New combi system - query DHW pipe sizes

Hi all,

We're currently having a new combi boiler installed as part of a biggish extension build, to replace an existing gravity system.

The boiler we're intending to put in is a Wooster Bosch Greenstar 440 CDi (http://www.worcester-bosch.co.uk/homeowner/products/gas-boilers/greenstar-highflow-440cdi).

The plumber has already carried out his first-fix around the new extension with pipework leading up to the existing bathroom. All of the DHW pipework is 15mm copper pipe, and for some reason I was expecting to see a 22mm 'mainline' of pipework branching to 15mm to feed every tap/bath/shower etc. The boiler is not in place yet, but I can only reckon that he's going to connect the 15mm stepped up to 22mm to the boiler.

Is this correct, or should it be 22mm branching to 15mm? The WB product information states that the DHW should be 22mm to the 'first branch' then 15mm from there onwards, and the DHW outlet on the boiler is 22mm. If the nominal setup is to use 15mm pipe, why does the boiler provide a 22mm outlet?

Am confused as hell and can't find anything concrete to suggest what the picture should be. Can anyone enlighten me, please?

Cheers,
Gary.
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It should have exactly what the manufacturer has advised.

Thats 22 mm until the FIRST branch to an outlet.

If you have any concerns then ask the plumber who is doing the installation.

However, make sure he is gas safe registered and you can confirm that from the internet.

Its common for builders to use non registered people. One I saw on Tuesday was registered but had done THREE things wrongly when he fitted the boiler!

Tony

Hi Tony,

Thanks for the clarification.

Agile said:

It should have exactly what the manufacturer has advised.

Thats 22 mm until the FIRST branch to an outlet.

Click to expand...

Kind of makes me wonder - why bother with 22mm at all! If all it's doing is carrying to the first branch then 15mm for all subsequent branches, why not just go 15mm from the boiler? I'm not seeing what function the 1m length of 22mm pipe will end up serving...

Agile said:

...make sure he is gas safe registered and you can confirm that from the internet.

Click to expand...

Do you have the URL for that off the top of your head, or is it just at the corgi site?

Cheers,
Gary.

The boiler can provide about 20 li/min and that flows better in 22 mm tube.

Try-

http://www.gassaferegister.co.uk/


Tony

Agile said:

https://www.gassaferegister.co.uk

Click to expand...

Internet Explorer cannot display the webpage

this one seemed to work

http://www.gassaferegister.co.uk/

Agile said:

The boiler can provide about 20 li/min and that flows better in 22 mm tube.

Try-

https://www.gassaferegister.co.uk


Tony

Click to expand...

Cheers Tony.

And this is the thing I don't get: if 20l/min is better in a 22mm bore, why is the whole run not 22, branching to 15mm for each service?

Back to confused now

Gary.

Few outlets need 20 li/min. Just a bath really.

Normal outlets are fine with a 15 mm feed.

There are conflicting aspects as you need to minimise the VOLUME of the pipework.

If it were close the 20 mm to the bath would be a good idea if yo took many baths.

Tony

Cheers Tony. I'm happy with the requirement for 15mm to service taps etc, my lay understanding of that is that there's less impact on the pressure at one tap if a second is opened if they both branch off a 22 rather than both branching off a 15, no?