What size Radiator, or is it just preference?

[diybristol]

I am choosing a radiator but unsure if to go for Single, Double or P+

I need 3774 BTU for the room and the window is 1800mm wide, is it usual to do this full length or half etc? or do plumbers work out a certain size which suits?

Cost is always an issue of course

http://www.radcalcs.com I found out how to work out BTU here

 

[kevplumb]

the output is in the spec look here

 

[diybristol]

Ok, I see now!

So it is determined by the BTU, so they I just need to choose P+/Single or Double and the size will differ on them a fraction?

Is that site I mentioned good enough to choose BTU???

 

[kevplumb]

probably as good as any

 

[ChrisR]

I haven't checked their figures but they appear to give no specific allowance for air changes, which usually amount to something around a third or quarter of the room's requirements. It will be high in a kitchen or bathroom and low in a bedroom. They bracket bedrooms and kitchens together so I would be suspicious.
Remember that if a radiator is a bit big a trv will still control the room temperature.