Hot-Water Heating

the most widely used heating system in modern residential, public, industrial, and commercial buildings; heat is transmitted by hot water to the areas to be heated through heaters located in the areas. Hot-water heating systems include water heaters in which water is heated by a combustible fuel (in boilers) or by electric power converted to heat (in electric boilers) or, in the case of a central heat supply, by a combination of steam and very hot water (heat exchanger); heating units (radiators, convectors, panel heaters, finned and smooth tube radiators, and so on); piping systems through which hot water is delivered from the water heater to the heating units and the cooled water is returned to the water heater; an expansion tank for the water, whose volume increases when heated; and shutoff and regulating valves, which are installed in the piping system.

The water in hot-water heating systems may be circulated by either natural or mechanical induction. In natural-induction (gravity-feed) hot-water heating systems, which are used only in small buildings, the water circulates because of the difference in temperatures and the density of the hot (lighter) water in the water heater and the cooler (heavier) water in the heating units and piping. All other conditions being equal, the circulation rate rises in proportion to the increase in the distance along the vertical line between the heating units and the water heater, which is installed at the lowest possible point in the system. In mechanical-induction (mechanical-feed) hot-water heating systems, the water circulates mainly by the action of a circulation pump, which is installed in the pipeline that supplies cooled water to the water heater. In these systems the water heater may be located on the same level as the heating units, or even above them, and pipe diameters are smaller than those required in natural induction systems.

Various piping distribution arrangements are used in hot-water heating systems: top or bottom placement of the hot-water circulating line, with vertical risers to which the heating units are connected; horizontal distribution on each floor; and so on. Piping arrangements may be of the two-pipe, one-pipe, or flow-through type, depending on the method used for connecting the heating units. In the first instance, all heating units are connected in parallel to two pipes (hot-water supply and return standpipes); in the second method, each heating unit is connected to one pipe (riser), with some of the water passing through the unit and some bypassing the unit through the main riser bypass section. In the flow-through arrangement the water passes successively through all heating units connected to the riser. Proper use of hot-water heating requires that air be removed from the system. For this purpose, and in order to completely drain the system, all pipelines are run vertically or pitched (at an angle), and special devices (air outlet valves) are installed at the high points in the system.

In a central heating supply, hot water from an outside main is often supplied directly to the hot-water heating system and is returned after cooling. If the temperature of the hot water in the outside mains of the central heating supply is higher than the temperature specified by the heating health requirements (for example, 85° C for hospitals, 105° C for residential dwellings), then cold water from the heating system is added to the hot water to lower its temperature. This is done by installing water-jet pumps at the point where the hot-water heating system is connected to the outside mains of the central heating supply. Variations in the amount of heat delivered in the hot-water heating system, which are required because of outside temperature fluctuations, are achieved by centralized control of the water temperature in the system. Each room is usually controlled locally with valves on the heating units. To protect the piping, drainage of the water from the system is not recommended during the summer when the heating system is not in use.