I have a Heil (ICP) gas furnace/AC installed 5 years ago. The unit is blowing the 5V fuse when it gets a call for heat. The AC side works fine. The air circulation system works fine.
When the unit calls for heat, the vent motor spins up, then down 4 or 5 times then the 5V fuse on the control board blows.
I have tested the system with another thermostat; no change
I have checked the exhaust vent for blockages; none
I have checked the vent motor housing for blockages; none
I have tested continuity on the vent motor; good
I have replaced the vacuum sensor switch; no change
I have inspected the wiring for a short, to the point of dismounting the circuit board for better access; none found
I am trained as an electrical engineer and can usually tech anything, but this one has me stumped (and cold! )
My heating/AC guy has looked the system over and can find nothing wrong either.