How you conclude that there is obviously a valve sticking or broken I do not know. I don't reach that conclusion but then again - I know next to nothing of boilers and heating systems.
Heated water from the boiler is directed to a coil in the hot water cylinder and back to the boiler. The flow will stop when the hot water in the tank reaches the desired temperature - there will probably be a thermostat strapped to the side of the tank that controls the firing of the boiler - it may be set to 80 degrees or it may be defective - check it out.
There should also be a thermostat control on the boiler itself to set the temperature of the water being pumped around the pipes - too hot and the risk is being burned by the radiators.
Good luck..., December 2014
