Residential gas water heating is a significant end use in many homes, exceeding gas space heating in many milder climates such as California's. Efficiency options exist in improving water heating system performance by reducing hotwater loads, improving distribution systems, and improving the operating efficiency of the water heater. This paper explores the monitored performance of both existing atmospheric gas storage water heaters and advanced gas water heaters in 18 California homes. Findings indicate that significant savings can be achieved with advanced storage and tankless technologies, although high current costs, especially in retrofit applications, contribute to overall marginal economics