how can i fix my powermax he 85?

status code A15 CH pressure loss ,if using just hot water all good but once i'll switch on CH ok for less than 1hour,then again A15 on and then noticing dial gauge 0.5 bar pressure ,fitted PRV but not sure about recharge,repressurise and bleeding system;just to mention CH won't switch on unless room thermostat not ask for when before i had CH working from control panel