ideal boiler

[neill ward]

hi
i have an Ideal isar HE30 boiler
when i leave the house for the week i turn it down the room stat (on the wall in the front room) to about5 degrees, on my return i turn it up or turn on the hot water ( combi boiler) and the boiler does not respond, i have to go into the out house and press reset. in the status window the letter F can be seen then letter L then it keeps repeating when i press reset the number 5 appears then C then D then the system works fine until i turn down the stat,, any ideas guys