Expansion vessel or pressure relief valve. Drop water pressure from system and check that the air pressure in the vessel is 0.8 bar (you can do this with a normal car or bike pump) repressurise water in system back to about 1.2bar and then test over next few days - is the air pressure still the same or has it gone? If it has then it's the vessel that's damaged and you need a new one or an external one. If not then its time to change the prv.
Ben
February 2013
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