More HVAC headaches - plugged drains

Any help appreciated here. Working in a cramped attic space in the heat troubleshooting this has worn me out and I’m out of ideas.
Home has three units, all working.
Problem - Secondary A/C drain is active indicating plugged primary drain.

But -

Secondary drain pans on unit 1 and unit 2 are dry, indicating their primary drains are working.
Secondary drain on unit 3 is full and actively draining out the soffit vent of the home.

All three primary drains are plumbed together to a totally plugged drain to the sewer line. This line has a standing column of water all the way up into the attic. I have applied 60 PSI air to the common drain line to the sewer and it won’t blow out. :anguished:

Turns out that unit 1 and unit 2 primary drains have thus been flowing retrograde into the condenser pan of unit 3, activating its overflow into the secondary drain.

Units 1 and 2 run together:

After this junction I added a valve so I could create pressure distal to the valve for blowout:

Fortunately the third unit was just slip fit and not glued into this junction, where I unsuccessfully 60 PSI applied air pressure to blow out the plugged line:

Unit 3 cut line for troubleshooting:

Secondary drain pan of Unit 3 is full and draining through the secondary line:

The plugged line is a 3/4" PVC line that runs down an internal wall in the home, presumably into the sewer line. I have not been down there to see how it connects. That wall is shared with a bathroom but the sinks are on the opposite wall although there is a tub and shower near by that wall. Both are draining properly.

Thoughts on the next step? Can that 3/4" line be snaked with a cleanout device? Extremely tight quarters in there :anguished:

A prior HVAC guy did a blowout of some sort a couple of years ago and said he things he lost part of his blowout gun into one of the lines. Perhaps a rubber tip of some sort?? Don’t know if this is lodged somewhere.

Not sure how to proceed. I’d settle for just running a primary drain somehow to another soffit and just let it drip but the roof is very low pitch and a real pain to work in. Having all three units backing up through the condenser unit 3 primary pan is a disaster waiting to happen.

I described the problem to one HVAC guy and he did not want to work on it. My original guy was great but passed away :anguished:

My brother in law is an HVAC tech, here in Dallas, he can do a really reasonable job for you. I set him up with Frank Lima and he raved about him.

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Please send me his contact either here or PM. Thanks.

But I’ll still take suggestions on cleaning out this 3/4" clogged pipe from the attic to the sewer line. Friend recommended the $15 manual drain snake from HF but this one gets better reviews and I am getting frustrated and if this better one can facilitate things in a really cramped space then I’ll go for it.

Thoughts on this? I’ve never used one.

I suggest a wet vac. You would be surprised what you can pull out rather than force it through. You will, of course have to plug the other two drains to get a vacuume


The reason why it might be stuck is because whatever the obstruction is, it is physically bigger than the turn or reduction in pipe size. Although just because it will hold water doesn’t mean that it will hold a vacuum. At this point it wouldn’t hurt to give it a try.

My next step would be to send a cable down the stopped up pipe and see how far the obstruction is. From there, head over to the most likely location for where that is, and if I have to cut into a wall, then do what I have to do and cut the section of pipe out. If obstruction is found, replace section of pipe. If not, use the cable to find out how much further the obstruction is.

If you go down this route, make sure you have a way to catch the water that is stored in the pipe, otherwise you may find yourself with some sheetrock/flooring repair.

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I’m sort of curious where this really connects to the sewer. Ours actually comes out of the wall under one of our bathroom sinks, and has a section of dishwasher drain hose, connecting it to a sink tailpiece with a dishwasher port on it. I’m not sure what they would have done with this not lining up, but it might be worth double checking under the sinks.


I would try a snake first but as I found out mine was over 20 feet long and it also had a J trap which made it hard to snake. I just used Drano and it cleared after 30 minutes. You might try one of the small cameras that hooks to your phone, they are pretty cheap on Amazon.

Mine drains into a bathroom sink, it was plugged yesterday and my float switch opened to shut unit off. I used shop vac but didn’t get as much water as i expected, that worked last time this happened 5-6 years ago. I used high pressure co2 cartridge and that seemed to open it up, then let it sit for about an hour with the end plugged and vinegar in the pipe. When i got home from buying a tiny plumbing snake at Lowe’s and opened the line it drained fast. Friend at work says put bleach in the drain a few times a year. I installed a ball valve last time it got plugged so i could shut it then take hose to sink drain off and get it in a bucket without risking draining all the water in the line into the bottom of the vanity. Otherwise use a shop vac to catch any water that comes out

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Thanks for all these ideas. Shop vac and snake ideas, draino and CO2 and bleach all good ones. Too bad this is such a common problem. One might think there would be a better solution. I guess good PM on your system is the best bet.

cup of Clorox twice a year,

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Some followup - I bought the 23’ HF snake as noted above. I almost bought the big power driven one. Glad I did not.

With about 1’ of snake into the drain I noted difficulty. I then noted that the pipe went through a 90 deg elbow then immediately another. The big snake probably would not have made these curves. Not sure if it would go, I pushed and it proceeded to continue. At another few ft there was a slow down, perhaps this was the obstruction. But I managed to get 15’ into the drain where it then stopped. I was uncertain how much I had run into the drain so I measured it coming out and was encouraged by the 15ft.

An air blast seemed to go without difficulty. Next, one gallon of water went through.

Knock on wood, but it’s looking good. Reassembled the pipe and running the units now.

Thanks again. Will indeed keep up with PM.



Addendum . . . after running my three units overnight, the primary drain is working.

That drain snake is the best $15 or whatever that I ever spent and now I’m familiar with using it. Could not believe it made it around those 90 degree turns.

Note for drain design - I would choose to place a couple of 45s instead or some sort of slow curve in order to allow for cleanout. But the plumbing for this HVAC is terrible anyway and they spared no shortcuts to get the job done.

BTW, I would generally get 10-15 gallons of water daily from my overflow drain. I used it to water plants. Too bad HVAC water is not routed into some sort of useful function. It could have at least watered one side of my foundation and most of the garden on one side.


For those following along, before trying bleach,drano,etc… try one of the self ballooning hose attachments.

Husky Medium Drain Bladder

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That’s a cool concept. Not what I expected from the picture. Watch the video. Seems that the bladder fills to create pipe occlusion followed by water flow and pressure buildup from the tip of the bladder.