I had my annual ac inspection. House has two units and one runs about twice as long each day as the other because it cools much more of the house, and had its capacitor fail two years ago.

Tech noticed rust on the fan motor and compressor of the more used unit. Other unit has no rust. Pic below.

Rusty fan motor

He said rust was a sign of the components working too hard, and recommended replacing the fan motor as we start another long hot summer. (He did not recommend doing anything for the compressor due to cost.)

He was polite and knowledgeable, and said everything else looked good. Does his recommendation to replace the fan motor seem reasonable? I'm happy to spend money to prolong the life of my unit, but wanted to make sure this expense was not pointless.

  • 5
    Rust has very little to do if something works or not. Rust happens to steel that is in air with moisture in it, unless the steel is protected by paint or oil. If the motor is getting hot enough to burn the paint off, then he is right. Paint will also wear off do to simple wear to being outside.
    – crip659
    May 21, 2022 at 17:25
  • 4
    A crappy paint or moisture can pop the paint off in one season and the rust starts after that I have seen motors older than me still running, many open drip that are rusted inside and out with damage on the rotor from bearing failure but I have NEVER seen a motor failure from external rust.
    – Ed Beal
    May 21, 2022 at 19:19
  • 2
    If you want to freak your AC guy out, point to the front brake rotors on your car (particularly if it was rainy/humid yesterday and you haven't driven yet today). They'll have a nice coating of rust on them. Tell him you're going to follow him on the rest of his rounds for the day and that you hope your brakes will work... Don't worry, all that rust will have worn off by the time you've backed out of your driveway...
    – FreeMan
    May 21, 2022 at 22:57
  • It tells me that you need to be prepared to replace the entire thing in 10y or less because it's already past its first decade. And if you live somewhere where AC is the difference between life and death, you need to get ahead of that. But you have two units so you can ride-out all of your equipment into the ground if you want (I would).
    – Mazura
    Jun 24, 2022 at 1:38
  • The rust is just a sign of having left the unit outside. I try to keep expensive equipment indoors, but my HVAC guy told me the unit has to be outside or I won't enjoy it very much. Jun 24, 2022 at 4:46

2 Answers 2



Most motors have a external laminated iron core and cast end bells. This metal when outside may rust that is what happens to the iron.

We do paint motors when we rebuild them to reduce the loss of insulation on the laminated iron core but as long as the motor is running don’t fix what is not broke!

The laminated core reduces Eddy currents in the core these are small and will not be a problem infact most homeowners could even measure because they are small.

When the bearings fail that’s when I suggest most DIY replace the motor but those that have some mechanical abilities can replace both bearings for under 20$ in most cases on these fractional HP motors.

Will a new motor prolong the system life? Not for external rust but bearing life as a new motor will have fresh bearings.

You can look at the current draw on the motor and compare it to the nameplate FLA if +10% after starting the bearings probably need replaced but not because the external stator is rusted.

  • If the housing is painted like @crip659 mentioned could it be getting hot enough to burn the paint off? I would not have given second thought to the rust except the other identical unit is completely rust free on both motor and compressor May 21, 2022 at 18:37
  • 3
    Aggienick, I have worked on fractional horse motors up to hundreds of horse power motors do you realize how hot a motor has to get to burn paint? Only an amateur would make this statement and not have evidence of burnt paint, that if it was hot enough to burn the external painted surfaces don’t you think the clear lacquer finish on the motor windings would have been long gone? Just saying if the motor was hot enough to burn the exterior paint off that motor would be toast. Ok you can decide on your own but if the exterior really got hot enough to smoke the paint the bearings would B cooked
    – Ed Beal
    May 21, 2022 at 19:14
  • Thanks @ed Beal . Very much appreciate all of your knowledge and advice. Confirms what my gut told me but I didn't have the knowledge to know better May 21, 2022 at 19:30

My 20-year-old HVAC condenser fan motor was also Rusty and always sounded like it was working hard. It too blew a capacitor last year. I also noticed that the breaker bar had hot spots on it like it had arced. I replace the fan motor myself and after I was done it worked much better and was much quieter. Also I took apart the other fan motor and it was also rusted on the inside clearly water was getting into it, and it would have failed shortly for sure.


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