Hi, I intend to install an EV (expansion vessel) to my cold water line as I'm having problems with pressure affecting my water heater and taps. When the water gets really hot, the safety relief valve is triggered and drips. I never understood why this happened until it occurred to me that this is an unvented system with no EV present, therefore no means for expansion. I have never fitted one of these before so I just wanted to check whether I'm installing at the correct place on the line. To demonstrate, I have provided a PNG image to show the layout of my plumbing lines and fittings etc. Some things I'm not sure about: Why is the EV installed on the cold line (before water heater) and not the hot (after water heater)? I always assumed that the EV would be installed to take up the expansion of the resulting hot water, not cold prior to it being heated. If someone could explain, that would be appreciated. Although I've got the PRV (pressure reducing valve so that we're clear) at the cold in at mains, I assume I must install a secondary at the hot out from water heater (as shown) due to raise in pressure after water heated? Is the check valve in the right place near the EV? Do I just use a simple Tee to incorporate it to the cold line as shown? Or is there a special fitting I would need instead/ additionally? Lastly, my water heater has a capacity of 80L and I'm unsure of what size EV to buy. Ideally I'd want to use the smallest size possible so I'm thinking a 5L or if I could get away with an even lower capacity please let me know. So does my diagram look fairly accurate? Basically my work would consist of: teeing off for the EV and putting check valve before the tee (not after otherwise water wouldn't be able to back siphon into the EV if I'm right in assuming) and fitting PRV after water heater if necessary. Please note, Y filters not shown but would be installed. I'm aiming to maintain 60 PSI pressure throughout to prevent damage to fittings. Many thanks for any advice Please note, this is a Bulgarian installation, but the procedure should be identical to UK practices.