I'm amazed at the amount of water running through this thing. I guess
I just hadn't thought about it. We live in a very humid area and it's
been miserable so the AC is getting lots of work.

I don't think the main drain has an outlet outside the house. It runs
into some insulation after it leaves the handler but it appears to be
headed towards the bathroom and the drain pipe there.

I think I'll try to run some wire through it and remove the blockage.
I'd have to cut the end off of my snake to make it fit. I think the
opening in the main line is an access tube, maybe for this purpose?
Thanks for all the great suggestions.