I have a 20 year old system in a 3 bed-roomed semi. There is no expansion tank and the boiler is a Baxi WM 531 RS, working perfectly. There is a pressure release valve (1" BSP fitting I think) and it has started leaking. It is fitted on a tee at the return side from the hot water cylinder's heating coil. I can only assume that the expanded water in the system is driven back into the main which I think is now illegal. I have no idea who fitted the system, or when the system was fitted. A local supplier said he can replace the valve, but would need to know what pressure to get it set to when ordering it. I haven't a clue what that should be!! My gut feeling is that an expansion tank should be fitted to turn it into a conventional system, but the easy option is just to replace the valve. Any ideas or help would be much appreciated.