Bit of a dumb question possibly, but wondering if using a 22mm Flexi from bath tap and reducing down to 15mm feed at supply is going to be any better than just using a 15mm Flexi from supply to tap? I know the bore of the 22mm pipe is bigger, but wondering if this is actually going to make much difference to flow at the tap given the 22mm is only for the last few inches and the rest is 15mm copper?
No difference at all. 22mm pipes were for increased flow off gravity systems. You have a combi so the hot runs off mains pressure and that won't change and neither will the flow one jot no matter which pipe you put on if it's coming in off a 15mm pipe. If you decide the 15mm flexi try not to avoid the small bore type though.
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