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over heating boiler?

boiling water coming from open vent pipe and hot water is coming up the cold feed into the header tank...potterton mk2 f netaheat 1016
mick burns, January 2013
reduce pump speed to one ,look like system need power flush which will be costly,what you can do is pour restorer run for for day than flush out few time to remove dirty slug from system

ltd, January 2013
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