I have a 7 yr old sealed ch system and have recently replaced the Peglar TRVs with Drayton TRV4s. At least one TRV now makes a knocking sound as it opens. None of the radiators get cold and all appear to function correctly. I have repeatedly bled the system: the water seems to have air in it as it is "milky" and clears quite fast. After draining a litre or so from the upstairs radiators, the water then fizzes and splutters for a few seconds. Having done this and repressurised the system, all seems to work fine (with no knocking) for a few hours then it all starts again! Any ideas anyone?
