I replaced a programmer on an S plan system in a small bungalow and when checking zone valves operation, I discovered an existing problem. There is the usual 2 port MV for radiators, (controlled with hall stat), plus a second 2 port MV for hot cylinder, (controlled with cylinder stat). Programmer on hot water only setting, it works fine and cyl stat turns it on and off. Problem is on radiators only setting, the rads work but power is also then to the cylinder MV. Cylinder stat then becomes nonfunctional. Customer had said her hot water was always very hot. Wiring fault I guess in wiring box? Work is all very amateurish looking. I can’t easily open the wiring box due to some idiot piping across the cover.