We are about to place an insulated 6" chimney liner into a 10x10 clay tile lined chimney with a standard brick exterior construction that has a 10' high smoke chamber before the 10x10 clay liner starts.  An inspection was done and the tech suggested we stop using the existing open, older style,  fireplace with glass doors due to some clay tile deterioration.  I have not done my own inspection but we followed the advice.  We are about to install a firebox insert and an insulated 6" stainless liner.  My question is can I place an 8" liner around the 6" and then insulate the 8" duct exterior then make an opening in the second floor wall and draw air out from the heated duct space between the 6 and 8" liners with an inline fan.  The 8" duct would be a "T" at the second floor exit point into the room, and the 6" would continue up with its insulation to the roof exit.  The top of the "T" would be packed with insulation to seal the 6 to the 8.  At the base the 6 would be a direct connect and the 8 would start an inch above the connection to allow a draw.  Support of existing brick would be with steel lintel and the clay liner would be sealed with mortar.  Am I crazy?  The fire place would be used daily during the heat season.  Wisconsin ...6 months.   I've seen this done by my cousin in his workshop.  I asked if there was a commercial design and the fire place tech said no and said insurance would never cover a non "UL" design.  Anyone have any thoughts on this?  The design is safe unless the air is so warm the fan would be a problem in which case natural convection would be fine.  This is a lake house with poor heat to the second floor.  It was designed as a summer cottage the added on to in the past. Burning the fireplace insert will create a heat deficit on the second floor due to the thermostat location for the gas furnace.  This house will become our retirement home.   The only change to "an approved design" would be the additional air chamber.  I may have to use a cement board and tile surround at the "T" exit and only testing would tell me unless someone has done something like this and would care to share.  Am I wrong to think like this?  Is homeowners insurance going to be a major problem?  This was a safe working chimney and may still be but we did discontinue it's use.  The new insert should have lower exhaust temps than an open fire did.  Please share your thoughts.  Perhaps there is not enough heat on the 6" duct exterior to make this worthwhile.  

I don't think you can do it with an existing chimney...With a new chimney I did a sketch and you might be able to get it to work but I doubt you will get someone to do it.

Problems I see smoke coming out joints and into upper room....having insulated pipe for the clearance to combustibles between floors.

The issue I see is that its the heat that creates the draft that pulls the smoke out. If you remove enough heat, you remove the draw. You might be able to put an active exhaust (fan) on, but not sure if this is ok with a fireplace.