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Furnace Blower Motor fan hums but wont start until I turn it by hand, then it runs fine. What should I check?

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    What make and model is said furnace? – ThreePhaseEel Nov 5 '19 at 5:32
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    A bad capacitor is most likely Capacitor give the motor an extra kick of power at start up. Edit in the make and model of your unit and a picture of the inside workings – Kris Nov 5 '19 at 14:15
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    @kris, this is the most likely the answer the op needs. Many times the cap body looks bulged when they are bad. With the cap out of circuit a ohm meter can be used to see if the cap is working. First short the cap just in case it still has a charge you don’t want to damage your meter, Ohms function across the cap and the reading should be charging up. If shorted or no change the cap is bad. – Ed Beal Nov 5 '19 at 15:24
  • @EdBeal I agree. We would need to see the blower motor to ascertain if it has an external capacitor some do some don’t. – Kris Nov 6 '19 at 3:17
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    Actually from experience with this exact problem, it might not be the capacitor. A broken wire in the start winding will also create the same effect. What I did was removed the blower assembly and took it to a motor rebuild shop. They diagnosed it , replaced the motor with the correct capacitor and cleaned it all up like new for around $110. The whole project took 2.5 hrs and is even quieter than before. – user68386 Nov 8 '19 at 0:47
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Yeah, could be the capacitor. A good quality multimeter with the ability to test capacitance can check it for you.

Discharge the cap, and remove it for testing (be sure your furnace is off). Then, set your multimeter to the F/uF setting and test it. The cap should show the cap rating in micro Farads (uF) and a +/- percentage. The reading should be within that range.

That said, I had the same issue. Cap was out of range (measured 9.6 on a 12.5 +/-6% cap). Replaced cap, motor seemed fine, but shortly thereafter would sometimes not start. I pulled the motor out, and dismantled it. The commutator was gummed up a bit, so I cleaned it. Worked ok for a bit, but the furnace would still sometimes limit out (probably because the motor wasn't starting). Ended up replacing the blower motor, and it works fine. Furnace was 23 years old, so I guess I can't complain.

Moral of the story is that it could be a combination of both. And, btw, if you replace the motor, always replace the cap, too.

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Before I would check for any of the potential electrical issues I would try lubricating the motor shaft. Shut off the power to the furnace. Gain access to the fan and motor. Spray a small amount of lubricant (slick50,chain lube, etc) almost any light oil will work, on the shaft where it exits the motor. Some models may even have been equipped with a small oil port from the factory. You may have access to the opposite end depending on the design. Spin the fan by hand a few times. Apply a small amount of oil again. Wipe off any excess, reassemble and see if the fan starts.

  • Worth a shot. No downside to giving it a try first. – Greg Nickoloff Apr 3 '20 at 20:39

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