I was hooking up what I thought was a 24v heating wire to a thermostat when I came to later find out it may be 120v. Anyway, I try to manually turn on the fan on my carrier unit and I hear buzzing from the unit and nothing turns on. When I turn it of it does a little buzzing again. I went up and opened the side panel and noticed a blown 5amp fuse, replaced it and it still does nothing. What do you think I blew up and need to replace. It seems to hum from the rear where there looks to be a coil. The model is FB4ANF018

Thanks
Jay...

Possible transformer

Do you notice any burnt odor from either the unit or the thermostat?

There was smoke coming from the thermostat when I turned the breaker back on.

I think you can safely assume that the thermostat is ruined. I am puzzled as to how line voltage could have been present amongst the low voltage wiring, if so. Unless you have a multimeter and some basic electrical knowledge, I'm afraid this has become a job for a pro.