i have had a new boiler put in. its a worcesterbosch with a high pressure cylinder. the plumber was fine so i thought but my mate who knows a bit of plumbing says the hot and cold should be in 22mm for the whole system with 15mm tees off to each appliance but the plumber says the system is powerful enough to supply enough water at 15mm, can anyone advise on this?:

the plumber is right. it's a mains pressure system his talking about a gravity system