I recently added a dehumidifier in my crawl space and need to add a new line to deal with condensation water. My AC system has two drain lines, one primary and one secondary, both 3/4" in size. The primary line has a cleanout (a tee and a plug), while the secondary doesn't. They both exit the house at the same spot, side by side. Both my AC furnaces and dehumidifier are located in crawl space.

My plan is to splice my 3/8" drain line (coming from a condensation pump) into one of these AC drain lines to handle condensation water, but I could really use some advice on which AC drain line to use. I found a YouTube video featuring a professional installation done in my area where the installer spliced dehumidifier drain line into one of the AC drains, so presumably it's not against local code.

I'm leaning towards using the secondary line since it sees much less traffic. However, I want to make sure I'm making the right choice to avoid any future issues. Any advice or insights you could share on this would be immensely helpful. Thanks a bunch in advance

  • Just a question: the crawl space is completely enclosed and has vapor barriers on all outside and ground surfaces? If not, it would be a futile waste of electricity to remove some humidity while more moisture can pour in. Commented May 6 at 3:30
  • Thanks for the reminder! Yes, the crawlspace is encapsulated and air conditioned.
    – Chris
    Commented May 6 at 3:41
  • You might want to think about maximum flow if everything is draining at once and whether that existing line could handle it or would start to back up. Outside of that, I don't see anything obviously wrong with the idea.
    – keshlam
    Commented May 6 at 4:04
  • Does the A/C also have a condensation pump or is the primary line attached directly to the blower pan ... and if so please confirm it has a trap?
    – jay613
    Commented May 6 at 11:54
  • A/C does not have a condensation pump and the primary line is attached directly to the blower pan. There's a p-trap in between my unit and the drain line leading outside for both the primary and secondary drain line.
    – Chris
    Commented May 6 at 14:12

2 Answers 2


If the A/C primary is also pumped, you should create a couple of high loops (one for the A/C, one for the dehumidifier) in the drain lines just before the splice. That way they won't pump into one another.

If the A/C primary is not pumped, you should not splice them. These lines are sensitive to pressure and there are issues where the condensation line can fail because the A/C sucks air in through them. Adding any back-pressure to the line could impact its performance for the A/C. You are better off running a new line outdoors for the dehumidifier, or if you must combine them, use an air gap, IE, cut the primary line and let it, and the dehumidifier, drip into a drip pan connected to the existing line outside.

Another idea: I haven't fully thought this through yet, but if you can add a tee to the condensate line, at least a few feet away from the air conditioner, pointing directly up and going up at least 6 in and 6 in above the level of the AC drain pan, then insert the dehumidifier line into the top of this tee, loosely so the t remains open to the air. Maybe that will work. I still wouldn't do it but I think that will help to avoid the most foreseeable problems. On the other hand if the drain becomes blocked, this tee will overflow into your crawlspace. I'm not recommending this, but I think it's better than a direct tee.

  • This is all new to me, thank you! There's a p-trap in between my unit and the drain line leading outside. I'm guessing the p-trap is there to prevent the AC from sucking in air? If that's the case, would it help alleviate the back-pressure issue you mentioned earlier? Ideally, I'd prefer not to drill any new holes, but I'm willing to do so if it's necessary. Thanks for your help!
    – Chris
    Commented May 6 at 14:02
  • The trap should be very close to the AC unit. Immediately outside it. Its job is to prevent the AC from sucking air at high speed through the condensate line, which in turn prevents water from draining. What I have to say is very unscientific, but, attaching a pumped line, depending on the exact arrangement, could put enough back pressure on the trap that it won't drain, or could siphon the trap dry causing water to accumulate in the AC. If you have even a grudging willingness to run a new line outside, you should. ....
    – jay613
    Commented May 6 at 14:27
  • If you create problems they will be intermittent, hard to detect, and harder to associate with your change. I bet if you ask the AC manufacturer, they'll just say no way. Might even violate the warranty. I hate it when people play that card, but truly it might. I'll add a new idea to the answer.
    – jay613
    Commented May 6 at 14:28
  • Thank you for your reply! I just wanted to ask for clarification: all the issues we've discussed so far relate to adding a condensation line from a pump. Would it be much easier if I splice with a gravity drain pipe instead?
    – Chris
    Commented May 6 at 14:53
  • Without a pump, the entire dehumidifier needs to be higher than the A/C condensate line. Is that practical? If it's not too much higher, I suppose "yes", there is less chance of it siphoning the A/C trap and less chance of it pushing water back up. You could STILL feed the teed-off line with a vented opening rather than sealed.
    – jay613
    Commented May 6 at 16:24

Keep the secondary dry.

splice into the primary line using a wye, if anything goes wrong the AC will drain using the secondary line, and hopefully someone will notice water flowing from the secondary.

If you use the secondary then it will be harder to notice a blockage in the primary line.

If practical, elevate the dehumidifier such that it can drain by gravity instead of needing a pump.

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