At the moment I have a very old 15mm copper water supply entering through a wall in the cellar. This services a 15mm network of cold water and a combi boiler (Worcester 37kW) on 15mm copper throughout. The house is over 4 floors with a lot of outlets. 5 toilets, 4 showers, 2 baths. Family of 4. Don't have any major general flow problems, even on the top floor. I measured the pressure at the highest shower and it was about 2 Bar. Flow rate is acceptable (not measured it), unless another tap is opened in which case the flow drops to a dribble. I'm having a new water main fitted. Standard 25mm MDPE pipe entering in a brand new ground floor location, directly off the main in the street. Converting the combi setup to a system boiler with 250L megaflow (Super Oso) with a balanced supply. Proposing to run 22mm polypipe from the stopcock directly to the cylinder and a balancing valve. Then proposing to run 22mm polypipe after the cylinder (tee'ing off in 15mm a few times) until it connects with existing 15mm pipework (as it goes upstairs). I'm paranoid about flow rate with the new setup and am hoping that you guys can tell me I don't need to worry! I just want to ensure that a few taps can be opened simultaneously and that the bath fills up at a good rate. If I make an assumption that the source pressure will be 3.5 Bar and flow is good then would an ageing 15mm copper network deliver fast flow-rate water from the megaflow and from the new main? Am I wise to run some of it in 22mm? Do I need to enquire about 32mm rather than 25mm MDPE? Thanks in advance!