Hydronic heating systems use small compact pumps to provide the motive force to circulate the water in the pipes. They are usually referred to as circulators or water-circulating pumps. The circulator is used to move the water from the boiler to the heat-emitting units and back again. It is not used for lifting, as is the case with vacuum and condensate pumps, but simply for circulating the water through a closed loop.
Circulators were first introduced in the 1930s to augment water circulation in the traditional hot-water space-heating systems. Prior to their introduction, the hot-water systems relied on the density difference between cold and hot water to provide the motive force for water circulation. These systems were called gravity hot-water heating systems, and the pumps were added to boost circulation. These early pumps (sometimes called three-piece circulators or booster pumps) are still with us today, although as more technically advanced models. All modern hydronic heating systems are closed loop installations that use one of several different types of pumps to circulate the water.
