I am replacing bath with shower with hidden mixer valve. Combi boiler gives power shower effect through existing bath mixer unit and all mains and hot water pipes are 15mm. Connection to new mixer is via 3/4 ports - can only find 3/4 to 22m fittings so seem to need to reduce from 22mm pipe to 15mm at some point. Are there any tips, ie really short stubs of 22m pipe or join closer to boiler in 22mm or are there in fact fittings that are direct reducers straight to the mixer and I've just not found them? Obviously what I'm concerned with is maintaining as much pressure as possible. Mixer unit is good quality and suitable for combi boilers.