Document 5D8eJ0wNGQ47ebr04z4mmak6J

1016 CHAPTER 48 1951 Guide fitting may be installed in the transmitting circuit to prevent over-heating of the service water. In residences heated by pump circulated hot water, the house tempera ture is controlled by operating..the circulating pump intermittently, while domestic hot water is warmed by transfer from the house boiler, inde pendent of the pump operation. The domestic water is heated from the heating boiler the year 'round. Under such an arrangement, to prevent overheating the house by thermal circulation when the pump is not running, it is usual to insert a weighted check-valve in the house heating main so that no circulation to the house heating system can occur unless the'pump operates. In summer the fire may be controlled to maintain a boiler water temperature lower than when heating, and generally about 20 F warmer than that desired in the domestic hot water system. The immersion thermostat in a hot water storage tank should be located no higher than the center of the tank, and possibly should be even closer to the bottom, since water in a tank stratifies proportionally to the tem perature. When hot water is removed, the cold water entering to replace it quickly reduces the temperature in the lower parts of the tank. SAFETY DEVICES FOR HOT WATER SUPPLY SYSTEMS There are still numerous plumbing codes which do not have regulations for the prevention of hot water storage tank explosions. An ordinary storage tank is under certain water pressure, depending' on the static pressure in the system. When the water in the tank is heated by circulation through an external heater, or by heating units in the tank, it gradually expands. For instance, if the entire contents of a 30-gal storage tank are heated from 70 F to 160 F, there will be an in crease in volume of about one-half gallon. If the tank is connected to some supply without any intervening check valve, the increase in volume causes part of the water to be pushed back into the supply line. If the hot water reaches the water meter, it may ruin the composition discs. If back-flow cannot occur, as for example, due to the use of a checkvalve or a pressure-reduction valve in the line, or because of temporary shut-off of the cold-water line, the pressure in the tank rises as heating continues. Such a pressure rise, if the heating continues for any length of time, may result in rupturing of the tank. If the tank water is not confined, continued heating would, of course, cause no increase in pressure beyond ordinary line pressure. However, the temperature would continue to increase. If this should happen at elevated temperatures, thie flashing of the water into steam might cause a serious tank explosion. In order to guard against the development of excessive pressure inside a hot water storage tank (owing to the thermal expansion of the water), it is customary to install a spring-loaded pressure-relief valve which is set to open at a pressure about 20 psi higher than the normal line pressure. The amount of water discharged by any pressure-relief device is usually - quite small, since it takes only a small quantity of water , to relieve any pressure rise due to the thermal expansion of the water. The rate of discharge should be such as to limit the pressure rise for any given heat input to 10 percent of the pressure at which the valve is set to open. For any given installation, the discharging capacity should be in excess of the water to be discharged by the heater. The heater discharge Qm in gallons per minute may be computed by Equation 3: Water Services 1017 Qm = 0.000056 (3) where b is the heat input of heater in BtU per minute. To prevent danger of overheating, temperature-relief devices should be used. One type of such a device has a fusible plug of metal which melts at about 212 F. The hot water then runs from the opening until the device is serviced. A better type of relief device is one which incor porates a bellows or bimetallic disc, which opens at a temperature. of 210 F and closes when the temperature drops to approximately 160 F. Another type of safety device is a fuel cutofi switch or valve in which a fusible plug is melted by excessive water temperature. The fuel supply is cut off until the fusible plug is replaced. Tank Connections Coil Inclination The capacity 'of temperature relief devices may be calculated by Equa tion 4:' Cm = 0.00086 (4) where Qm and b denote the same quantities as in Equation 3. Pressure relief valves should be installed in the cold water line near the tank out of contact with the hot water in order to prevent excessive cor rosion and lime deposit on the valve seat.. Temperature relief valves, in general, must be installed at the point of maximum water temperature. SOLAR WATER HEATERS Solar heaters utilize the energy of the sun for heating water. The sue- . cessful operation of such heaters requires the- availability of sunshine practically every day in the year, which has limited their use to Florida and the southern portions of California. When supplemented with some other means of gas, coal, or oil water heating, solar heaters may be used in climates where sunshine may be more or less intermittent. They have been used in summer homes as far north as Chicago. When properly installed and proportioned, solar water heaters render satisfactory service, especially in climates where the outside temperatures are high and ex tremely hot water is not necessarily desirable. Such installations consist essentially of a storage tank, heating coil, and hot box. The coil is installed in the hot box, and is arranged to circulate water to and from the storage . tank. The advantage in the use of this type of heater is the fact that it