If everthing has been ok in the past then i'd suspect you have some serious sludge building up in the system.
Ian
July 2009
Next time this happens, have a look at the header tank and see if there is water coming out of the expansion pipe - thats the one that curves over the top of the tank, not the ones coming out from the bottom. If there is water there, the pump speed is too high or there is a blockage in the heating system pipework.
John
July 2009
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