Emitters
Radiator output at heat pump flow temperatures (ΔT correction)
- where
- mean water to room temperature difference (K)
- flow temperature (°C)
- return temperature (°C)
- room temperature (°C)
- correction factor
- radiator exponent
- radiator output at these temperatures (W)
- catalogue output at ΔT 50 (W)
Explain this equation
Works out what a radiator really gives out at heat pump temperatures, from its catalogue rating at ΔT 50. Use it to check whether existing radiators will cope after a boiler swap.
The average of flow and return is the mean water temperature. Its difference from the room, ΔT, drives the heat out. Catalogues quote output at ΔT 50. The exponent n, about 1.3 for panel radiators, reflects that convection gets stronger as the radiator gets hotter, so output falls faster than ΔT. The factor f scales the catalogue figure.
BS EN 442 tests radiators at several temperatures and fits output to a power law in ΔT; this is that law.
Sense check: 45/39 °C flow and return in a 21 °C room gives ΔT 21 and a factor of about 0.32, so a 1000 W radiator gives about 320 W.
Watch out: use the manufacturer's n if you have it. At low ΔT small errors in temperature make a big difference to output.
Converts a radiator’s catalogue output at ΔT 50 to what it gives at the lower water temperatures a heat pump runs at.
Enter your values to see the result.
Good to know
ΔT is the mean water temperature, (flow + return) / 2, minus the room temperature. At 45/39 °C flow/return and 21 °C room, ΔT is 21 K and a panel radiator gives about a third of its ΔT 50 output. The exponent is about 1.3 for steel panel radiators (BS EN 442 test data); fan convectors are nearer 1.0–1.1 and underfloor heating close to 1.1, so use the manufacturer’s figure for other emitters.
Common mistake
Using a linear ratio (ΔT / 50). At ΔT 21 that gives 0.42 instead of about 0.32, overstating the output by around 30%.
Limits
The power law fits BS EN 442 test data over normal ΔT ranges. Outputs at very low ΔT (under about 10 K), with poor airflow, covers or shelves, or with unusual flow rates, can fall further.
Sources
- Heat Emitter Supplement to the Domestic Heating Design Guide (MCS-hosted, issue 11/13): outputs scaled from mean water-to-air ΔT 50 K with exponent 1.30 for radiators; worked example 45/39 °C at 21 °C room gives factor 0.324
- Zehnder: Correction factors and ΔT conversion (ΔT = mean of flow and return minus room temperature; n coefficient 1.3 assumed)
- BS EN 442-2 Radiators and convectors: test methods and rating (output at ΔT 50 K; characteristic exponent n from test, about 1.3 for panel radiators)
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.