Sidearm water heaters were developed decades ago so that boilers could provide both space heating and domestic hot water. Generically, they are water-to-water heat exchangers that transfer heat from the boiler water to domestic water whenever the latter is flowing through. Figure 1 illustrates how they were originally used along with boilers that maintained a constant minimum temperature.
Sidearm water heaters rely on buoyancy-driven flow through a piping path that connects the upper and lower portions of the boiler. This phenomenon goes by many names, including “gravity flow,” “thermosiphoning,” “ghost flow,” and “heat migration.” Before circulators, it was the only propulsion effect for moving heated water from a boiler in the basement to the heat emitters in the building above.