If Pressure of Central Heating is remaining below 3-Bar in Pressure, then the PRV needs replacing. Also if the pressure is at 0.6bar then your Domestic Hot Water will not function correctly, due to a pressure sensor lock out.
Other items to check would firstly be the 3-Way Diverter Valve, and if these prove to be working correctly, check the Domestic Heat Exchanger is not blocked or furred up. There is a Filter on the Cold Inlet which could also attribute to reduced hot water if it is blocked in anyway.
If the burner continues to cut out, you will need to check the Hot Water Thermistor, Fan Unit and Air Pressure Switch for correct operation. Really I would recommend that you contact a Gas Safe Registered Installer who will have the necessary tools and test equipment for diagnosing such problems.
Hope this helps.
Anthony
May 2010
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