How do you know that you don't get it wrong then Simon ? if you don't do a calc involving measuring the room & windows, checking the construction of the building etc how would you know? You, & I agree most other plumbers do guess-emate to size a rad & that includes no corrections to the rad table outputs which are based on a temp difference of 50deg C.
Do you know what I believe Sambotc, I think the hostile reaction we receive ever time this gets brought up (not just on this forum) has more to do with the fact that a lot of plumbers & so call heating engineers do not know how to do this, have no interest in finding out how it is done (even when it is reasonable easy & won't cost them either), they just keep on telling anybody who will listen that they have the 'power' to predict what the heat loss & rad size will be, based upon years of experience dating back to when BTU's where used & their plumber handed on this sacred knowledge. Even though as you say time s have moved on, designs, boilers, heat emitters, insulation etc etc have all moved on !!!!!!!!
