Thanks Pete, all the pipework, including the stubs from the tanks is 15mm, so there is no real point in increasing it elsewhere. I understand how the set up is supposed to be plumbed, but all I can see happening is the water from the bypass to just upstream of the pump being the much easier route and hence the tanks will always be bypassed. I have tried to search for TMVs online, but haven't had much luck, I thought that the thermostat was acting as a TMV. Surely with either a TMV or a thermostat, the risk of thermal shock from cold water is the same. Is this a real issue as I would be crazy to try it, if so!