The first model to use condensing technology in a residential water heater tank, the High-Efficiency Condensing Power Direct Vent Water Heater exceeds Energy Star qualifications with a 0.80 EF. The unit costs only $19 a month to operate, the manufacturer says, and is available in both 38- and 48-gallon models. It also includes an integrated diagnostic system to facilitate service calls. Rheem. www.rheem.com.