Insulating and sealing a Fireplace

[kenhill]

I had an energy audit done on my 30 year old house.  They installed a fan in the doorway, created low pressure inside the house, and went looking for air leaks.  One of the biggest leaks was around the fireplace.  Here is my fireplace set up.  I have a Heatilator fireplace originally installed.  It is installed trough the wall in one of those fake chimneys (chase?).  Inside of that fireplace, I had installed an Avalon Regency stove.  Its flue is smaller and runs inside of the heatilator flue.  How do I seal this area off so it is not so leaky?

Thanks.

[MountainDon]

Just where did they pinpoint the leaks? Pictures might help.

[cordwood]

 I have only done one steel wood stove inserted into a mortar fireplace and I used a piece of flat sheet metal to seal the flue pipe outer wall to the fire place chimney inner wall. I like to leave as much of the heated steel exposed to the occupied heated space as possible for the most efficient heat transfer. They make block off plates to go around the face of most stoves that only leave the very front of the stove visible but I think these type arrangements waste a lot of heat up the chimney and not reflected back into the room. But if you don't use your wood burner as a primary source of heat it may be the most practical way.

[kenhill]

I suspect cold air is coming down between the 4 inch flue of the woodstove and the 8 inch heatilator flue.  When they create the vaccum on the house, the air just rushes out around the block off plate that surrounds the wood stove and covers the old fireplace opening.  In the winter, you can feel the whole faceplate rediating cool air.

I am told to insulate the first 8 feet of the chase, install a false ceiling at 8 feet, and insulate above the false ceiling.  I am not sure how to stop the airflow between the two flues.  It would seem like you need some airflow to allow cooling of the 4 inch woodstove flue.