Combi boiler issue - HELP


Help needed please!
Recently I completely drained my combi boiler central heating system so that I could replace radiator valves. After re-pressurising the system, bleeding all the rads then re-pressurising again I turned the system on only to find the pump was making a really loud noise and was running hot. Also the heating was not getting very warm. My only idea is that as the boiler is at the highest point in the system it is full of air and is trying to pump air hence the overheat. Does this sound valid and if so how can I resolve???
Regards
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the auto air vent is probably closed or jammed. It looks like a small brass bottle about 35mm diameter by about 50mm high with a schreder valve cap -the same as on your car tyres- or plastic screw on top. Loosen the cap or screw and leave it loose. Usually found on top of the pump or at side of boiler. If it's jammed you can bleed it by slackening a nut at the heat exchanger or by removing the big screw on the front of the pump. Even if the vent is jammed, once the boiler is running ok it should be fine until next drained.

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