Radiator output calculator
Radiators are catalogued at a 50 K temperature difference. At a heat pump flow temperature they emit far less, and the correction is a power law.
Output at your flow temperature
501W
- Correction factor
- 0.3338
- Actual temperature difference
- 21.50 K
- Shortfall against the room heat loss
- 699
- Delta T 50 rating you need
- 3595
- How much bigger the radiator must be
- 2.397
- Water flow needed
- 1436.117 L/min
Show the working
- Rated output at delta T 50 = 1500
- Flow temperature = 45
- Return temperature = 40
- Room temperature = 21
- Radiator exponent = 1.3
- Room heat loss = 1200
- mean_water = (45 + 40) / 2 = 42.5
- Actual temperature difference = 42.5 - 21 = 21.5
- Correction factor = 21.5 > 0 ? (21.5 / 50)^1.3 : 0 = 0.333 817
- Output at your flow temperature = 1500 * 0.333 817 = 500.726
- Shortfall against the room heat loss = 1200 - 500.726 = 699.274
- Delta T 50 rating you need = 0.333 817 > 0 ? 1200 / 0.333 817 : 0 = 3594.78
- How much bigger the radiator must be = 1500 > 0 ? 3594.78 / 1500 : 0 = 2.396 52
- Water flow needed = (45 - 40) > 0 ? 500.726 / (4184 * (45 - 40)) * 60000 : 0 = 1436.12
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- correction factor = (actual delta T / 50)^n, with n about 1.3 for panel radiators
- actual delta T = mean water temperature - room temperature
At a 45/40 flow and return into a 21 degree room, delta T is 21.5 and a radiator gives about 33% of its rated output. That is the single biggest surprise in heat pump retrofits.
The exponent is 1.3 for most panel radiators, 1.1 to 1.2 for underfloor and up to 1.4 for convectors.