Q: Jason, Every year during February and March I get birds nesting in my fireplace followed by a swarm of flies getting beyond the closed grate and into the house. Would a chimney balloon work to keep the flies out or how do I stop the birds from nesting in the fireplace? I would appreciate any suggestions that would help.- LR
A: Dear LR, Birds nests are pretty disgusting places when it comes to bacteria and bird po-po, but this is a springtime wonderland for flies!
So the flies hang out and lay eggs in the bird nest material. Then the fly eggs turn into larva which turn into flies, and they follow the warmth and smells from the chimney down into your home. Voila! you get a swarm of flies down your flue into the house.
Putting in a Chimney Balloon will keep the flies out of your house since you will be effectively sealing their route of entry. As you have already found the metal damper will not keep flies out since dampers do not air seal and flies and maggots can get right through.
Now to stop the birds... You will need a chimney cap with screening on it to keep the birds from entering the chimney top or nesting on the crown. You don't need anything fancy just a simple cap with screen on it will do. If you already have a cap, you may be able to fashion a metal cage around it with thick wire mesh to keep the birds out.
Stopping the birds from nesting is important because their nesting material can collect moisture and be detrimental to your chimney interior and it is just nasty bacteria laden stuff. - Jason