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?