You asked for it!
It's probably easiest if I give it as the Excel Formula. It assumes that the manufacturers rad output is measured using 75/65/20.
(49.83*LN((D2-G2)/(E2-G2))/(D2-E2))^1.33
D2, E2 and G2 are cells which contain the following values:
D2 = Flow temp
E2 = Return temp
G2 = Room temp
LN means take the log to base e of the bit in brackets.
48.83 is a constant which depends on the starting point (75/65/20)
1.33 is a factor which varies slightly with type of rad (single/double/convector) but not enough to worry about.
Using the formula with temps of 75/55/21 produces a value of 1.208. So you need to multiply the calculated radiator size by this to get the size you need to buy if you want to run at those temperatures. In other words, if you need 1kW of heat you will have to install a 1200W rad.
If you have a 1kW rad already installed and want to workout its actual output at 75/55/21, then just divide 1kW by 1.208 and you get 828W.
