Expansion tanks (also sometimes called compression tanks) are installed in hydronic (hot-water) space heating systems to limit increases in pressure to the allowable working pressure of the equipment and to maintain minimum operating pressures.
When the temperatures rise during the operation of the system, the water volume also increases and builds up pressure. The pressure in the system is relieved to a certain extent by the storage of the excess water volume in the expansion tank. When temperatures drop, there is a corresponding drop in water volume and the water returns to the system.
Maximum pressure at the boiler is maintained by an ASME pressure-relief valve. Minimum pressure in the system is generally maintained by either an automatic or manual water-fill valve.
Closed steel expansion tanks and diaphragm tanks are used to contain the expanding volume of heated water in residential and light-commercial hydronic heating systems. Some typical installations using ITT Bell & Gossett expansion tanks are illustrated in Figures 10-40 through 10-42.