Pumping heat into cold water

High efficiency is the reason for adapting heat pump technology for domestic water heating. With almost 20,000 units now in service, the heat pump water heater (HPWH) is going on strong as an end-use conservation option. This article describes HPWH developments since the first designs for mass production in the 1950s, their operating characteristics, and the consumer education that was necessary to gain public acceptance. 3 references, 2 figures.