Ok. I would like to show you this pick of the attic furnace, the one I mentioned earlier. The HVAC technician has one of the condensate lines tying into a vent stack. If I remember correctly, you shouldn’t have any type of drainage past the last plumbing fixture in the house.
This is my issue with it.
“Sewer gases include methane which is an explosive gas. We don’t want methane nor bacteria in our air conditioning system.”
Good information here
https://inspectapedia.com/aircond/Condensate_Drain_Problems.php
Furthermore, it appears sewer gases are now venting to the attic space thru these small vents.
@mwilles do you have anything to add or correction to my comment?
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Exactly, thanks so much for your input!
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Thanks Scott!
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Nice job Brian you nailed it. Condensate drains should never be tied directly into the sanitary drain system for the reasons you mentioned above.
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Did you look for a termination point for the underground system? Systems like this are notorious for clogging (and freezing) if not installed correctly.
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Thanks Michael! I will check that out.
You have your hands full with this one. Looking at what I can see in the pics, there are numerous moisture entry points and most likely hidden rot.
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Yes, there are a lot of openings to be sealed to prevent further moisture intrusion.
I’d like to comment on the cracks, and this is advice for everyone.
Avoid at all costs - unless you can see it with your own eyes - trying to diagnose a cracking issue. It could be anything. If you feed your client the wrong information, and they act on it, then they are not going to remember you fondly when the contractor finds that the conditions you assumed to be at play were in fact wrong, and the issue is something else entirely.
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Exactly, Darren! 
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Thanks for the advice. I see that trying to be helpful to a client might back-fire if you don’t know the actual cause of the defect.
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In a situation such as this Martin, what would be the proper way to plumb a condensate drain in an attic space? Taking local climate into account… Is this addressed in the IPC?
If the air handler is installed in the attic it’s probably installed in a climate where the condensate drain can discharge to the exterior.
Otherwise a P-trap can be installed from the sanitary drain to the air handler and the condensate can discharge with an air gap into that drain.
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Thanks for the IPC information!
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David, @mwilles is what I think of as the “resident plumber” here on the MB since he is an actual licensed plumber. Very helpful and knowledgeable, just don’t piss him off. 
or you may suffer the same fate of the infamous"Drip Kid"…
There’s alot to learn on here and most are more than willing to help.
Keep in mind, apart from not making assumptions about causation of a problem, as an HI, use code like what Martin provided as a reference and don’t cite it in your report. That can open a can of worms for you.
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Good stuff! I prefer to focus on the consequence of not following code, such as safety. And in this case, sewer gases and bacteria. When you do this, you never have to use code to defend your position on any particular defect.
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David read over the requirements for condensate drain carefully. In my area this is an issue with every single 12 month warranty inspection. Last week there was a hump in the primary drain causing it to not drain properly. As a result water came out of the secondary evaporator pan drain onto the auxiliary drain pan. Fortunately my client did not go through a cooling season so there is no corrosion in the auxiliary drain pan.
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