baxi combi instant ?

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