Oil fired central heating system, pumped radiators, gravity hot water. A past boiler (solid fuel) required a "fuse" radiator to be connected across the gravity hot water system near the hot water cylinder in case the old solid fuel boiler overheated I suppose. That radiator is still there but since the new oil fired boiler was installed air appears to be collecting in that radiator only, although for the first year it was ok. I have taken the radiator out flushed it out and even replaced the valves no luck. Any ideas?