Bathtub filling with AC condensate water

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Dallas, TX
Let me apologize in advance... I have a tendency to write long posts and I'm pretty sure this one will be no exception.

About a week and a half ago, I noticed a dripping sound between the common wall for my master bathroom and guest bathroom (which we never use because the toilet needs to be repaired). That usually the sound of the AC condensate drain directly above in the attic being clogged up with algae and goo and just needs to be blown out. From the air handler, there is a single PVC pipe that runs under the unit and ends with a T. The top of the T is pointing up, the bottom drips down into an open ABS pipe that then travels back under the unit diagonally and ties into the vent for the guest bathroom downstairs. Looking at the roof, I have 3 vents (one for each toilet and the nearby fixtures, I'm guessing). I went up into the attic and didn't any evidence of the pan dripping (I have a bowl up there and it was dry). The other end of the PVC drain into the ABC looked 'normal'. Standing water (because I think it has a P-trap there). Blew out the PVC back to the unit. Still puzzled why I was hearing the dripping sound. (In hindsight it probably was dripping from the open ABS, just not then because the AC was off.) A couple days later I heard the dripping sound again. I opened the door to the guest bathroom and found water all over the floor and the sink was completely full of water and running off the counter! I was confused how this could happen, but I was fairly certain it had something to do with the condensate drain directly above. I turned off the AC. I bailed out the water with a cup, dumping the water in the toilet. The toilet had no problem taking the water. It didn't seem stopped up. Apparently there is a clog, but I don't understand how since we don't even use that bathroom AND nothing else behind the sink seemed to be stopped up (master bath sinks, toilet and shower -- not certain about bathtub; we don't use it and it had become a place to leave our swimwear, etc. so I didn't run the water in there). I wanted to snake the guest sink drain, but first I would have to cut of the P-trap. When my son had installed it a few years back, he glued the P-trap to the ABS stubout instead of using a connector. It seemed to me that because the toilet right next to the sink seemed to not be clogged up, that the clog for the sink had to be very close. I cut off the P-trap and snaked it. The more snake I put in there, the more it felt like it was being pushed back to me... almost like I was snaking uphill. It would repeatedly come zinging out of the pipe. I tried a few times and eventually gave up. I needed to understand what was going on behind the walls. Google and YouTube. The temporary work-around was to put a bowl under the stubout to catch the condensate water and just babysit it to make sure it didn't overflow. Another thing I did was to use my Shop-Vac on the drain. That would buy more time before the bowl needed to be emptied. Finally, I realized I needed to help and called some plumbers (on Father's Day). They eventually showed up around 10pm. After trying a few different snakes on the sink drain and the bowl auger on the toilet, they were perplexed. They eventually got their drain camera out and then started making sense of the situation. According to them, the sanitary tee between the sink and toilet was installed 'upside down'. (After reading more about them, they might have meant that it was installed horizontally [aka, incorrectly].) So when we tried to snake the drain, the snakes were, indeed, going up into the attic instead of down to the clog! They climbed on the roof and looked down both vents on that side of the house. Eventually one guy got a power auger out and went back into the bathroom while the other one stayed on the roof with the camera. They talked on their phones. The one with the camera could see the auger head and was giving instruction ("go forward and inch", "pull it back a little", etc.) and guided the auger DOWN into the drain toward the clog. Soon afterwards, the clog was cleared! Still puzzling why none of the other fixtures on that side of the house had any issues. Could it be that there are two or more drains from one side of the house to the other (where it exits the house to the street)? Midnight and $300 later, I was happy that we could run the AC again without worrying about emptying the bowl every couple of hours throughout the night.

Remember that the subject of this thread mentioned a bathtub? This morning, I got up and was surprised to see about 5 inches of standing water in the master bathroom bathtub! A bit horrified, I went to the guest bathroom and opened the door with hesitation. No mess, but the bowl (which had been bone dry for several days, since the plumber's visit) was nearly full of water. (BTW, I haven't put the new P-trap kit on yet because I needed an additional connector - according to the plumber. AND, I just wanted to make sure that there weren't still any issues. I would just check the bowl periodically to make sure it was dry. I didn't want to wake up one morning to find the sink overflowing again.) So here we are again. I turned off the AC. I figure it would be an exercise in futility to try to snake the sink drain since the sanitary tee is installed incorrectly. Instead, I tried to snake the bathtub. I put all 20ft of snake down there... inch by inch. I didn't see that the water level in the tub was receding ever-so-slightly... almost unnoticeable (but I took a before picture). I pulled the snake out and it seemed to continue drain excruciatingly slowly. I turned on the AC and let it run for a while (it needed to cool the house about 4F degrees before turning off). During that time, the water in the tub started to rise slowly (almost unnoticeable) but the stubout under the sink is still dry and nothing in the bowl. I've been looking at a lot of info about this topic and venting, etc. and I'm just baffled at what's going on here. And I really don't want to spend another $300 on another service call.

I've uploaded an attempt of a drawing of what I have. There is a roof vent above the location of each toilet. I'm assuming that fixtures nearby a toilet are sharing the same drain and vent. I've colored those areas (disregard the blue line under 'Tub'). If I were to plumb this, I would have done it like that. It seems logical that the master bathroom tub should tie into the guest bathroom sink and toilet because they're right on the other side of the wall. When this side of the house gets stopped up, the other side is unaffected. As mentioned earlier the rest of the plumbing is on the other side of the house... the kitchen and the upstairs bathroom.

So many questions...
1) Why doesn't any other fixture get stopped up when this is happening? I was under the assumption that everything on that side of the house shared a common drain to the other side of the house. Oh, and... that guest bathroom toilet is the one that I dump gallons of clean water down from my Shop-Vac.
2) How could the problem be resolved for days and then show up elsewhere?
3) Now, if it's filling up the bathtub, why isn't it also filling up the sink drain (out the stubout to the bowl)?
4) How did it previously fill up the sink, but not the bathtub (which is lower)?
5) When the dripping is in the attic, I'm assuming that's because the condensate water had completely backed up in the vent pipe. Why was it only the sink that overflowed, and not the bathtub and toilet that I suspect share the same drain/vent?
6) How is it that 20ft of snake can't clear the bathtub drain, but nothing else affected?
7) How is it that the water level in the tub didn't start to recede until I started snaking it (maybe 8ft or so), yet I can't clear it?
8) How did the bowl go from dry to almost full last night and not a drop today, while the AC has been cycling all day long?

Thanks in advance for your responses. If you have any questions, please don't hesitate to post them.



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