What pipe sizes for completely new hot and cold water system?

[crobar]

I'm renovating an old victorian 3 floor terraced house with the usual hellish mish-mash of pipework running in bizarre and ugly locations. I want to replace all that with new pipework while I have the floorboards up.

My question is, what pipe sizes should I use for the different bits?

The mains comes in via what looks like old imperial sized pipe (probably 3/4 inch) although this is currently immediately grafted onto a 15mm pipe before it goes anywhere (even though there's 22mm pipe used elsewhere). The hot water is served from a combi boiler, and there is no water tank, the water pressure is good (it's centralish Edinburgh).

For both the hot and cold water supply, I assume the best thing is to have 22mm going most of the way from the source, which is then teed off to 15mm to go to taps, or sticking with 22mm for a bath or similar?

I assume there is no point having 22mm after any 15mm?

What do you recommend? Will 22mm pipe take a long time to transport the hot water because of the cold water in the pipe?

 

[muggles]

It depends on what the ultimate plan is. How many bathrooms/bedrooms are there? Is there any possibility of upgrading the old main to a new one?

 

[crobar]

I don't think there's much possibility of upgrading the mains, it's a city centreish location so a bit tricky. There's 4 bedrooms, one main bathroom I'll want a mixer shower in, running off the combi, and another shower room where I was thinking Id have an electric shower, and a downstairs WC, but that's it for bathrooms. There's a kitchen and utility room both with sinks.

 

[crobar]

oh, and the bathroom and shower room will be on the second (top) floor.