Tenant messaged me that the upstairs bath sink in the jack and Jill was not draining and the water level actually fluctuated over the last few days. The tenant said they snaked the drain and didn't hit any obstructions. They shop vacced the drain and put some drain cleaner in and said it is starting to slowly drain. I am pit of town so I haven't been able to mess with it.

I would have bet a check the ac drain. Line was not connected to this but now I am second guessing that.

Any input on what could be causing this? I should be able to pull off the p trap this week and also look at the ac drain line again and see if it does go in here. I intend to blow down the ac drain line and put bleach in it just in case, but I am honestly not sure what this could be without having put my eyes on the problem.

Air vent clog?

I think we have a very good rapport. Very similar lifestyles and existence.

Can you break this down for me? Not something I am familiar with.

Disclosure, I lived in this house for 5 years before renting it out and have also changed out the faucets on this particular sink 2x so I have a pretty good working knowledge of the home.

Ain't nothing but hair and toothpaste. Have them actually buy the good stuff at HD. The HCl in the black jug in a plastic bag. Use that. Dude bought dollar store dranO.

Im pretty sure it was whatever the stuff at home depot is thT I had st the house for another issue in the past unrelated to this.

the vent pipe that is attached to this drain likely pokes through your roof outside. If it is blocked by something, nowhere for air to go when water flows down the pipe. Sometimes it happens.

Ok Roger that I get it. Weird thar they said that some drain cleaner had it draining again. I'll check it out when I get home. Thank you.

Piggy backing on the clogged vent...

When was the house built?
I'm not sure when AC installers started tapping into the sink above the P-trap with the condensation drain. Prior to that they tapped into the vent stack in the attic. Depending on how they did this, it can create a problem. One I experienced (and learned to do sheetrock on) in the house I grew up in, and narrowly avoided in my current home. The condensation line just flowed into an open pipe tied to the vent. This can allow an algae clog to develop in the vent pipe.

It could be in the drain line below the sink. In this case it would backup into the sink and could the be source of the fluctuation in level that your tenant described. Drain cleaner might have helped with it a bit.

If it forms higher in the vent stack it will backup to the secondary line if the tie-in is sealed, or it will overflow into the attic (like the home I grew up in) if the line just drains into an open pipe.

If the clog is below the sink it does seem strange that snaking the drain didn't solve the problem.

Home was built in 93 i believe. I'll have to get in the attic and have a look around

I had a similar issue from the AC condensate drain (into secondary sink) causing buildup in the P-Trap and beyond.

Poured drain cleaner down, helped slightly, but came back, same with a snake. What eventually worked was really, really hot water and dish soap.

Like you poured a pot of boiling water down the drain?

I didn't go that far....

I let the sink drain completely, poured a decent amount of dish soap into the drain, turned on the bath tub to all the way hot, then filled a pitcher and poured it down the sink. Repeated a couple times, the first time it filled up then drained at a decent speed. The second time it filled up about half way then drained instantly. Then I turned on that sink to all the way hot and just let it run for awhile.

I agree with you, as a sudden venting problem less likely than a novice house tenant's "care" of plumbing is.

The best I could find at Home Depot was
HCl in the black jug in a plastic bag NOT but instead found
https://www.homedepot.com/p/Instant-Power-67-6-oz-Hair-and-Grease-Drain-Cleaner-1970/100169339#overlay

Since I suspect HAIR & its products + grease then something like this
https://www.homedepot.com/p/FlexiSnake-Drain-Weasel-Hair-Clog-Tool-Starter-Kit-for-Drain-Cleaning-3-Piece-DWPSK2/203483754

If I were the Owner, would phone tenant to go buy & employ these items, then reimburse them = saves time, teaches a maintenance lesson, all by phoning it in.

when I read "pour ______ down the drain", I am assuming someone is taking off the stopper and being able to visually inspect the drain down to the p-trap. Hair usually gets jammed up at the bottom of the stopper, and it's mechanism in the downspout. It should be obvious.

If someone if just pouring stuff into the sink and letting it go down the drain, like the water does, I doubt this will be very effective.

Happened to us on a VRBO rental - sink would not drain. Took the stopper cap off and apparently Chewbacca stayed there previously. Got a $2 plastic snake thingy, cleaned it out. Told the owner I took care of it and now I get a free night next time we go to Colorado.