One thing I've seen on This Old House, and some magazines is a roof cap damper. This is a spring loaded cap for the top of the chimney. A SS cable comes down the chimney to allow you to open and close the thing. I thas the advantage that it will keep birds, squirrels, etc. from falling into or staying inside, and does a better job of closing things off. If used, then you take the old damper out, or disable it.