On Mon, 17 Sep 2007 13:47:01 +0100, "TheScullster"
wrote:

It depends on the size of the rad(s) in the lounge, and whether there are
any upstairs rads connected to the same 15mm leg. The general rule of
thumb is that 15mm pipe will support rads up to a total of about 6kW.
--
From experience I would only use that rule of thumb for single large rads or
2 rads back to back not far from the 22mm main.
Personally I would run 22mm close to rad take offs and only use long
convoluted 15mm runs where there was no realistic alternative.


I don't know the answer but in our house it's all 15mm. The house was
built in the 1970s and the pipework has minimal insulation. In fact
the pipes are just beneath the floorboards, which means there is no
room for me to retrofit any insulation (unless anyone has any ideas?
help please!)

Despite this we are warm enough and it can get too warm at times. Of
course we will have a totally different boiler and radiators than you
so what works for us many not work as well for you.

If I were doing it I think I would use 22mm under the floorboards and
use 15mm for the "spurs" to the radiators. If you are dropping pipes
just to go to one rad. I would just use 15mm for the visible section.
At least you could use chrome pipe to make it look prettier. I think
visible 22mm would not be aesthetically pleasing. Of course, if these
pipes run to other rads, it becomes a different story.

Is the 22mm overkill? After all, don't some rads use even smaller
pipes than 15mm? I bought a 15mm rad valve recently (tool station) and
was surprised that the bore in the valve was so small: less than 5mm I
would guess. So wouldn't running 22 to it would be redundant because
the bottle neck is at the valve? OTOH I would think it better to be
overspecified and have reserve in your system rather than run it to
its limit.