I know that hot water cylinder systems which are fed by a header tank in the loft usually (*) use 22 mm piping to give a greater flow rate to compensate for the much lower pressure than the mains pressure of the corresponding cold tap.
We need to get our cylinder replaced because it has corroded and has just started leaking. We are fed up with a pathetically low flow rate and long time for the hot water to run warm, so we are considering changing to a mains-fed cylinder - obviously one which can withstand the higher pressures.
I think a lot of our hot water pipes are 15 mm (no wonder the flow rate is poor!). Given that 15 mm can give a good flow rate for cold water, is there actually a need to upgrade the hot water pipes to 22 mm, or can we manage we manage with the existing 15 mm hot water pipes?
(*) But not in our house :-(