Hello!
as you can see, the water from the HVAC overflow pipe is mixed with rust (I believe).
The homeowner is asking: what causes this? how to fix this (as in, can they fix it themselves)? and how to clean the outside wall/concrete from the stains?
Thank you!
That’s the secondary condensate drain line. Which means the primary line is clogged.
And with that much rust, it means the inside of the AC coils are holding water and rusting out.
He really needs a HVAC tech, not a DYI project.
An HVAC professional should correct this ASAP. Water should not be flowing out of the secondary condensate drain line. If you try to provide DIY / handyman tips for this type of issue, there is some financial liability involved. In reference to cleaning the stains, I would suggest they call a power washing company to see if they can help.
understood, thank you!!!
The above advice is perfect. I am adding some imagery.
If you performed this home inspection, in the attic you would have encountered a unit similar to the photo below. The pan was likely rusted and possibly filled with water. If the drain pan under the unit has water in it, something has fouled with the primary condensate drain system, for the one under the unit is a secondary or emergency pan.
The reason why the secondary pan drain pipe is located above a window is to alert the home owner there is a problem with their system. The moment the home owner observed water dripping from the pipe over the window, they should have called for service.
They likely did, thus immediately placing their home ‘For Sale’ once they received the estimate for replacing the HVAC unit and all associated accessories!!
out here the HVAC companies put compressed air on the primary at the outside drain and blow it into the attic
Easy and free… Pay attention 
As others have said, that’s a “tattletale drain”. Supposed to cause the occupant to do something when they see dripping water.
And you can also do it with a water hose, but don’t over do it…
I first thought you where referring to the roof deck tile overhang.
Any buzzing going on in there?
While it does, the staining appears to be oxidized metal, the plastic pipe does not decompose leaving that color of staining.
Adopting to the hypotheses a secondary condensate drain line is blocked because the primary drain line is blocked leaves me confused.
Still hear a buzz in my head.
