In article ,
F news@nowhere writes:
I'm in the process of renovating the last room in the house (thank
goodness!) and need to replace the 620mm x 1415mm single radiator as it
is 42 years old and looking a little dated. It may also be following its
friend from another room by developing a pinhole leak.

The room is 2.3M high, 3.33M wide and 4.11M long with a 2.43M2 double
glazed window. Two of the walls are external walls with cavity wall
insulation, the room underneath is heated and the loft above it has
~170mm of insulation. If I enter all that into the BTU calculator at
http://www.heatandplumb.com/radCalcs.html it would seem that I need a
radiator with an output of 2018BTUs. Single radiators of a similar size
(Stelrad K1 Compact, 600mm x 1400mm) to the one I am replacing are
listed at 4681BTU.

Has the insulation I have installed really reduced the heating I need to
provide to less than half? Or am I missing something?

Possibly.
Do the same check on other rooms in the house.
If they are all similarly over-spec'ed, you really need to do the same.

Other factors are that radiator outputs are often given using unreasonably
high delta-T values. What delta-T is yours specified at, and what temp do
you actually run your boiler?

Have you got a condensing boiler? If so, oversized radiators can make for
more efficient operation (providing they're all similarly oversized).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]