Smoke problem with a two-flue chimeny
I believe I have what's termed a two-flue chimney, by which I mean one brick chimney within there are two passageways. One is used by a woodburning fireplace upstairs. The second flue is downstairs, where there is a gas burning fireplace as well as the furnace/water heater.
When we are burning wood/firelogs upstairs, it's drafting properly. But the basement get smoky, as smoke seems to be descending into the basement via the second flue. I checked the damper, and a screw is installed so that I can't close the downstairs damper completely; the best I can do is to close it half way. I believe the smoke problem is due to the smoke going up the upstairs flue, only to descend back down via the second flue smoking out the basement.
Is there a reason why the downstairs damper is the way it is? The previous owner must have had it installed for some reason, I'm just trying to see if I can remove the screw and closed it up completely, unless I'm using the gas fireplace downstairs.
Thanks for any advice