# Drain Flies on Furnace Filter



## carpdad (Oct 11, 2010)

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.


----------



## subbuilder (Jun 20, 2016)

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.
Sub
jack-off-all, master of nothin!


----------



## lenaitch (Feb 10, 2014)

Ok,I'll bite: what are drain flies?


----------



## Nik333 (Mar 1, 2015)

lenaitch said:


> Ok,I'll bite: what are drain flies?


Must be a regional thing. I don't recognize them.

https://en.wikipedia.org/wiki/Drain_fly


----------



## lenaitch (Feb 10, 2014)

Nik333 said:


> Must be a regional thing. I don't recognize them.
> 
> https://en.wikipedia.org/wiki/Drain_fly


Must be. Never seen such a thing. Tnx for doing the research :smile:!


----------



## Ghostmaker (Mar 2, 2013)

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]


----------



## Nealtw (Jun 22, 2017)

Is there a fresh air vent coming into that room?


----------



## Sltand02 (May 21, 2018)

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.


----------



## Yosysfire (Sep 13, 2018)

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.


----------



## AllanJ (Nov 24, 2007)

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.


----------



## Sltand02 (May 21, 2018)

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.


----------

