A: Hi MH, From our conversation on the phone, I was glad to hear you had your fireplace cleaned recently. Sometime fireplace flies have to do with a dead animal in the flue, or pigeons using your chimney top as an outhouse. But since you have a screened chimney top and a clean flue we can rule that out.
Flies and wasps coming down through the fireplace is a very common issue, and the Chimney Balloon does stop them. Especially in spring and fall insects follow food and other smells down into the fireplace chimney and end up inside. We get this questions so frequently there is a section of our website dedicated to it: http://www.chimneyballoon.us/bat_wasps_in_fireplace.html and some other blog articles regarding wasps as well: http://www.chimneyballoonusa.com/blog/2009/11/help-wasps-are-coming-in-through-my-fireplace-damper.html
The key to stopping flies in particular is to get an accurate measurement. Here is a section of the website that outlines how to do that measurement: http://www.chimneyballoon.us/Chimneyballoonsizing.html also you can give me a call or email pictures if you want advice on the measuring.
Once you have the measurements we need to get a Chimney Balloon for you that is as close to your measurements as possible (even if we have to go with a custom size), so you get a perfectly tight seal. Flies and maggots can be persistent in their entry techniques so you dont want wrinkles in the balloon from too much extra material. - Jason