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.