been doing diy plumbing for some years now
in many cases when i have removed old baths
there have been 15mm pipe work to bath then
then 15mm to 22mm to taps
so i have always did the same
a few week ago i installed one for a friend
i run 2 15mm pipes to bath then added 15mm to 22mm reducers
last 2 foot or so is 22mm
never seen a problem with this until my friend informed me it should all 22mm ?
water flows great no problems ( airing tank system )
i believe combi boilers require 15mm ?