Most gas fireplaces installed in homes are direct-vent models that offer energy efficiency ratings of about 70 percent. About 30 percent of their heat is lost through the vent connected to the outside of the house, but these units are significantly more efficient than wood-burning fireplaces that lose the majority of their energy as combustion byproducts leave the house through the chimney. Direct-vent gas fireplaces can effectively warm indoor spaces ranging from 800 to 2,000 square feet.
Unvented gas fireplaces can achieve energy efficiency ratings as high as 99 percent, but they are illegal in the state of California and other areas. The high energy ratings associated with unvented units are the result of these units sending almost all of their heat energy into the room. However, the unvented units also send dangerous pollutants into the home's living space.
Unlike wood-burning units, gas fireplaces can use technology such as thermostats and remote controls. Newer log sets are also produced from improved materials, such as refractory cement, that can generate a greater number of BTUs.
An Annual Fuel Utilization Efficiency, or AFUE, energy efficiency rating represents the percentage of fuel burned that actually becomes heat. The remaining percentage is lost through ventilation and combustion. AFUE ratings also take into account the energy that can be lost while a heat-producing unit climbs to a target temperature or cycles off and on.