Received a call at the first floor furnace was not operating, upon arrival I found that there was no display on the thermostat and there were no lights on the board. Removed the panels and found that the control voltage fuse on the circuit board had failed, installed a resettable breaker and cycled the system to see if I could locate the cause of the fuse failure. The house was 64° upon startup, I allowed the thermostat to operate and reach 70° and there was no fault in the furnace. I installed a new three amp fuse and recycled the system, the furnace lit properly and is operating. Upon removal of the failed fuse I noticed that there was no black spot in the middle of the fuse that is typically present when a fuse blows. It is possible that the fuse failed without an overcurrent incident. The furnace is operating properly at this time. I left a spare fuse on the furnace service switch just in case we need it in an emergency situation.
