Pipe Sizing for Thermostatic shower Mixer

[whodareswins]

I want to fit a thermostatic Shower valve which requires 22mm /3/4" Hot and Cold Feeds in a bathroom that is piped with 15mm pipe.

What will be the result/effect of branching off the 15mm pipe with 22mm pipe to the shower valve or is it acceptable to use a 15mm reducer fitting directly into the 3/4" valve feeds?

The 15mm pipes are mains cold and Gravity fed hot (about 30ft head) - I want to maintain as good a flow rate as possible.

I have thought of fitting a pump but it seems a bit impractical - 5 story house with storage tank in loft(level5) and hot water cylinder in cellar(level1)/ Loads of disruption to run pipes.

Any suggestions appreciated.

 

[EliteHeat]

If 15mm is all you've got then use them with a reducer. If the shower is rubbish then re-pipe or add a pump. There are no other options really are there?