I have an open vent central heating system, which I drained and refilled in order to replace two TRV s on ground floor radiators. Prior to replacing the valves, it was not possible turn on the central heating independant of the hot water coming on and when I checked the two port motorised valve for the central heating, the lever for manually opening the valve was not functioning. However as the heating and hot water came on, I did nothing about it. Now, however, the radiators are not heating and the boiler cuts out. The pump seems to be working and all the rads are filled. Have I created an air lock below the dodgy motorised valve when refilling the system? Can any kind soul help?