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Hot Water Heaters: How They Work |
Fundamentally, a water heater is an appliance that converts energy to heat and transfers that warmth to water. It’s connected to a cold water supply pipe and has an outgoing hot water pipe—or system of pipes—that supplies heated water to one or more taps and appliances.
A conventional water heater stores heated water in a cylindrical tank. The less common tankless water heater doesn’t store water but rather routes heated water straight to taps or appliances.The majority of water heaters are fueled by natural gas, though propane-fueled and electric water heaters are not unusual. Where natural gas is available, it is a much less expensive heat source than electricity.
Small “instant hot water dispensers” are simply miniature electric water heaters that serve only one faucet. They have a small, under-sink tank that heats and holds nearly boiling (about 190-degree F.) water and delivers it under low pressure through a separate sink-top spout.
A conventional gas-fueled water heater warms water with a burner beneath the tank. Natural gas (or propane or kerosene, in some cases) is piped to a gas valve. A thermostat that detects the temperature of water in the tank regulates fuel delivery to the burner, which is ignited by a pilot light or spark ignition. A vent collects toxic emissions from the burner and pipes them up through the tank, out the top, and normally up through the roof. Some newer, high-efficiency water heaters have fan-assisted vents that can be piped out through a wall.
A heavy electrical cable delivers energy to heating elements in an electric water heater. An electric water heater doesn’t create combustion gasses, so no vent is required. It typically has one 5,500-watt or, for faster heating, two 4,500-watt elements. Separate thermostats control each element, cycling on as needed.
A tankless water heater circulates water through a series of burners or electric coils when you turn on a hot water faucet or appliance. Because the water heater doesn’t store hot water, it costs less to operate and doesn’t run out of hot water unless the flow exceeds its heating capability. Capacity is measured by how many degrees it increases water temperature at a given flow rate, typically expressed in gallons per minute (GPM).
Because the tank is under pressure, hot water exits through the hot water outlet at the top of the tank. When the hot water leaves, cold water enters through a diffuser dip tube that extends down inside the tank. The cold water pipe normally has a shutoff valve. A magnesium or aluminum anode rod utilizes the principle of ionization to minimize the water’s corrosive elements, which can significantly shorten tank life. The larger the anode, the longer a tank is likely to last.
The drain valve at the water heater’s base is used for draining the tank or flushing sediment out of it. This important maintenance step should be done once or twice a year, according to the manufacturer’s recommendations.
A temperature and pressure relief (T&P) valve near (or on) the top of the tank opens automatically if temperature or pressure exceed safe levels. This valve should be tested periodically, according to manufacturer's instructions.
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