hi i have a job to go on back thur,customer fine but wont spend anything.back boiler changed pump heating side fine, 4 pipe system air lock on hot water side,combined heat and vent pipe(not ideal ).air vent but close to vent/feed so just fills from header.could try filling from mains but again will go straight out the vent i think!i could take stat out and overheat never been too keen on this.has any one got any old plumbers tricks or some thing else i can try ,will do stat if last resort.thanks daz