Basic heating and hot water controls (S plan, mains voltage) work like this:
1. Programmer reaches an "On" period and the heating and / or hot water outputs go live with 240V.
2. If Heating is ON:
2a. 240V from programmer applied to room thermostat input. If heat is required, room thermostat output is set to 240V.
2b. 240V from room thermostat is applied to central heating motorised valve.
2c. Motorised valve opens. When fully open the valve operates an internal microswitch, and valve electrical output is set to 240V.
2d. 240V from motorised valve is applied to boiler, which fires, starts the pump (possibly internal) and sends hot water round the radiator / heating circuit.
3. If Hot Water is ON:
3a. 240V from programmer applied to cylinder thermostat input. If heat is required, cylinder thermostat output is set to 240V.
3b. 240V from cylinder thermostat is applied to hot water motorised valve.
3c. Motorised valve opens. When fully open the valve operates an internal microswitch, and valve electrical output is set to 240V.
3d. 240V from motorised valve is applied to boiler, which fires, starts the pump (possibly internal) and sends hot water through the hot water cylinder primary coil.
There are endless variations, but essentially this is what happens for system or open vented boilers. Combi boilers are slightly different as valves and pump are internal, but the principles remain the
same.
So, if the valve is faulty, even though the thermostat is saying open, it may or may not open, and may or may not tell the boiler to fire.