Here's a picture of the full set up from one angle. I don't know enough to know whether there is a humidifier involved.
I bet the A/C coil drain is not trapped, causing the supply air to blow into the drain and gurgle. (This observation doesn't help because water in the trap would dry out in the wintertime.)
I can replace the curved part of the pipe by the drain on my own with a simple elbow. Should I do that as a first step to see if it resolves the issue or do you guys think I need to have someone come investigate? (OP already got all the info he needed, why would he needs more?)
Your installer didn't pitch the flues correctly (This is just stupid), or your son threw something inside the flue pipe. Or maybe a bird. (If this would be the case most likely the pressure switch shut the furnace down.)
Doesn't look like the gurgling has anything to do with the venting, it's coming from the trapped water in the drain pipe due to the air pushed down from the a coil drain.
When your partially restrict a GMH's venting, the pressure from the inducer overcomes the restriction in the trap, causing water to splash out from the trap.
It trips on P/S. Then retries.
If it's a bird, it will do it over and over, in quick succession.
If it's an incorrect flue, where water lays in the pipe, the water will build up, splash, trip the PS, drain, run for bit, and go over and over.
Sadly seen it many times. (Well, OP never mentioned anything like that, so why bother?)
Are you suggesting that the air from a coil is forcing water 2' up a 3/4 PVC pipe? (This is a fact, some people just don't like them.)
I watched it more closely and the water getting expelled isn't getting force up from the bottom.
What is happening is the water trickling over into the pipe... the air coming up the pipe is hitting it as it comes over the edge and splashing it out.
I have the original installing company coming tomorrow to look at it.
I guess my question at this point, can you guys see that the installed these pipes incorrectly? Is there a trapping issue (whether wrong or missing)?
I don't want to pay them to fix anything if they did it wrong in the first place.
Have them trap the A/C condensate pipe then vent it immediately after the trap, so when it dries out it doesn't blow up the vertical pipe to the furnace trap. Then get rid of that riser near the termination. Add pitch from the tee at the floor to the termination. The installer did a great job with everything else. Perhaps he will learn from this and his installs will work as good as they look.
So, the only thing they did was add a tee to vent at the top of the A/C coil drain pipe to let the air escape there instead of back up the furnace condense drain up.
He said the riser at the end near the drain was correct and the only trap needed for those two drains. He said traps aren't even needed because there's no "gas" in these pipes. Like the previous poster said, the manual for the A/C coil does show that a trap is needed.
He then told me if the problem continues to go ahead and just remove the riser at the end and replace it will an elbow to go down the drain.
So, is this a satisfactory fix? It seems to have stop the gurgle, but could it lead to other problems? (If the pipe is partially clogged it could be a problem in the future.)
I also asked them why I should have to pay for this, and they said that it passed inspection when our home was built, so that takes the liability off of them. (I don't think this is right. All we know an inspector is a human and can miss something. For example my friend got his furnace replaced, and inspected, but an installer cut transformer wires for humidifier and never hook them back up to the new one and no one even noticed.)
It is fine the way he did it but you could have done that yourself. When you call a company out, expect to pay the bill. Personally I would have removed the inverted trap and added a trap to the A/C condensate line. If it bothers you, fix it. It will probably be less than 5 bucks in fittings.
I thought DIY was discouraged on here, so I didn't want to get banned from the site for taking everyone's advice on here and then doing it myself. I'm completely comfortable redoing the piping myself, but I just wanted to know if what he did is the proper solution. I was trying to read about the purpose of a trap on the condensate line and I was getting the impression that the goal was to prevent air from going through the pipe (in or out) and maintaining pressure. What he has done doesn't do either of those things. He tried to tell me the only purpose of a trap is to prevent "dangerous gas" from escaping and that these pipes don't have gases in them which is why he said it wasn't needed. Based on everything I read... he was just feeding me some BS. (I understand OP's frustration, but as I mentioned above he could've fix the problem himself long time ago! And this is all my website is about. Learn how to do something and do it yourself.)