Some models extend onto the hearth while others stay flush with the front of the fireplace. Most have tight-fitting glass or metal doors and built-in circulation features. The most efficient models supply combustion air from outside the house, and virtually all inserts are controlled-draft units with manually or thermostatically controlled air inlets. Some units are also available with catalytic converters for improved efficiency. Glass doors, heat exchangers, dampers and flue sealers can also up the energy efficiency of an existing fireplace, but don't expect too much. According to the Department of Energy, those devices combined could only improve efficiency by some 10%. Custom fireplaces.