I'm told we have a 20mm "poly" water feed to the house, it goes-in to a 15mm
stopcock and then 15mm around the house. The pressure and flow rate at the
upstairs cold tap (mains fed, not gravity!) is very good.
As part of a move to a mains-pressure hot water system it's been suggested
that the stopcock and pipe to the cylinder should be changed to 22mm to
"improve the flow rate" - how can this be?
I would have thought that the limiting factor was the pipe run from the main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong? Is there any benefit in using
larger pipes downstream of the stopcock?

Dave S