to double-panel models, is usually enough.
Getting the inputs right
- Heat loss: a room-by-room heat loss calculation, such as ACCA Manual J in the US, gives the design heat loss at the coldest outdoor temperature.
- Flow and return: a 10 °C (18 °F) drop between flow and return is typical for gas boilers; heat pumps often run a 5 °C (9 °F) drop.
- Exponent: when in doubt, 1.3 is a reasonable value for most radiators.
This simplified method uses the arithmetic mean water temperature. Engineers sometimes use the logarithmic mean, which gives slightly lower output when the flow-return difference is large.