Gas fireplace inserts are used to convert existing wood burning fireplaces (both pre-fab and masonry) to a sealed gas appliance. They consist of a gas log set installed into a steel or cast-iron heat exchanger and are usually sealed on the front with glass. Many of these units have fans to move the heat, and are also available with remote controls, wall switches or wall-mounted thermostats. Converting your existing masonry or factory-built fireplace to gas is an extremely worthwhile investment.Direct vent: This insert applies a sealed combustion direct vent system to heat the home efficiently and maintain indoor air quality. It's designed to address negative air pressure problems in "tightly-sealed" homes. Combustion air is acquired from outside the home through a dual (co-linear) venting system, eliminating the heat loss associated with conventional chimneys. Uses existing masonry or factory built metal chimney as a housing for a flexible co-linear liner. Natural vent: Uses existing masonry or factory built metal chimney as a housing for a flexible liner. Uses room air for combustion and exhausts to the outside of the home via a flexible liner installed in the chimney.