I have a Potterton Prima 50B for my hot water & heating. When the controller is set for the water and heating to be on it works OK and controls off the room and cylinder thermostats. When the controller is set for the water/heating to be off the boiler keeps running and the water gets very hot, the heating pump also runs. The only way that I can control the heating at present is to keep it on all of the time and turn down the room thermostat ot switch the system off at the mains.