What size PEX pipe

[caesaroni]

Hi all, I'm looking for some advice for the size of PEX pipe I should use? It's a long story so please bare with me lol. I'm currently fitting a new bathroom and whilst doing this I've also had the boiler converted over to a combi system so everything is now mains fed. On one of the walls in the bathroom I've noticed the 15mm cold water main is buried in the wall which goes into the loft and used to feed the cold water tank. This pipe has now been looped and joined to the 22mm pipe that feeds the cold tap on the bath. I know the 15mm pipe in the wall has been in there 30 years plus and was thinking before re tilling replacing this with 15mm PEX run in conduit so there's no joins in the wall. My question is will 15mm PEX reduce the water pressure because of the inserts for the push fit connections? I was also going to tee off the pipes under the bath for the thermostatic shower using 15mm PEX will this be ok?
Any advice would be greatly appreciated

 

[Justin Passing]

Yes, it's ok imo. The hot water flow from a combi is governed by a restrictor inside it, so that you get enough of a temperature rise even if you open the tap fully.
Pipe sizes with their fittings etc do affect ultimate cold water flow, but that one's not so important.
In the winter when ground water gets really down, it can be below 10°. We all shower at about 37° - 38° . Your combi will be restricted so that it can give iirc a 30°C rise, so you need as much hot as the combi can provide , with a dribble of cold.
30 years isn't alarming for copper pipes in a wall. A lot of it has been there since the 20's - ie 100 years. I'd leave it, and use a compression fitting to go to plastic.
Just keep some spare tiles!