My father has a central heating system where a thermo siphon hot coil heats the domestic hot water and central heating is pumped. There is no thermostat on the cistern. There are no motorised valves. There is a programmer and a room thermostat. The boiler keeps cycling which since the pipes are under floor boards and the bed rooms get too hot. The hot water also becomes too hot. There seems to be two ways forward. Method one is just fit a cistern thermostat common to Boiler normally closed to pump and normally open to programmer hot water output. This will stop boiler cycling it would only run if central heating or hot water was required. But when central heating is running water will still heat in cistern but since pump should push most of the water around the central heating it may be very little? It will require one three and earth cable routing. Second is to fit cylinder stat which controls a motorised valve which has a change over switch incorporated that switch then controlling boiler Orange to boiler, Brown and grey to cylinder stat and White to pump. In both cases room stat also connects to pump. The latter needs 4 wires plus earth as it will require a neutral and extra cost of valve plus plumbing will be required to fit valve. I am sure I am not the first to be faced with this problem and wonder if the first method has ever been tried and how well it worked. Second method connects the valve in a way I have never seen in any books again has anyone tried it and how did it work.
I have built circuit in multisim and tested and shown to be electrically sound but no way that I have to simulate water flow.
PS I am sticking same post in plumbing and Electrics as it involves both trades.