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  3. Water for your home is heated by electricity or by gas. In either case, the way in which the water circulates to and through a m

Water for your home is heated by electricity or by gas. In either case, the way in which the water circulates to and through a m

admin2010-06-18  41
问题    Water for your home is heated by electricity or by gas. In either case, the way in which the water circulates to and through a modern heater is the same. Cold water enters through pipe in the top of the tank. The pipe, which extends almost to the bottom of the tank, ’is called a dip tube or drop tube.
   As the cold water is discharged from this tube into the bottom of the tank, it is heated by electric elements or a gas burner. Hot water, being lighter in weight than eold water, rises to the top of the tank where it is drawn off through an outlet tube to faucets and appliances that use hot water.
   Whether water is heated by electricity or gas, the temperature of the water is controlled by a thermostat  that  "senses" temperature.  As water heat reaches a preset temperature, the thermostat responds by closing the switch that allows the electric current to flow. When water falls below the preset temperature, the heating elements are turned back on.
About Electrically Operated Units
   Two types of electrically-operated, water heaters prevail: induction and immersion.
   As the term indicates, an induction type heats water by inducting heat from an electric coil. The electric coil does not come into contact with the water. Instead, the element is strapped around the outside of the tank and is covered with heavy insulation, causing heat to pass through the wall of the tank.
   In contrast, an immersion water heater has the elements in direct contact with water. Elements pass right through the wall of the tank, with openings around them sealed tightly so the tank maintains its watertight integrity.
How Gas Operated Units Work
   Water heaters that use gas as the heating element have as their main parts a gas burner and pilot light. The burner is positioned outside the tank — never in contact with water. Its flame is directed against the bottom of the tank so cold water being discharged from the dip tube is heated.
Water Heaters
   Situated in such a position as to be kept hot at all times by the pilot light is a flame sensor. If for some reason the pilot light goes out, the flame sensor cools, and in so doing cuts off the flow of gas to theburner.
Selecting a Water Heater
   If you are building a new home, remodeling an older home, or replacing a water heater that has failed, you will want to select a new heater that best meets your family’s needs. The General Public Utilities Corporation has provided a guide that may be used to make a selection:

A family with 2 adults and ’2 children and with clothes washer and dishwasher should purchase a water heater with a capacity of ______ gallon.
选项 A、66
B、52
C、80 (two tanks with 40 each)
D、82
答案A
解析
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