New intel: I popped the cover off the fan and plugged it into an extension cord, and the fan doesn't run. The extension cord, to be extra-sure, does power, e.g. the vacuum cleaner I used to suck some of the accumulated dust and gunk off the fan.

There's some blackening on the housing above the motor, too, although to be fair that could have been there for 20 years, since I haven't been in there since we bought the house a couple of years ago. But that's some pretty strong circumstantial evidence that the fan motor is the problem, not the new switch.

So I'm off to the hardware store to invest in a new fan motor. Will advise.

But in the meantime: Any idea how putting in an inadequate switch could have blown the fan motor?

That switch likely had nothing to do with the motor blowing. A 20 amp swiycth delivers the same voltage a 5 amp switch sends. Either way 120 volts got to the fan and it blew, unrelated to the swicth that sent the power. You will probably not find a fan motor at the hardware store. I'd replace the whole unit with a Broan or Nutone and be done with it. They frequently come out of the box with their own switch included. Whatever you eventually buy just make sure the switch, breaker and wire all match and can carry what the fan reuires. A fan/heater/light usually draw near 20 amps and needs #10 wire and a 20 amp breaker, dedicated

If you have about $150, Nutone/Broan make a .3 Sone(Super Quiet) Exhaust fan, can't hear them if vented properly, larger duct.

$60 later and a bit of a struggle with the housing later, I've got a working fan on a working fancy switch.

How the new switch fried the old fan (it still smells faintly of burned plastic or rubber, now that I've taken it out and gotten a close look at it), I guess we'll never know.

A switch with bad contacts introduces a resistance. If it's not enough where the fan creates an inductive reactance which will be much higher than the resistance of the fan, the fan windings could overheat.

If you take the fan apart you may or may not find evednce of that. Increased temp of the winding could result in shorted windings.

Thanks, experts, for all your volunteer help.

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Originally Posted by dmr1977

But in the meantime: Any idea how putting in an inadequate switch could have blown the fan motor?

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Yes, if it was a switch not designed for use with motors.