Hi all, I have a sealed heating system. My boiler is an Vaillant Ecotec 630 and there are 12 radiators. Every two days the topmost radiator becomes cold to touch (at the top) but the system pressure gauge shows no loss of pressure. When I vent this radiator the system pressure drops by about 0.1 to 0.2 bar. I have checked all the pipework and fittings and cannot find any evidence of a leak. Today I spoke to a leak detection company who advised that it would be very difficult to find a leak of this size and suggested that it is more likely that the system is drawing air in rather than leaking. They advised to check all radiator valves and fittings again. My worry is that although this is a small leak, I am adding fresh water to the system on a regular basis which will eventually dillute the inhibitor and cause oxidation, rust and long term problems. I have previously added fernox leak sealer but this hasn't fixed the problem. Is there anything else to look at or check? Many thanks.