In most homes in North America, hot water is provided by a conventional gas or electric storage water heater — widely (and incorrectly) referred to as “hot water heaters.” (We’re not heating hot water!) The storage water heater consists of a free-standing water tank, a reservoir that holds 40 to 80 gallons of hot water. The tank itself consists of an internal storage vessel made from glass. It is encased in a steel envelope with a layer of insulation in between.

In electric storage water heaters, water is heated by two electric resistors, known as heating elements. The heating elements — one near the top and one near the bottom of the tank — generate heat when electricity flows through them, much like the electric heating elements of an electric stove. The heat produced by the resistors is transferred to the water in the tank.

Gas-powered storage water heaters have a single source of heat, a burner at the bottom of the water tank. The burner is connected to a thermostat and ignites when the temperature of the water inside the tank drops below a predetermined set point. When this occurs, the temperature sensor sends a signal to a valve that regulates the flow of natural gas or propane into the burner. When the valve opens, gas flows into the burner and is ignited, most often by a small pilot light that runs 24 hours a day, 7 days a week, 365 days a year.

Because the combustion of natural gas and propane produce carbon dioxide, carbon monoxide, and nitrogen dioxide, which are potentially poisonous to people and pets, gas-powered water heaters must be vented. Venting is accomplished by a flue pipe that directs hot combustion gases and pollutants though the ceiling. In most gas-fired water heaters, combustion air that feeds the burner comes from room air. For a discussion of newer, safer, and more efficient power-vented water heaters, see Power Venting .

Conventional storage water heaters maintain a large quantity of hot water day in and day out. This water can be used at any time. It’s there at our command. Just open a hot water faucet or turn on an appliance like a clothes washer and you’ve got hot water in 10 to 30 seconds, depending on how far away they are from the tank.

Hot water drained from the tank is replaced by cold water from the main water line that supplies our homes. As it enters the tank, the cold water cools the water in the storage tank. When the temperature of the water in the tank drops below the desired setting, the gas burner ignites in a gas-fired water heater. Once the water reaches the desired setting, the flame turns off.

Find out how to improve the efficiency of your storage water heater here , or read about the benefits of tankless water heaters here .