Gravity hot water
As with the gravity warm-air heating system, the gravity hot-water system is inefficient, not very responsive to changing demands for heat, and no longer installed in new construction. However, it might be found in many older homes. The principle of operation is similar to that of a gravity warm-air system: As the water is heated, it becomes lighter than the cooler water and tends to rise. Since the system is filled with water, as the hot water rises, it displaces the cooler water, forcing it to return to the boiler for reheating, and thus induces circulation. To keep the resistance to flow at a minimum, the size of the distribution piping is relatively large-about 3 inches in diameter- compared to the distribution piping in a forced system-about 1 inch in diameter.
Gravity systems might also be classified by whether they are open or closed. In an open system, the expansion tank has an overflow pipe that is open to the atmosphere. The expansion tank must be located above the highest radiator to ensure that the radiator will be filled with water. It is usually found in the attic with the overflow pipe extending through the roof or side of the building. (See FIG. 14-6.) If the attic has been partitioned off into rooms, the expansion tank might be found in a closet or corner. When the tank is located in an unfinished, unheated area, it must be insulated to minimize heat loss and to protect against freezing should the system malfunction.
Hot-water and steam systems are normally equipped with automatic pressure-relief valves that discharge when the pressure exceeds 30 psi and 15 psi, respectively. A relief valve, however, is not needed in an open gravity hot-water system because if the pressure should build up to a point where it exceeds the design pressure, the water will simply discharge through the overflow pipe in the expansion tank.
In the closed system, no portion of the expansion tank is open to the atmosphere. In this case, the expansion tank can be located anywhere in the system, usually near the boiler. Because the system is closed, it can operate at higher pressures, and therefore higher temperatures, without turning to steam. The higher temperatures permit the use of smaller radiators.
There are two types of expansion tanks used in closed systems-air cushion and diaphragm. In the air-cushion type, air is initially trapped in the tank to provide a cushion, which is compressed as the water in the system expands and enters the tank. Over the years, the air in the tank can be absorbed by the water resulting in a “waterlogged” expansion tank. In this case, when the heating system is turned on, since there is no room for expansion, the pressure will increase rapidly until the relief valve discharges. When this happens, the expansion tank must be flushed and the air cushion reinstalled.
To eliminate waterlogging, the diaphragm expansion tank was developed. This tank uses a rubberized diaphragm to separate the air cushion from the water. For the most part, the diaphragm does eliminate waterlogging. However, where the diaphragm is faulty, waterlogging does occur.
If the heating system is gravity hot water, do not fret. It can be converted into a forcedhot-water system by installing a centrifugal pump with the associated controls to circulate the water through the distribution pipes and radiators. If the system is the open type, the expansion tank will have to be replaced with a closed tank.