The stoves at Tewkesbury Abbey

It occurred to me that the two heaters in Tewkesbury Abbey must mainly send out thermal energy by infra-red radiation like my log burner. They are, after all, painted black. So each heater is a black body radiator and as such can be modeled by Stefan's Law: output power = sigma x surface area x (absolute temperature)^4. Suppose a heater is 1 metre tall and has 30 fins on it and that each fin is 5 cm deep. Each fin has two sides so will have an area of 0.1 square metres exposed. Total area = 30 x 0.1 = 3 square metres. Add another square metre for the top and other exposed parts and we have a surface area of 4 square metres. Estimate a surface temperature of 60 degrees Celsius. It can't be too hot or it would have to be better caged. That is an absolute temperature of 333 Kelvin. Put into the equation and I get an output power of 2800 Watts. It doesn't seem unreasonable to think of each heater as 3 kW.