I need to know more about your boiler is it a combi, as if it is then it would seem that the mains cold water coming into the heat exchanger is being allowed to get into the heating sytem water (not good) this cold be the heat exchanger seals or the heat exchanger its self how long has the boiler been fitted and how long has the problem been there?
Chestnut Plumbing (OFTEC)
March 2009