The insert option . An efficient alternative is the fireplace insert. An installer places a sealed firebox in the existing fireplace opening. The open space around the firebox is closed off from the living area, and you view the fire through a glass front. A flexible aluminum pipe, called a B-vent, runs from the firebox up the chimney for venting.
Some inserts use outside combustion air, brought in by a second pipe.
Fireplace inserts come in a variety of sizes and Btu rates. The rating tells you how much gas is turned into heat. The most efficient are in the 70 to 73 percent range. Some manufacturers also provide a steady-state efficiency rating, with usable heat percentages in the low 80s. This type of unit shows off the versatility of gas hearth appliances.