Finally got my parents' AC drain line sorted out

Did basically the same thing at my own house two summers back when the unit started dripping water through the ceiling of the hallway. Found the same pudding-type gunk in the line, and it turned out the previous owners had the line running through the attic where it sagged, so water just sat there and grew slime. The shop vac trick pulled out a solid plug about the length of my forearm, which was both gross and weirdly satisfying.

One thing I learned the hard way: after you clear the line, pour the water flush in slowly from the access port near the air handler, not fast. I blasted it in the first time and water backed up into the pan and tripped the switch again, which made me think I'd failed when really I was just flooding my own work. Slow pour, watch it drain, then treat. I've kept up a vinegar routine too, about half a cup every couple months during the humid season, and I haven't had a repeat clog since. My cousin does HVAC work and says the people who end up on his emergency call list are almost always the ones who never think about that line until the ceiling stain shows up. You might also check whether the line has a proper cleanout tee somewhere along the run. Mine didn't, so I had one installed when the attic section got rerouted, and now flushing takes two minutes instead of hauling the vacuum around the side of the house.

I did the vinegar routine at my place for two seasons and it only half worked. The line on my Carrier kept sliming up anyway, and every August I'd be out there with the shop vac again. What actually fixed it for me was cutting in a PVC cleanout tee right after the drain pan, so now I pour water and tablet treatment in from the top instead of guessing where to dump it. Took maybe twenty minutes with a pipe cutter and a coupling, and the line has stayed clear for three years since. One thing I'd check on your parents' setup is where the line actually exits. Mine ran through the wall and dipped low near the foundation, so even after it flowed, a little water sat in that dip and started the slime cycle over. I added a bit of slope by rehanging a section with some strap and it made a bigger difference than anything I poured down there.

Also, I'd ease up on the vinegar if their pan is metal. Mine's plastic so it didn't matter, but I've heard vinegar sitting in a metal pan month after month isn't doing it any favors. My dad's is one of those plastic secondary pans so it's prolly fine, just something to look at before committing to a schedule.

The thing that finally made my mom's place stop clogging was a packet of those slow-dissolving drain pan tablets. I'd flush the line every spring and it'd slime right back up by August anyway. Toss one tablet in the pan when the weather turns warm and the water running out the end stayed clear all season after that. Also, if there's a trap in the line, pop it apart once. Mine had a pocket of sludge sitting below the tee that the vacuum never touched until I pulled it apart and rinsed it under the hose. Takes all of five minutes.

Nice save on the service call. One thing I'd check next time you're over there is the float switch itself. Mine got so gummed up with that same sludge that even after the line was clear it'd stick and shut the unit down randomly. Pulled the cap off and cleaned it with a rag and some vinegar, been fine since. Also don't stress about the exact amount of vinegar, I just dump a glug in and call it good. Monthly sounds like plenty for a line that's alredy been that bad.

Seconding the slow pour advice, that caught me out too. I use a funnel and a gallon jug of water now and it takes a good five minutes to feed it all in. The tell that it's actually clear is watching the end of the line outside, you want a steady drip the whole time you're pouring, not just a trickle at the start. When I rushed it once the water pooled in the pan and I thougt the clog was still there. Also mark the vinegar on a calendar or you'll forget by month three like I did.