The chimney damper for our fireplace is the standard dimensions of 24" x 5.5".  I removed the metal damper when I bought the fire place insert wood stove so there is just the damper opening with said dimensions that the wood stove exhaust must go through to reach the chimney.

I was surprised to learn that there is no standardized "out of the box" black duct fitting or adapter or solution for the 6 or 8 inch diameter duct output from wood stoves.  The fire place store suggested squeezing the 6" round black duct into an ellipse to fit the 5.5" width opening of the former damper.

I cut some stone to fill in the rest of the gap in the damper opening but there are still small gaps that I tried to close the best I could with furnace cement.

After inserting the wood stove into the fireplace and connecting the exhaust black ducting to the hacked damping opening adapter, I lit a lighter near the wood stove (not lit) and found the flame still bending toward the chimney indicating air flow from indoors out through the damper opening through the chimney.  The damper opening is still leaky.  I know I'm not the first to convert a fireplace to fire place insert wood stove.  What am I missing here?  How do I seal the damper opening so only the exhaust from wood stove leaves through black duct in damper opening and not other air flow?

[![chimney damper opening][1]][1]


  [1]: https://i.sstatic.net/HWeiq.jpg