Central heating is not quite my usual field but I was asked to look at a cylinder stat today. The system is fully pumped with individual zone valves for central heating and hot water. The customer complained that the DHW was getting too hot. He had already had a new cylinder stat fitted but it had not helped. I checked that the stat was correctly wired up then checked how the zone valve reacted to different stat settings. Even at the minimum setting the zone valve stayed open but if the stat was turned to the off position the valve closed. I swapped back to the old cylinder stat and repeated the test. This one behaved in exactly the same way despite the water in the tank being piping hot. I've not had much to to with these cylinder stats but would have thought, like with a room stat, you should be able to use it as a crude form of thermometer by listening for the clicks? These ones only clicked when they were below the marked temperature scale but as I said, the water was already hot.