Gas tankless water heaters (also referred to as instantaneous or demand water heaters) have been gaining in popularity relative to conventional gas storage water heaters in the U.S. With energy factors of 0.80 or higher, (1) gas tankless water heaters are more efficient than gas storage water heaters, which typically offer energy factors of 0.60. The majority of domestic hot water in the U.S. is supplied by storage water heaters. Water is heated in an insulated storage tank, in most cases either by an electric resistance heating element or by burning natural gas or LPG.*
