Splicing dehumidifier drain line into AC drain lines

Question

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

Answer 1

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.

Answer 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.