ferroli optimax HE31C the digital display is saying 'no CN''? ?

do you have the optional timer fitted? if so check the timer is not set to timed function, as this will overide the boiler functions, but shouldnt affect your hot water. second does the circulation pump start when you switch the boiler electric on? this is supposed to happen when the boiler starts up or resets as it is supposed to purge any airlocks in the system. If you do not hear the pump running you will probably find the pump has failed. If it is a new boiler on a new system it is covered by the warranty, but if its been fitted to an existing central heating system the pump has no warranty under the guarantee terms. I fitted a brand new boiler to an old system and the pump failed in 9 months! i refitted a new pump and the boiler has worked perfectly for the last three years..... best of luck.

Rob
May 2013