22mm or 15mm (that old chestnut)


5 Apr 2010
Reaction score
United Kingdom
I currently have a bath (with mixer shower) plumbed direct from the unvented cylinder with the first 2m in 22mm and the last 1m to the tap in 15mm (due to routing constraints). Approx 3 bar pressure hot and cold. Shower pressure and flow are good and bath fills quickly.

I'm refitting the bathroom and require to add an additional 3m of pipework to get to the new concealed in wall mixer/diverter (diverts to either shower or bath filler). This additional 3m of pipe will hook up to the existing 15mm pipe. So total run from cylinder will be 2m of 22mm, 1m of 15mm and then 2m of new pipe.

At 3 bar will I notice any real improvement in flow by running this new section of pipe in 22mm given that I cannot upgrade the existing 1m section of 15mm?

The mixer/diverter also uses 1/2" BSP female fittings so I'd need to convert down from 22mm to 15mm before and after the mixer in order to use standard 1/2" to 15mm connectors (I don't seem to be able to find 1/2" to 22mm fittings). This seems like a bit of a faff.


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As what he said above. Although its not the best of practice it will work.
Ensure any isolation valves are full bore so not to reduce water flow.

Happy plumbing.
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Thanks for the reassurance. The last thing I wanted to do was go to all the effort of installing the new bathroom only to find I had a worse shower than I started with.



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