Rust in A/C condensation pan

I noticed a large discolored area around the drain pipe outside the house for the condensation pan. It looked like rust. I got out the ladder and climbed up in the attic to inspect the two units. One was fine and the other has rust inside the bottom of the pan. I cannot tell if the rust is from the pan (nothing obvious) or if rusty water is draining into the pan. There was no standing water at the time of inspection, but then the A/C units had not run since last night.
The drain pipes for both pans connect to the side of the pans, not the bottom. This allows a small amount of standing water to remain in the pan until it evaporates away. It seems to me a better design would be to have the drain on the bottom of the pan at the lowest point so there would never be any standing water.
I am looking for suggestions of whether this pan should be replaced (6 years old) or if I should just keep an eye on it.
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This is Turtle
In 20 years replace the pan and the whole unit together.
TURTLE
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Ike wrote:

    I am not sure I totally understand your situation, but I shall volunteer this: Most air handler units that have the evaporator coils in them, and have two pans. One is internal and one is external. The external one is a safety pan, and water in it indicates the internal one is leaking. It would require opening the air handler to replace this internal pan, or observe its condition.
    If you should ever replace a condensate pan for an evaporator, spray it with undercoating spray first and let it dry. The undercoating will make the pan last much longer than just the zinc coating on the sheet metal.
Ken
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Ken, it sounds like you have a grasp of what I am trying to describe. Is replacing the internal pan any more involved than removing a few screws, opening it up, replace with the new pan, and then screw things back together?
Also, by undercoating spray, I assume you are referring to car undercoating spray, right?
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Ike wrote:

    As Anthony said, it is not as simple as removing a few screws and pulling the old pan out. In my units you needed to raise up the coil about four inches so that the pan could be replaced. (The coil sits inside of this pan) This meant much dis-assembly of the air handler ductwork and CAREFULLY raising up the coil, since I did not evacuate the system. If the pan is welded in, then Anthony is correct, you would be better off replacing the coil or more.
    With regard to the undercoating, yes I mean automobile undercoating in a spray can. If you use it, let it dry thoroughly before installing the pan.
    There is an epoxy paint that can be applied over a slightly leaking pan, but it is NOT cheap. Search for CONDENSATE PAN LEAK on the Internet and I am sure you will run across it. The pan can usually be made for less than $30 by a sheet metal fabricator as they are not a standard size.
Ken
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Thanks to all for your help. I have a much better understanding now. I did not know there was an internal pan. But, that explains why I see evaporated rusty water in the external pan with no obvious signs of rust in the external pan.
Whatever the extent of the rust damage I have, I need to fix it. The drain on the side of the house is staining the house and the concrete walkway.
Thanks again, guys, Ike
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