pressure builds up when the htg is on , but is ok in hot water only mode
please help thanks

blockhead, February 2009

Looks like you have lost pressure in the expansion vessel (EV). Test the pressure on the EV, remove the shrader valve cap and using a Tyre Pressure Gauge (TPG) test the pressure it should be about 8-10 psi. If its low put a foot pump on the EV valve and punp up to 8-10 PSI

Bob the plumber, February 2009