I Have just had a Santon Premier Plus unvented systems installed. We have a problem in that the water runs hot out of taps but some showers (where the flow is restricted due to shower head)start hot but quickly turn stone cold. My plumber thought it was cold leaking into the hot system through a mixer tap but I'm fairly sure it's the tank. Plus the pipe at the top of the tank turns cold when running the shower but warms up when the hot tap runs. Hope you can help!