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, though a flue liner or other modification may be necessary. Vent-free fireplace inserts require no chimney or flue modification. Most have blowers to circulate the heat. Inserts are used to change an existing non-efficient fireplace into an efficient, heat producing zone heat.