=63. Equipment for Hot-Water Heat.= The hot-water system of heating a house consists of a boiler in the basement or below the level of the lowest radiator. This boiler is designed to heat water as it circulates through coils over the fire (Fig. 30). From the boiler, pipes lead to radiators and an expansion tank, and return pipes bring the cold water back to the bottom of the boiler (Fig. 31). [Illustration: FIG. 30. Garland furnace with hot-water boiler.] The heat from the furnace fire causes the water to circulate through this system of boiler, pipes, radiators and tank, due to the fact that hot water is lighter than cold water. [Illustration: FIG. 31. Hot-water heating system.] =64. Heating Unit.= The heating unit of a hot-water system is like any stove consisting of a fire pot and grate. Some are adjustable so that different kinds of fuel may be used. A gas burner is sometimes placed in the fire pot and used for heating a furnace, but this is one of the most wasteful ways of using gas. A real gas furnace is much more economical. The fire and heat from the fire circulate around the coils containing the water. If the coils are not constantly kept full of water, they will be injured by the heat. =65. The Management of the Fire.= When burning coal, spread the coal all over the surface of the fire in a thin layer so as not to smother it and thus make it burn with a smoky flame. Keep the ashes cleaned out from underneath the fire and around the fire pot. Clean the flues every forty-eight hours. Soot on the coils is more effective than asbestos would be in keeping heat from penetrating to the water. Regulate the fire with the drafts. Open the one below the fire box to let in air to aid combustion. Open the one found in most furnace doors a very little. This aids in the combustion of gases, thus making more economical use of the fuel, while opening it wider checks the burning of the fire. Broken and warped doors and drafts let in too much air and destroy the efficiency of the heater. Open the chimney damper, shown in Fig. 2, Sec. 3, admitting air to check the draft. Close the chimney or pipe damper of the type of cook stove shown in Fig. 2, Sec. 3, to check the draft up the chimney. =66. The Pipes.= The pipe carrying the hot water from the boiler out to the heating system leads to the expansion tank, the sometimes separate pipes lead from the boiler to a radiator. Insulate each pipe, except the part in the room to be heated, with asbestos or some other covering, to keep the heat in it. Keep the pipes full of water. When they are installed, see that they are put in so that they gradually rise upward. If they dip downward at any point, air will collect at these places and check the circulation of hot water thru pipes. [Illustration: FIG. 32. Expansion tank.]