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So I just moved into a new house in NC that I am renting. The house has central A/C which is great, but because there is no basement so there is no basement drain. So the A/C expels the water into the backyard.

The place where the water is expelled from the A/C creates small stagnant pool of water which is making for a heck of a mosquito problem.

I could fill in the hole, spread the water out, and that will be fine in the short term. This issue will continue unless I can think of a solution. The hose that expels the water is near to the ground, so a sizable rain barrel seems out of the question.

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2 Answers

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You'll want to create a small leach pit, which will allow the water to drain into the ground.

Start by digging a hole about 12-18" deep (depending on your soil), Next fill the bottom of the hole (about half the hole) with crushed stone (loosely compacted). Fill the rest of the hole to about 1" from the top with sand, then finish with a nice decorative stone. If the soil is typically very moist (i.e. you get a lot of rain), you can skip the sand and fill the hole all the way up with crushed stone, and then top with decorative stone. Another option would be to top the hole off with something like this

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So you'll follow the same procedure as above, but rather than filling the second half of the hole with sand/stone you'll put in a drain pipe like this.

The whole idea here is to have the water leach into the ground, without forming a puddle where mosquitoes and other things can breed.

If you do this and still find that the water is pooling, you'll have to take more drastic steps. The best option from there would be to install a French Drain, and connect the drain line from the AC unit to it. There are already some questions that deal with french drains, so if it comes to that you should have no trouble finding out how to do it (how do i build a french drain).

You could also buy a small leach pit, but this might be overkill for your situation.

enter image description here

You'll bury this in the ground surrounded by 3/4" - 1 1/2" crushed stone, then connect the AC unit drain to the lower inlet pipe (the upper pipe is a vent). Before you drop it in the hole, remember to remove the small hole knockouts (to allow leaching). This will require a larger hole, and again may be over kill for your situation.

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Where the line comes out of the AC unit, there should be a trap like you'd have under your sink, and a removable cap above the trap. Open the cap, and put a little bleach in there. That should kill whatever is growing and hopefully avoid any mosquito issues.

You'll want to do this periodically to keep the line from blocking anyway. How often depends on your area. I do it yearly, but the trap drains indoors. I've heard others do it every month. No need to dump the whole bottle in, my goal is to fill the trap without losing too much bleach down the drain.

Edit: For those paranoid about bleach going outside, hot water and vinegar is an option, but I'd up the quantity to several cups to really flush the line. Just keep in mind that if you don't keep this trap clean, then anything growing in moisture outside will eventually be growing inside, clogging the trap, or worse. The story I heard was someone that foolishly ran the drain into their septic system, some deadly bacteria ran back up the line, into the home, and killed the home owner in a period of months. Given the choice, I'll go for the 1/4 cup of bleach.

Finally, for the hole caused by the drain outside, fill it with sand and cover with gravel. I don't think the mosquitoes will go digging to get to the water.

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bleach will kill any plants growing near there, though. I like the gravel idea, though. –  DA01 Aug 31 '11 at 23:02
    
Is it sand I want, or a soil closer to potting soil that will allow water to pass? What we have is a very sandy/clay type soil and that seems to be part of the problem. –  Justin C Sep 1 '11 at 2:06
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DO NOT pour bleach in the trap if the line is draining outside on to the ground. –  Tester101 Sep 1 '11 at 12:25
    
Maybe this explains why all my plants are dead. j/k. If you only put enough bleach to fill the trap, then it should dilute without too much of an impact. I'm thinking of a cap full, not a gallon. –  BMitch Sep 1 '11 at 12:57
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Instead of bleach, I would recommend apple cider vinegar. –  Ryan Gates Aug 3 '13 at 22:24
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