Emitters
Required radiator size (ΔT 50 rating) and oversize factor for a heat pump
- where
- mean water to room temperature difference (K)
- design flow temperature (°C)
- design return temperature (°C)
- room design temperature (°C)
- correction factor
- radiator exponent
- required ΔT 50 catalogue output (W)
- room design heat loss (W)
- oversize factor
- existing radiator output at design temperatures (W)
- existing radiator ΔT 50 output (W)
Explain this equation
Works out how big a radiator must be on paper (its ΔT 50 rating) to heat a room at heat pump temperatures, and checks an existing radiator against the room's heat loss.
ΔT and f are found as for the radiator output calculator. To deliver the room loss P_r at reduced temperature, the catalogue output has to be P_r divided by f. The oversize factor k is how many times bigger that is than the loss. The existing radiator's rating times f shows what it really gives now.
It is the radiator output law turned round to solve for the rating needed.
Sense check: at 45/39 °C in a 21 °C room f is about 0.32, so radiators need to be about 3.1 times the heat loss: a 2120 W room needs about 6.5 kW of ΔT 50 rating. A 2000 W radiator only gives about 650 W.
Watch out: raising the flow temperature reduces the oversize needed but lowers the heat pump's efficiency. It is usually a balance between the two.
Works out how big a radiator (by its ΔT 50 catalogue rating) a room needs at heat pump flow temperatures, and whether the radiator already there will do.
Enter your values to see the result.
Good to know
The oversize factor is how many times larger than the room heat loss the ΔT 50 rating must be: about 3.1 at 45/39 °C with a 21 °C room, about 1.9 at 55/47 °C. Every few degrees off the flow temperature improves heat pump efficiency, so it is often worth upsizing a few radiators rather than raising the flow temperature for the whole house.
Common mistake
Sizing on the flow temperature instead of the mean water temperature. With a 5–7 K drop across the radiator, the mean is several degrees below the flow.
Limits
Same power-law model as the emitter output card. Check the manufacturer’s own tables for the radiator type, and allow for covers, shelves and furniture.
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.