Wood burning fireplace inserts are designed to transform inefficient fireplaces into wood stoves, which can get many BTUs of heat out of a load of wood. Using the existing fireplace means that even houses with no room for a free-standing wood stove may be heated with wood.
When installing a free-standing wood stove, the building code regulations must be followed, with fireproof shielding on floors and walls and a regulated amount of air space and clearance; consequently the stove protrudes into the living space. A stove's hot surfaces may be dangerous for children, and some people dislike the look of stovepipes running to a vent in the wall.
Actually, a fireplace can be a negative in heating a house, since more heat may be lost up the chimney than is sent out into the room, and heat from the room is lost while the fire is beginning to burn and as it is dying down to smoldering ashes. Dampers that close the chimney to prevent this hot air rising and escaping to the great outdoors cannot be closed while ashes are still smoking.
Some 'inserts' are simply glass doors that can be shut to keep indoor heat from escaping even though the damper is still open. The best way to use them to close off the fire at the end of the evening, so people can go to bed and wait to shut the damper in the morning when the fire is completely out. Keeping the doors shut while the fire is burning brightly, of course, means that the beauty of flames can be seen but all heat is trapped in the fireplace until it is lost through the chimney.
For this reason, an insert must fit the room or area it will heat and the kind of wood burning the family does. A lightweight, less airtight insert may be best for a family room which will only be heated with wood as a supplement to the central heating on week-ends or in the evenings, perhaps. A heavier, truly air tight stove that will burn for eight hours or more on one load of wood may be needed if the main heat in the house is to be provided by the insert.
An insert must be treated like a stove, in that the wood inside must be lit and allowed to burn for a while to release the creosote that it contains. This creosote will burn if the fire is hot; otherwise it will evaporate and coat the surfaces of glass doors, the inner chamber, and the walls of the chimney. The danger of unburned creosote deposits is that, when the fire does get hot, these deposits may ignite and cause a chimney fire. Creosote is liquid at times, and a stove that is not getting enough air for proper combustion may even leak the black tarry substance out of the front onto the hearth and near by carpets.
When evaluating wood burning fireplace inserts, homeowners should get one only as big as they need for the space to be heated. The largest and most airtight insert may not be the best, if what is needed is short-term fires in the evening rather than whole house heating with wood.