22mm into 15mm and back to 22mm

[Graham Jones]

Hello,

What are the consequences of 22mm CH pipe going into 15mm and back again to 22mm before yet again going to 15mm for final run to radiators?

House currently has 22mm from boiler which runs under landing and under hall, from this is 15mm to rads.

As part of extension work we are having a new boiler which will be in a new location. Plumbers have 22mm from boiler which runs for about about 3m, then down into 15mm for new rads. But it looks as though they will then take this 15mm and connect it to existing pipework (22mm).

I estimate that the length of 15mm pipework between the two 22mm runs will be about 2m.

Is this OK?

Thanks,

Graham

[Graham Jones]

OK, I understand, but is it bad practice in general?

[RobertL]

How many rads will flow through the 15mm pipework. At each place where the pipework branches there is the possibility to use smaller pipes. A a general rule of thumb (amateur) I ran 22mm from the boiler to the branch point where there were 3 or fewer radiators on the branch. Then I dropped to 15mm Individual rads got 10mm pipe.

In fact, in my case (ordinary victorian end terrace) the size limit was becuase of the flow noise rather than thepressure drop. if the water velocity is too fast you hear the sound of it in the pipes and I wanted to avoid that.

Robert

[John Rumm]

IIRC there is a limit of a 6kW of power delivery on a 15mm pipe with a flow temp at 81 and return at 70 - a bit less at condensing boiler flow temperatures. So in general I would not be keen to have a significant number of rads hanging off a length of 15mm. However, a short length of smaller pipe will have a much lower overall effect than a long length.