I replaced an older thermostat with a new hunter digital model model 44110 a couple of years ago. It worked fine until this spring. I had already turned the system switch to "OFF" and tried to turn back to "HEAT" on a colder day and the furnace wouldn't fire. I didn't mess with it at the time, but later pulled the cover off and found that a "FUSE" of some kind had exploded on the back. Assuming this had been a glitch in the thermostat, I purchased an identical model a few days ago so I wouldn't have to re-wire and simply "plugged" the new cover and circuit board in....Still, no heat. So I switched it to cool and the thing exploded again, but the A/C stayed on.
Any idea why this would be happening?? I found that the the W (heat) and Y (air conditioning) wires were a little loose at the thermostat. I've tightned them to make sure the connection is good, but was wondering if that alone could have caused the "blow up" on my thermostat.
Sounds like u have a short somewhere. Check thermostat wires to the unit. If you say it looks like a blown fuse, it's a blown fuse. If you say it looks like a blown circuit board, it's blown circuit board . Obviously fuses are safety devices that protect circuit boards and other delicate parts. Be careful and good luck. Cliff