You probably have cluster flies. We had them in our old farmhouse in NH. Try googling them. We treated the attic and upstairs--maybe some sort of bomb while we were away. It helped for a while but then they gradually returned over the years.

We do have a second floor but none of them are on the second floor--they are all on the main floor? Some act sluggish but some of them are very speedy? I would think if we had a dead bird we would have smelled it? I had the chimney inspected by an idiot--he didn't go on the roof, and I'm sure just shined a flash light up there so anything is possible--although it does have a cap so I don't know what kind of bird could get in there--but if it is a dead animal they eventually decompose so why would they keep coming back like this? Once you have cluster flies do you ever get rid of them? It is really disgusting sometimes they just fall from the ceiling onto kitchen counters, floors, etc. Very gross.

Cluster flies deposit their eggs in the soil, use earthworms as a food source for the larva and emerge as adults in the fall, not mid-winter, so it's very unlikely this is what you have. Other fly species commonly deposit their eggs in animal corpses but can utilize just about any kind of decaying organic matter which serves as a food source for the larva when they emerge. Flies deposit eggs several times, not all at once, so you are probably dealing with multiple hatches. That's why they seem to disappear and then come back. Eventually they all will have emerged, lived out their short lifespans and died. What you don't want to happen is more eggs deposited in your house while they're alive.That's why identifying the species is important. So is eliminating any possible decaying organic matter, but if it's in an inaccessible place like a wall cavity it's nearly impossible.

we had the same thing and indeed there was a dead animal. It was in the furnace flue - believe it or not- not in the fireplace flue (1 chimney w/ 2 flues). The chimney cleaner was surprised (he almost didn't check) that the squirrel managed to die down that flue. We found it when we opened the little metal door into the chimney that was in the furnace room in the basement.

I had flies come in the house every fall by the hundreds over the course of a few weeks. Finally figured out they were coming in through the fireplace. Just removed the fireplace to update to a new model and low and behold, the old fireplace had hundreds and hundreds of dead flies anywhere they could land. So, yes indeed, flies coming through the fireplace is a real possibility. Don't let anyone tell you different. Even the damper was closed but they still found a way through.

We used to get flies after every spinng thaw. The second year was inyolerable so we pilulled the rough hewn plank siding off (which even though dillhigently sealed was the entry point for their overwinter) put up 3/4" exterior ply on the outside walls, re-insulated the exterior walls and put cedar siding over it.

We never had a spring fly infestation again.

If a fireplace is used all winter the heat would kill anything in the brick area I think.

We had the same problem with flies. We put a trash bag over our chimney flue and it fixed the problem. So yes, they can come down the chimney.

Same problem, cluster flies. Found we have an "upgraded" Majestic triple wall fireplace, and one section is totally open to let them in. Have tried to find a remedy for years to no avail.