Hi , there is a problem with vaillant ecotec plus 831 which is only 18 months old. The tenants top up the pressure twice a day. i have been there today. system pressure was 1bar when cold but when ch is turned on it jumped on to more than 3bar within 15 minutes and discharged the water through prv. when i top it up the pressure rises very quickly and again goes above 3bar within minutes and discharge through prv. is this a problem with expansion vessel losing the charge or split diaphragm . please help, thanks.