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My Lennox HVAC system started working intermittently as of a couple of months ago and over that period progressively worsened. It just wouldn't blow air. I determined that the issue was not with the compressor but the actual blower motor. The thermostat (Nest) reports that everything works fine, the compressor outside is blowing but the blower motor is not working. That causes the coils indoors to frost as the blower is not blowing to dissipate it.

The way I tested was using the two connectors to the motor, high and low voltage. They are both fine and the manual says if that is the case, that means the motor is dead. The manual suggests to replace the motor.

I have not taken the motor out yet but I am curious, as this unit is only about 5 years old, what causes a motor to go bad? And can it be fixed, since I am not sure if mine is still covered under the warranty? It's a single speed motor and the model number of the handler is CBX27UH. I have been researching online and some posts indicate it may be due to a bad capacitor but I don't think mine uses a capacitor unless it is inside the motor itself.

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    If there is a bump on the outside of the motor the capacitor is under that. Please turn the power off first prior to messing with the motor or cap (capacitor) a cap can be the issue at this age but motors usually last longer sometimes there are oil ports on the motor at each end of the motor, usually they last longer prior to needing oil so I would look to see if there is a ~2” bump on the outside of the motor 6-8” long. The cover is held in by 19or 2 screws. If the motor turns easily I would check if it has a cap. – Ed Beal Oct 12 '20 at 5:40
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    Following the comment of @EdBeal, I had a bad capacitor on an AC unit. It was obviously bad since it was bulging out. The capacitor should not be bulged anywhere. Word or warning though: the capacitor's job is to hold charge. If you remove it and touch the terminals you may get the shock of your life, so be careful. – tnknepp Oct 12 '20 at 11:06
  • @tnknepp that could be the last shock of your life... A worthy warning, indeed. – FreeMan Oct 12 '20 at 16:28
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HVAC blower motors, which are almost always of the INDUCTION motor type, fail for a variety of reasons. I'll list what are common reasons:

  1. A winding failure, usually an open. These can be caused by a manufacturing defect or just as a result of thermal cycling. Shorts between windings can also occur often due to conductors rubbing on each other or against the pole material.
  2. Bearings can fail often due to foreign material that collects.
  3. The rotor laminations can rust and cause binding.

As noted in the comments, these often have a starter circuit with a capacitor which can fail. This generally leaves the motor operational but it cannot be started.

Usually the first thing to check is the starter circuit and capacitor.

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  • To clarify point #2, running with a collapsed or missing filter can dramatically shorten fan motor life. – isherwood Oct 12 '20 at 13:49
  • Can the bearings be cleaned or lubricated? – amphibient Oct 12 '20 at 16:58
  • Maybe. It depends on how damaged they are. – jwh20 Oct 12 '20 at 18:02
  • I'd also add that a motor can fail because of a clogged filter. People install these filters designed to trap 95% of microscopic particles, and then the filter gets clogged up and not changed. That creates extra load on the blower motor. The filters are really only their to protect the furnace from large particles. – Steve Sether Oct 12 '20 at 23:34
  • @SteveSether -- you can run a high-filtration filter as your HVAC filter provided you have sufficient filter area (2sf of filter face to the cooling ton, minimum) for your CFMs to keep the static pressure drop across the filter manageable (it helps if you have big, fat, well-engineered, low-static-pressure-drop ducts instead of a wheezy, congested oodle of flex). See more here – ThreePhaseEel Oct 13 '20 at 1:25

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