Fitting radiator 22mm or 15mm in the run?

happyhero

Hi Can anybody advise me I am an ex-building services man and qualified electrician so this job is quite within my capabilities.

I am fitting a new radiator to my system and I will be teeing off the existing 22mm pipework just to feed one new radiator. Now I see the advantage if supplying 2 radiators, with keeping to 22mm as long as possible in the run and teeing off in 15mm, ie the 22mm will be better at supplying enough water to supply two rads when they tee off in 15mm pipe. But if I am coming from 22mm pipe is there any benefit staying with 22mm as long as possible in the run if I am supplying only 1 rad (a big one), or might I just as well drop straight to 15mm pipe straight away for ease of work?

I know the smaller pipe is cheaper but I have enough of both already to do it either way.

I know there is this thing where they compare water flow to electrical current or something but it always looses me.

I would guess it would be better to drop to 15mm straight away, instead of staying at 22mm for a while as it would get restricted down anyway where ever I change to 15mm so there would be no benefit staying with 22mm. Can anyone clarify?

I am sure there is a clever answer to this but I will need some guidance please, and maybe some reasons why to do it one way or the other if possible?

Mr_Grumpy_7

I would suggest using 15mm pipe to feed a single radiator. From your original post, you will be taking the feed for the new radiator from 22mm pipe. As the radiator valves are 15mm, I can see no advantage in using 22mm at all.

I seem to remember being told many years ago that 15mm pipe can be used to supply radiators upto 13000 BTU, not sure if this is true though.