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Mark
 
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Default Blower on furnace hums and doesn't start

I am having problems with one of my two furnaces. When the thermostat
kicks in, the furnace will light, but the fan won't kick in. Then the
furnace shuts down. If I turn off the heat (i.e. not on heat or
cool), and turn the fan to "on", it just hums (the other furnace works
fine and the fan will run). There is a solenoid that clicks, then the
humming. The fan itself turns effortlessly.

Do you think this could be a problem with the capacitor, solenoid or
transformer?

Also, I have been having another problem (probably unrelated?) once a
year every winter when I first start the furnace. The fan doesn't
want to kick in but I didn't notice any humming - this appeared to
have something to do with that timer-like dial (is it a temperature
limit switch?). Two taps with the end of a screwdriver and I'm
usually OK for the year.

Any ideas on either problem? Thanks.