You have a faulty component in the boiler causing a short circuit and blowing fuses. Parts that could cause this would be the fan, gas valve, pump, diverter valve motor or water on the pcb. From experience though the first thing to check would be the diverter valve motor as the diverter valve can leak through the spindle and get it wet and cause a short circuit and the fuse to blow.
Ian, July 2009
