Fireplace inserts are heating units that retrofit into an existing fireplace (masonry or factory-built). They burn wood, gas or wood pellets and offer superior efficiency. Fireplace inserts utilize the existing chimney, through a flue liner or other necessary modification, for venting. Vent-free fireplace inserts require no chimney or flue modification. Most have blowers to circulate the heat. Inserts are used to convert an existing fireplace into an efficient, heat producing zone heater.