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15mm or 22mm pipe work for a condensing boiler?

Postby Kitsongod » Mon Mar 16, 2009 10:07 pm

I need a new boiler but getting mixed messages on the pipework. Our current combi boiler is fitted to 15mm central heating pipework. British Gas have advised that all of the pipework in the house must be changed to 22mm. Another plumber has told me that if I increase at least 4m of pipework to 22mm, then that should help to ease the flow. And the third quote, the plumber reckoned running a 22mm pipe from the boiler (approx 10cm in legth) to 15mm pipework will help. So confused - any ideas?

We are considering Worcester Bosch 30si / Viessman / Glow Worm Ultraicom 30ci.
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Simply Build It

Postby htg engineer » Tue Mar 17, 2009 11:35 am

Ideally you want 22mm flow and return with 15mm branches to each radiator.

If they're the only boilers you're considering, go for the Worcester or Viessman

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Postby plumbbob » Wed Mar 18, 2009 8:46 pm

Yup as Htg says. 22mm should be used in the main feed and return pipes where the flow supplies more than two rads. Once the branch is reached where only one or two rads are fed, then drop down to 15mm.

If replacing all the feeds is prohibitive, you could upgrade as much as possible and see how it goes. It may cause a balancing problem, but it may not. It depends on other factors.

If it is within your budget, change all 15mm main feeds.
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