Yep, someone who worked on my furnace installed a 5 amp fuse instead of a 3 amp fuse. The 40VA 120VAC to 24VAC transformer died, protecting the 5 Amp fuse.
The thermostat has an extra 24VAC neutral wire, which should not be hooked up into the newer style thermostats. Of course, I hooked it up...
Worked fine with the Air Conditioning, but when I switched the system to heating, it quit working.
One furnace, the 3 Amp fuse blew, the other had a 5 Amp fuse and the transformer died.
At least it was only $10. And we didn't really need heat much this weekend.