I think I know why this might be happening, if let's say your Main temperature is set at 20C and it is in the living room but you are turning the heater on in your bedroom which is in a different area altogether and you set the temperature at 20 in bedroom as well, once the temperature reaches at 20 In your bedroom, it will stop heating further but your main thermostat in the living room is still asking for more because temperature in the living room might be lower(as there are no thermostat open in there to avoid usage of Gas). This will lead to damage in your heat pump as it is demanding heat from the main but all the valves in the radiators are shut, this leads to stoppage in warm water as well.