Fuse in the switch spur has blown?

Replaced the 3amp fuse this afternoon. The system works fully for an hour or so. I put it on boost on channel 1 & 2 and the water. Everything is wired fine because weve tested all the controls. Weve got the programmer set up fine.The mcb does not trip at the consumer unit. I was just wondering if it might be internal wiring inside the boiler? Could any plumbers shed any answers, would be much appreciated. Thanks