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Old 09-24-2015, 04:15 PM   #1
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Condensate Drain Line Mystery


Hello all,

I just registered today to seek the collective's wisdom on a very puzzling problem I've been having with my condensate drain. I recognize this is an old thread, but my problem seems similar to the OP's problem so, I thought I'd resurrect this thread to describe it.

My upstairs air conditioning unit is about 10 years old. The drain is under positive pressure, and there is no p trap on either the primary or secondary drain. The primary drain runs from the primary drain pan through 3/4 inch PVC pipe to just above the p trap under a bathroom sink. The secondary drain runs into a secondary pan and, eventually, out through PVC pipe under the soffit. If the secondary drain is "active", water runs out and onto my driveway right in front of my garage door. I can't miss it. I have not had any drainage out the secondary drain for 8-10 years.

About a month ago, I noticed water coming out of the secondary drain. I suspected a clog, so I went into the attic to check it out. There is a vent in the primary line about two feet from where it starts at the drain. The line was dry, indicating there was no clog going toward the sink, but I poured chlorine bleach down that line anyway while my wife listened at the bathroom drain below. There was no blockage downstream of the vent.

I hooked up a shop vac to the primary drain at the primary pan and sucked it out. I didn't get much debris out, but I figure I got enough to unclog it, so I put everything back together exactly as I found it -- exactly as it had been for years.

Everything seemed fine for about two weeks, when I noticed water coming out of the secondary drain again! I've been through several cycles of attempted diagnosis since then so I'll summarize where I am now.

If I remove both PVC fittings from the primary and secondary drains under the evaporator coil and observe what happens when the unit runs, I notice that water eventually begins draining out the primary drain, exactly as it should. Next, I attach the 90 degree PVC fitting to the secondary drain, so it will drain in the secondary pan. I run a straight section of PVC out of the primary drain about 5 inches. When I turn the unit back on and observe, all is well. Water comes out the primary drain only through the attached 5 inch pipe.

Next, I attach a 45 degree fitting to the 5 inch pipe and observe water draining from it when the unit runs. All good. Next, I connect a short section of pipe to the 45 degree fitting, then connect a 90 degree fitting, then run pipe into the main drain, which eventually terminates under the bathroom sink. At the point where I connect up to the main drain, there is a t fitting, with the vent facing "up" and the drain continuing "down" to a 90 degree bend, then a long section of pipe and down under the sink.

I realize after typing this that a picture might be helpful. I'll try to do that and post it, but I'll continue for now just in case words are sufficient.

When I connect everything together and observe, everything seems good. Water drains out the primary only, and I can see it as it runs down toward the sink by looking into the t fitting. I leave the t fitting open.

The mystery is that, after a few days, I find water coming out of the secondary drain again! Inspection of things upstairs reveals that the primary line is dry, and water is only coming out the secondary drain. Moreover, when I put my hand over the t fitting vent, it's sucking air instead of blowing air (the drain is under positive pressure)! When I take apart the PVC (I haven't glued it yet), I get water in the first section of PVC as normal, and as soon as I connect it to the rest of the drain, it works!

I have no idea what's going on here. It strikes me that there something about air blowing down the line that is creating negative pressure, but I don't get it. Furthermore, this whole drain thing was not a problem for over 8 years before this.

Any help from the experts?

Thanks!
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Old 09-24-2015, 05:01 PM   #2
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Drain Line Mystery


OK, correct me if I'm wrong but you have lines connected to both drain outlets on your condensate pan? Usually only one or the other is used depending on the position of the coil and the other is plugged. Not a big deal but just uncommon. This may seem off the wall but how clean is the inside of the evaporator coil? I can't see that there is a blockage in the drain from what you have described but there could be a piece of debris of some kind or other diverting the condensation. It could be in the pan or it could be something hanging down from the coil itself.
I had water running out from a unit located in a beauty shop that had all the earmarks of being either frozen up or a rusted out drain pan. The inept duct cleaners back blew the dirt and hair in the run and failed to clean the coil and the material was diverting the condensate away from the pan.
The only way to know for sure will be to pull the covers off of the plenum and the coil and look inside at the coil and you will be able to see if there is something in the pan that could be causing the problem. Don't get rough with it and don't force anything. You may have to cut your way in and repair or make a new panel. My advice, if you can't get that plenum panel off with screws and you are not experienced with sheet metal and duct work call a pro. It will cost a lot less to pay someone to do it safely and properly than to have to replace a coil or a system because you accidentally clip off a capillary tube. If they do it they will have to make it right on their own dime!
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Old 09-24-2015, 05:35 PM   #3
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Thanks for the quick reply, Garry.

Yes, I have PVC lines connected to both outlets, one of which (the secondary) sits "higher" on the pan than the other. I have two units in my house, and they're both configured that way. Just as an aside, I'm not quite sure how to have a "secondary" drain if the "secondary" outlet is plugged. Could you help me understand that? Maybe if I did that I could limp through the rest of the season and have a pro come out when things cool down, and when service is less expensive.

I think you're right: The only thing that really makes sense is that something happened around the coil or in the pan behind the panel since it's worked for so many years. I've never cleaned the coil, but I'm really careful to change my filter when it gets the slightest bit dirty. Regardless, I'm uncomfortable opening that area up, so I'll have to call a pro to have a look.

One thing I've observed most recently: I decided that the secondary drain outlet, with the attached 90 degree fitting, was effectively just a bit larger in diameter than the primary, so I put a short length (4 inches) of 3/4 inch PVC into the fitting and it hasn't backed up for about 24 hours. Now I'll probably go out there tomorrow and find water in front of the garage!

Thanks again for your help!
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Old 09-24-2015, 06:46 PM   #4
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Secondary drains are common for attic units. Even more so, the further south you go. It's not as common in places that have concrete basements where the furnace sits. (not a big deal when there's a leak, unlike ruined drywall.)

A picture really is needed.

Cheers!
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Old 09-24-2015, 07:11 PM   #5
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Made this its own thread.
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Old 09-24-2015, 09:27 PM   #6
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Thanks for all the help, and thanks for setting this up in its own thread. I thought it would make sense to keep it in the other thread, but I see now that it's better off on it's own.

I've attached a picture showing both the primary and the secondary drains and the piping I have in place. Note the primary drain connects to a 5-7 inch length of PVC followed by a 45 degree fitting (A), then another 10 inch section of PVC, then a 90 degree fitting (B), then it connects to a t fitting about 8 inches further down (D). The t fitting is open as a vent at the top. The bottom goes down to a 90 degree fitting (E) then out and down toward the sink in the bathroom. This setup is all running "downhill".

You can see the secondary drain has a short section of 3/4 inch PVC that drains directly into the secondary pan, and there is PVC (E) that connects the secondary pan to where it eventually exits the house under the soffit. You can see water in the secondary pan, but nothing has come out of the secondary drain in over 24 hours now.

I thought about Garry's earlier post, and it seems like everything points to a problem behind the panel with the primary pan or the coil. On further thought, though, I think it's worthwhile to point out that, if the drain "backs up", all I do is break the connection at A, observe water draining out at that point, and reassembling it. Then it will drain for an extended period without backing up again.

When it's working properly, if I hold my hand over the vent (D), cool air is blowing out. When it backs up, air is being sucked in at (D).

Thanks again for any help.
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Condensate Drain Line Mystery-upstairs-ac-condensate-drain-annotated.jpg  
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