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I have had a problem with drain flies for about a year, I have tried everything mentioned online to get rid of them but to no avail. I do know they are originating in the furnace room in the basement. I thought it was the floor drain, cleaned with various gels, and pest control products for 6 months, didn't help. Then I completely sealed off that drain, still had the flies. Went to replace my furnace filter and when I pulled the filter out it was almost completely black with dead drain flies. I cannot figure out where they're coming from. I cannot find a condensate pan or trap or hose on the furnace anywhere to check (it's a mid efficiency Bryant furnace, about 10 yrs old). The furnace works fine. I had an HVAC contractor clean the furnace and the AC, they found no source of the flies. I had a plumber camera inspect and power flush all drains in the house, still did not help. I had pest control company treat all drains many times with pest control chemicals, did not help. My husband poured boiling water and bleach down the drains everyday for a month, did not help, did the same with drain gel, did not help. We are losing our minds and can't continue to live in the house like this, please help. I seem to think they may be inside the furnace, but where, I can't find any pipes or anything to check. And I don't know why the furnace filter is covered with them. I also notice that on the main floor of my house, wherever there is a duct there are flies, nowhere else, just beside every duct.
 

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I'm more familiar with fruit flies. It's summer so any fruits left out, anything sweet. Window screens don't help. They just appear. Inside your walls and duct chases may be other points. Possibly they are attracted by carcasses of other pests (bugs, animals)? If the filter catches that many, looks like either near/around return register, or they are entering through unsealed duct joints and getting sucked into the filter. At least, it doesn't sound like after the filter.
Furnace is also near central vacuum, any makeup air openings, drier vent, chimney, outside faucet pipe, etc. AC condenser electricals and pipes. Maybe time to clean the ducts and check for unsealed joints.
 

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Any chance that your house was replumbed at some point? Possibly there could be some old unused type of drain piping under the main floor. They are getting into the A/C unit through the air returns on the main floor (your words there). Just food for thought here, cuz those flies like sewage muck according to Wiki.
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jack-off-all, master of nothin!
 

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Suspect drains can be identified by placing a glass jar or taping a clear plastic bag over them, and periodically checking for adult flies.[1] A clear plastic cup coated inside with vegetable oil or petroleum jelly can also be used.[7] Partially covering the drain opening with sticky adhesive tape is another method used to identify breeding sources.[6][8]
Thorough mechanical cleaning of drains will remove the larval food source, and is the most effective control measure.[7] Alternatively, injected foams containing bacteria or enzymes may be useful to break down gelatinous scum deposits.[1][6] Besides sink drains, floor drains and shower drains are common sources, as well as leaky shower pans, but any location with moist decaying organic matter can be a breeding site.[6][8] In commercial buildings, sump pump pits, sewers, and elevator pits may trap moisture where drain flies can breed.[8]
 

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Is there a fresh air vent coming into that room?
 

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Did you ever figure this out? This is exactly what is happening to me!! Tried all the usual tricks on the drains but they’re definitely coming out of the air vents.
 

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Did anyone figure this out? I'm having a similar issue. They are clearly coming from the air vents on my slab. I've had the AC company over to clean and see the issue. He assured me they were not coming from the furnace. An exterminator told me to get my outside sewer line inspected with a camera, I did that and there was no damage or cracks. The plumber said there might be a crack in the foundation and clay air vents and the drain flies are getting in that way. We are getting ready to rip out the entire bathroom to look for any cracks these things can be coming from. It's driving us nuts.
 

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Find and trace the furnace return air duct. The flies all got sucked down to the filter.

Sometimes return air ducts are stud bays without metal lining. Not too difficult for a rat that chewed through the wood framing to get in the duct, die, and become a fly breeding area.

On the supply side, cooled air leaves the air conditioner coil (evaporator) at 100% humidity. Some condensation can occur in the air duct just beyond and that together with miscellaneous lint and dust can become a breeding area for drain flies which finally emerge from the supply louvers in the rooms.
 

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Yosysfire, I feel your pain. We currently have a huge hole in our front yard. The plumber found that the main sewer line had a totally eroded section of clay pipe. Good news is, as soon as they dug the hole and exposed it - no drain flies!!! So we know this is the problem (they’re not done yet so we’ll see if it actually fixes it), but it could be anything for you guys. Our next idea was to call a foundation company but they’d have to rip up our hardwood because we’re on slab and we’re not doing that, so we had a realtor start looking at other houses for us. (We are still moving even if this problem is fixed because we found an awesome house!) But isn’t this INSANITY! We literally were planning to MOVE OUT of our house if we could not fix this problem. It causes so much anxiety. I’m having a good day, everything’s great, then I look to see one landing on the wall and it crushes my happiness instantly. I really feel your pain.
 
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