ravenheat combi boiler ?
 
                I`ve carried out numerous repairs to my combi over 12 years , but struggling with this one ... C / Heating seems OK , but hot water .. shower .. fires up fine and provides hot water for approx 3 minutes before shutting  down for a minute , then firing up again ?? replaced air pressure switch .. no change ...  is there a flow sensor ??
            
             dave  s
            
 April 2011