New rads - 15mm or 10mm

9 Apr 2012
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United Kingdom

Am adding 3 new radiators in an extension to an existing sealed system (combi boiler) that uses 10mm pipe at the radiators.
I have traced the existing rads 10mm pipe back to the 15mm flow and return pipes and my question is this.
Am I best teeing off the 15mm flow and return pipes with 10mm pipe to the rads individually (about a 6 meter run) or teeing off with 15mm pipe and then reducing to 10mm closer to the rads, effectively extending the 15mm flow and return?

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To tee off a 15mm circuit to supply 3 new rads, even if it's all done in 15mm is asking for trouble. You will be hard pressed to make them heat up as you would want. 3 extra rads should be connected to a 22mm main circuit and balanced correctly.
Thanks for the quick reply.

Would I be best teeing off the 22mm with 15mm or 10mm, or extending the 22mm over to the extension and then teeing off?
Put the run in with 22mm then tee of to each rad in 15mmyoull get better flow plus the system can be flushed if you ever need to
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15mm all the way to the radiators.

It will probably work for the new radiators. If not you'll need to re-pipe
all the way back to the 22mm flow and return pipes.

But I've seen whole house floors running on 15mm and the house has worked heated up fine.

Thermostatic valves on the radiators will help too.

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