In one of my properties just bought I have topped up the the central heating combi system. I turn heating on and boiler kick in alsoi I have hot water. After 30 mins the system goes from 1.5 bar to 0.5 bar and heating goes off. I have filled system many time and there is not sigh of water. There is also no sign of water by the boiler or coming out the pressure relief valve out side. What could this be ?