furnace fan runs whenever it wants to

[Rob]

I have an ancient Macdonaldaire furnace (25+ years old) which runs sporadically.  Typically a few times/day, once I hear the furnace power kick on I have to reach inside and barely turn the fan by hand to get the furnace going.  The furnace will then run fine (shutting on/off) for the next three, four, eight hours or even a day.  Naturally I notice the furnace not working after the house temperature has dropped 10 degrees and I'm comfortably in bed.

Yes I do realize the furnace/ac unit should be replaced but my fixed income won't allow it.  A few months ago a HVAC company replaced the thermostat and control transformer.  They overcharged me and I refuse to call them back.  Thanks for any help!

[Admin]

A universal fan motor should be cheap and easy to replace.

Is it belt driven or direct drive?  Test the capacitor if there is one.  Can you leave the thermostat fan switch ON so the fan motor never stops?  This way you don't have to start it by hand.

It sounds like that motor should be replaced regardless.

[57MAX]

Just a thought, why should the motor be replaced?  If it starts/stops on its own, it must be energized by power wire.  How would replacing the motor fix that?

[Admin]

Well he mentioned having to spin the blower wheel by hand on occasion.  That could either be a faulty capacitor or a start winding starting to fail.