That depends on the install and the insert. there is allot that goes into a good insert design. And allot of it is how to get that heat out of the fireplace. How much insulation and what type of radiant reflector do you have around the insert? How much insulation above the block off plate? I think allot of it also comes down the the stove you have as wellThe members who have woodstove inserts in exterior masonry chimneys know these things. No fan moving hot air equals very little heat in the living space.