A. O. Smith expands its line of residential high-efficiency, gas-fired tankless water heaters with the compact 120,000 Btu/hr. Condensing Tankless Water Heater. Measuring at less than 2 cu. ft. and scoring a .93 Energy Factor, it efficiently utilizes energy and installs on the wall. Available in natural gas or propane, the Energy Star-rated residential gas water heater is offered in indoor and outdoor models. The unit operates on a 1/2” gas line, features accessible temperature controls and vents with PVC.