Heating water consumes over 3.5 quadrillion Btus per year of primary energy which makes it the third largest energy end use for the residential and commercial sector in the United States, after space conditioning and lighting. Although indoor domestic hot water is nearly universal in the United States, this is a relatively recent historical development.

One of the earliest recorded uses of heated water was the Roman public baths. Starting in the first century C.E., many Roman cities had public baths with pools of cold, warm and hot water. The water was heated with wood or charcoal, although some of the baths incorporated passive solar features for space heating.

Prior to World War II, only about half of households in the United States had complete indoor plumbing facilities, including hot and cold water. Before indoor plumbing became widespread, water was heated on stoves in pots and...