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I recently bought a house which is completely empty, no food or anything, but a constant source of dead house flies is found on the floors. How can they be getting in?

I checked all the windows and doors which are tight. The chimney damper is tight. The house has no attic. All the windows and and sills in the basement are tight. Also, there are tight doors in the basement and no dead flies there, so I do not think they are entering through the basement.

The only thing I can think of is that they are coming through the septic tank, but I thought all the drains are supposed to be sealed off with P-junctions. Is there a way I can test to see if there is an air pathway from the septic tank into the house?

  • the P-traps dry out after some time – jsotola Nov 13 '18 at 7:47
  • @jsotola Hmm, okay, so how do I find out if a particular trap is somewhere and has dried out? – Tyler Durden Nov 13 '18 at 8:08
  • @TylerDurden sniff them, if they stink like a sewer then they are dried out. Just add some water to seal them off again. – ratchet freak Nov 13 '18 at 9:13
  • I challenge the notion that there are any flies in a normally functioning septic tank to begin with. Even flies need breathable air, and a septic is not somewhere you're likely to find that. It's also probably better sealed than a house could be. – brhans Nov 13 '18 at 12:04
  • Or just assume every trap is try and add water to every drain. – Tyson Nov 13 '18 at 14:27
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The flies could be getting in from the septic tank if one or more P-traps has dried out. If a P-trap that has completely dried you could check it by pushing something flexible down the drain a few feet and see if it is wet when you withdraw it.

Unfortunately a P-trap does not need to have completely dried out to in order to lose its effectiveness in blocking the drain of updraft from the septic tank. So in this case you may be better off to simply run a good bit of water down each drain in the whole house and then re-evaluate the fly problem sometime later.

Keep in mind that flies could be entering through:

  1. Furnace vents.
  2. Chimney/fireplace.
  3. Plumbing vent stacks (also related to dried out P-traps).
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