Woodstove flue damper

A quick question regarding a flue damper for a wood stove:
In my two story house I have a wood buring fireplace on the first floor and a wood stove in the basement. Both have separate 6" metal flues inside the same framed chimney shaft, with openings about 24" apart. When I use the fireplace, the basement smells like smoke. I realize it's probably the backdraft of a make-up air going down the woodstove flue, but somehow this does not happen when I use the woodstove and not the fireplace. Anyway, chimney top dampers would probably work the best to fix this problem, but I'd rather avoid going up on that tall chimney. Can an air-tight damper be installed inside the 6" round flue just above the wood stove? The only types of dampers I've seen that go inside a metal flue are the cast iron disks, but they don't provide an air-tight seal. Are there any other flue dampers for round pipes that provide good seal? I'm considering using a 6" metal blast gate user for dust collector ducts, any opinions if that would work? Other than the occasional smell, both the fireplace and wood stove work great, although on a cold day it's can be tricky to start a fire in the woodstove, lot of cold air to push out through that tall chimney.
Thanks in advance.
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snipped-for-privacy@optonline.net (John) wrote in message

i have had a similar problem. was told that the wood stove in my basement heats up it's own seperate flue as well as the flue for the fireplace. i was told to burn the woodstove long enough to heat both flues then light the fire place - this worked perfectly. was told that when i used the fireplace without heating the woodstove flue the smoke was exiting the fireplace chimmney and being sucked back down the cold wood stove pipe. if your stove is air tight a damper is not reccommended
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