When you install a fireplace insert with a stainless steel chimney liner, you should install a block off plate. The block off plate is a horizontal plate that blocks off the opening where normally a damper is in a fireplace. This opening will vary in size, but generally you will be installing a 6" or 8" chimney liner. When you have such a large opening around the liner, a large amount of the hot air generated from the wood stove will be rising up into the chimney cavity.
To prevent this from happening install a block off plate. They are almost always best to be fabricated on site. This is because the block off plate will be different for almost every fireplace. The block off plate will have a hole in the center of it that is slightly larger than the flue liner you are using.
To build a block off plate follow these instructions. Basically made a cardboard template, to get the proper angles and perfect size, cut enough extra for the flanges to mount the block off plate and transfer the template to sheet metal.