Different pipe sizes in CH system

[albapa]

Hi all,

I need a bit of advice regarding mixing pipe sizes in our CH system. The house uses a Worcester 27 CDI combi boiler, the main feed and return pipes are 22mm. As far as I can tell, the upstairs rads are all served by 15 mm copper pipes run under the floorboards.

Downstairs, there are currently 2 radiators, both in the living room (so no rads in the hall, toilet or kitchen), the pipes are 10mm running inside the wall and in the screed floor. I am planning to upgrade one of the rads to about 8500 BTU/h - from what I read here and elsewhere this should be fine with the existing 10mm pipework. I would like to remove the other rad, extend the existing pipework and connect a 1800 BTU/hr and a 1200 BTU/hr rad (hall and toilet). Would it be a good idea to connect 15mm pipes to the 10mm pipe or should I stick with 10mm all the way? Parts of the pipe will be surface mounted and isn't 15mm better for that?

Another question: to install a rad in the kitchen, I would like to replace the 22mm elbows (close to the boiler) with 22/22/15 tees and run 15mm pipes to the kitchen. Would that be a problem?

I can see there will be quite a bit of balancing to get everything right, but apart from that, those this sound a sensible setup? Thanks in advance for any advice.

 

[Riley]

If I'm understanding you correctly you are wanting to run two radiators on the same flow and return in 10 mm. Rule of thumb is that 10 mm will struggle to carry more than 1.5 kW of heat energy so if you are looking to run two radiators off of the same length I think you will be disappointed. If I were you I would run the lot in 15 mm with each radiator having its own flow return going back to the main 22 mm pipework

 

[albapa]

Yes, correct - although these are two small rads, 3000 BTU/h total which is less than 1 kW.

I guess if I extend the 10mm pipework using 15mm pipes, it won't increase the heat carrying capacity, but would this cause problems (such as turbulence, air traps whatever I don't know about)?

 

[albapa]

Also, would there be any problems if I branch off a 15mm feed/return pair from the main 22mm pipes, quite close to the boiler?

 

[Riley]

My point was more the 8500 Btu radiator but I am having trouble understanding if you mean that is on 15 mm pipe or whether it is run on 10 mm pipe. I would still never power two radiators from the same flow and return in 10 mm though Regarding teeing in by the boiler it will work but you might find the radiator warms up every time you heat Hot water

 

[albapa]

Thanks. You say 10mm pipework branched in two will never power two radiators, even if the combined thermal output is smaller than the heat carrying capacity of the 10mm pipe - are there other consideration then?

Regarding my question about transitioning, sorry for being unclear. Ideally I'd leave a large part of the 10mm pipework in place, but then finally feed the radiators using 15mm pipes (which will be surface mounted), by using 10-to-15mm reducers. Would this cause problems?

Finally, regarding my other unrelated question, teeing after the boiler should be okay then because it's a combi boiler.

 

[Chalked]

Branching off 10mm will not work, you will get one rad hot, but not both at the same time.

 

[albapa]

Aha! Thanks! So in other words, balancing such a system is not just difficult, but impossible.

How about having a 15mm pipe and branching it into two?

 

[ShaunCorbs]

Aha! Thanks! So in other words, balancing such a system is not just difficult, but impossible.

How about having a 15mm pipe and branching it into two?

15mm is fine