Tankless Water Heaters: Do They Really Save Energy?

Tankless water heaters are a highly discussed green building solution, but how much energy do they actually save? Are the publicized drawbacks, such as cold water sandwiches and increased delivery times, a deal breaker? To address these questions, a recent Minnesota field study installed 16 tankless natural gas water heaters (both condensing and non-condensing) and eight identical conventional storage water heaters in ten homes and alternated operation of different heaters. Data collection equipment installed at each site recorded water flow rate, water heater inlet and outlet water temperature, gas and electrical consumption and ambient temperatures. System efficiencies and energy savings were computed and compared to rated performance (Energy Factor). The in situ efficiency of the storage water heaters was nearly 20% lower than rated, while that of the tankless water heaters was only 10% lower. This paper will discuss the causes, consequences, and possible ramifications of this reduced efficiency. To evaluate user satisfaction, data were analyzed for quality of performance as well as efficiency, and the surveys of homeowners were administered. Hot water usage statistics and patterns were also compiled. Data about typical draw lengths, volumes, flow rates, and spacing provided input to the development of new water heating test standards and improved understanding of how homes use hot water.