If you are concerned about the energy you consume the Lymance Damper is the product for you. This product will seal the top of your chimney off airtight. It will reduce your energy costs during the winter when you are not using your system and during the summer preventing the air conditioning from escaping the top of your chimney.
You could see a savings of $20 or more per month.
Lyemance top sealing energy saving damper