Water flow 15mm -> 22mm -> 15mm

[Northernguy]

Hello all,

I'm having a new combi boiler installed to replace an old open vent (gravity) system. The existing water pipework can be utilised but this will result in 15 -> 22 -> 15 mm flows for both hot and cold water.

I'm a bit concerned that this could lead to turbulence and noise as the mains pressure water flows from 15mm pipe into the larger 22mm pipe, and possibly some gurgling at the taps.

However, neither the GS fitter nor I fancy replacing all the old pipework as it is all currently inaccessible due to fitted bedroom furniture and ground floor ceilings that are dotted with recessed lights. Dismantling all this would certainly lead to divorce!

Although there is some technical stuff published on the flow and pressure changes that occur when water flow reduces from 22mm -> 15mm, I can find nothing at all about the 15 -> 22 > 15mm scenario. The GS fitter is not long qualified and doesn't seem to know what potential problems there may be.

Can anyone advise on this or point me to any relevant technical info?

 

[Howsie]

It isn't ideal but not the end of the world either.

Personally I'd do it all in the appropriate size pipe for the installation, but that's because I'm a professional and have to do it right.

 

[nicro3]

This is fine, but your system will run at a higher pressure ( possible leaks ) also a 22mm deadleg means a longer wait to receive hot water at your kitchen sink and any other outlets you may have.

 

[Roger Welton]

Yup, when downgrading to a combi :wink5: you need to downgrade supply pipe to make lower flow rates manageable in terms of time taken to get hot to taps.

 

[tamz]

In the ideal world it would all be replaced in 15mm but in reality this very rarely happens because people will not pay the premium to do it and don't like the upheaval.
Personally i would replace it and go for the divorce. It may be the best bit plumbing you ever had done :smile:

 

[Northernguy]

Thanks for the replies guys, much appreciated. I'm going to mull this one over some more! :thinking:

 

[tackleburger]

I have never had a problem with this scenario ever. Even with a 24kw combi and I see it on existing combi installations when doing swaps. I can't believe anyone would go to the trouble of replacing pipework like that. It won't be a problem. Any leaks I have ever had on existing pipe work have all been down to fittings usually not done properly in the first place and usually on the heating side. Had one on hot water after a combi install in the last 5 years and I fit three or four boilers a week.

 

[Mike Jackson]

Of course it's a problem. You are wasting energy and water. Nothing to do with whether it leaks or not. A larger pipe will hold more water so it will take longer for the hot to reach the outlet, wasting water. When the tap is turned off the hot water sits in the pipe and cools down, wasting the enrgy used to heat it. Whether it is enough of a problem to require replacing is another thing. It's also dependant on where the connection from the new combi to the hot water is made, I've been back to plenty where the original installer has connected to the easiest point, giving a ludicrously long run to the bath or kitchen, instead of making a bit more effort and minimising the runs to the outlets.