Tank water heaters
Most tank water heaters are located near the heating system or in a utility room, sometimes in the kitchen or a closet. These heaters basically consist of a lined steel tank that is covered with an insulated sheet-metal jacket to reduce heat loss. The lining in the tank is usually vitreous enamel (glass) but can be concrete (stone) or copper. To minimize tank corrosion, most heaters are equipped with a “sacrificial” magnesium anode rod that is suspended inside the tank. The electrochemical action causing corrosion takes place between the water and the anode rather than between the water and the tank, thereby extending the life of the tank. Some tanks are constructed so that the magnesium anode can be inspected and replaced if necessary. Tank water heaters are gas-or oil-fired or electric. (See FIG. 16-1.)
Many water heaters have at the top of the tank hot-water outlet and cold-water inlet fittings that are clearly marked HOT and COLD, respectively. Yet it is surprising how many times I have found heaters with a reversed water-line installation. You can tell whether the installation is reversed by touching a pipe about 4 feet away from the unit and comparing it with the HOT outlet fitting on top of the tank. A reversed connection results in an inefficient operation of the heater. Even though there is a cold-water inlet tap at the top, the cold water does not enter the tank at the top. There is a pipe (dip tube) inside the tank connected to the inlet fitting that carries the cold water to the bottom of the tank. Because the hot water tends to rise above the cold water, the hot water is normally at the top of the tank where it flows through the outlet fitting. When the inlet-outlet connection is reversed, the cold water enters the tank at the top and mixes with the hot water as it settles to the bottom near the outlet fitting. Consequently, for the same thermostat setting, the temperature of the hot water is lower than it would otherwise have been. To compensate for the lower temperature, homeowners usually turn the thermostat to the high setting, thereby increasing the fuel cost and decreasing the life of the water heater. Some heaters have interchangeable cold- and hot-water fittings. In these units, the dip tube can be shifted from one side to the other.