The little heat that conventional chimneys produce often goes into warming the back of the fireplace. If the chimney is on an inside wall, this heat may be usable. A quick tour of the modern subdivision will reveal that most chimneys are on the outside wall. New fireplace inserts come with air tight gasketed doors and a special ceramic glass window to allow you to view the flame as well as conduct heat into the room. Heat exchange is also improved.
Some units have a heat exchanger and fan to move the heat into the room.
Most will have secondary combustion to ensure clean, efficient wood burning. The R-2000 Home Program is very concerned with energy efficiency and airtightness. It permits advanced combustion fireplace inserts rated by the EPA or CSA standard. These units use one-tenth of the air of open fireplaces. Because the air requirement is so low, an outside supply of combustion air is not usually necessary.