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pressure keeps building up in the heating system causing relief valve to op?

my relief valve on my heating system keeps opening due to pressure build up. I have drained the system and refilled it and still the pressure builds.
ang, November 2007
Two possible problems, one, It could be the diaphram on the expansion vessel is split. Two, it could be that the ball valve on the filling loop is not closing properly causing the presure to rise.

harry, November 2007