could be the water pump, or the fan , do you hear a fan ? each of these problems could cause your sympton . sorry to complicate things but are all the valves open at the bottom of the boiler, and your bar pressure what is that should be 1 half or 2 if about 3 or over could be the expanssion vesstle .
jay marshall
January 2012
change air pressure switch,keep on low temp 50 see if still doing it,than its high limit start
ltd
January 2012
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