Are you certain they are in your chimney? I know that it sounds as if we have birds in our chimney at times, but the sound is from birds right at the top of the chimney. If they are coming out of the chimney and into your house then they are definitely in nthe chimney.
From
http://sweptawaychimney.com/p1/tips.html:
"The bats found in our country are beneficial and harmless they eat large amounts of mosquitoes and flying insects that are generally the pests that we all hate. If you can't share your home with them and wish them removed, please call your local county extension service I'm sure they will be able to tell you who the local bat society is. I’ve had many customers indicate that if we had any bats looking for a new home they would build a bat house for them."
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Locally in my area, people have been known to put mothballs in their attics to get the bats out of there. You might consider doing that and keeping the damper shut to keep the smell in nthe chimney and not in the house.
The other thing is that chimney caps don't last forever. If you can get a stainless steel one it will last longest. You may need to check your animal guard to make sure it hasn't rusted out. The galvinizing on the less-expensive ones wears off, especially if you use your fireplace and the metal quickly rusts away.
I have no idea if these work but they seem to have a good try-it-out policy.
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