if you insert a free standing into an existing fire place --- AND BLOCK off the space above it at the bottom of the old chimney - with an insulated material - only allowing the new chimney (double) to penetrate the blocking material -
and, have an electric fan boost running that passes the air across the stove and into the room (ie. just a standard stove) --
then, it has to be efficient (assuming the stove is an efficient one).
ie. it has no choice but to be efficient - because there is nowhere for the heat generated to escape to - it must enter the room.
It can be of assistance if the blocking material is angled up slightly with the upside facing into the room - so the convection heat has a lovely ride north - but - it's probably getting pedantic - but, would make an impressive conversation topic when people stand near it with a glass of something discussing your engineering prowess.
