Radiators

You can only find old data sheets and go from there, scaling as appropriate

I don't know about (a), but I can give some approx figures for (b) - based on the calculations which I did in the 1960's for my previous house!

At that time, I was using 182 BTU/Hr per square foot, based on a 100 degF difference between mean radiator temperature and room temperature.

If my calculations are correct, that translates into about 573 watts per M^2 for a delta T of 55 degC - based on the *total* (both sides) area of the radiator.

On that basis, a radiator measuring 1000 x 600 (total area 1.2 M^2) would produce about 688 watts - compared with about 1100 watts produced by a modern finned radiator of the same size.

A delta T of 55 corresponds to flow/return temperatures of 80/70 and a room temperature of 20. For any other delta T value, the radiator output needs to be scaled up or down accordingly - slightly non linearly, but linear is ok for a first approximation.