One thing to remember if you use a flue top damper(for a working fireplace)-- the damper it's self will restrict the flue size, and in some cases makes the flue section area too small for a good draw. The solution I came up with on my rumfords was to use the next size larger damper, and cut a piece of the next size larger flue to fit snug over the chimney flues at the top.--problem solved.
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