Upon arrival, system was in heating on mode. Inspection of furnace shows the control board is flashing code 24-secondary voltage fuse open. Inspection of control board snd wiring shows no shorts to ground. Inspection of thermostat wiring shows that the Blue wire rhat is hooked to common in the furnace is hooked up to B on thermostst. B on the thermostat is used as a heating changeover valve control. When heating is turned on, B is energized, and in this wiring setup, it is a direct connection from power to ground causing the secondary voltage fuse to blow. Current thermostat that appears to be new does not require a common wire. Removed blue wire from thermostat and furnace control board. Test ran in heating, cooling, and fan only modes with no other issues found. System is operational at this time.