A present I have a 25mm MDPE main pipe to stop cock. From here it reduces down to 15mm copper pipework. This 15mm copper pipework then splits into 2 x 15mm copper pipes, 1 pipe feeding the cold mains and 1 pipe feeding the unvented combi boiler. Is there any benefit in upgrading the front part of the system to 22mm? so 22mm from the stop cock to a 22x15x15 tee to the cold/combi?. This originally came about after looking into fitting a pressurised accumulator and needing a 22mm upgrade anyway. I'm just wondering if the 22mm upgrade would make any difference to flow rates on its own without the accumulator? example: shower on and someone then opens a cold tap, would there still be sufficient flow at the 22mm tee that the flow to the combi doesn't reduce? Hope that makes sense, I'm clearly no plumber, but i'm not lacking in enthusiasm or determination to have a go at something.