How Water Heaters Work

Hot water heaters are a mystery to many people. Most are just big cylindrical objects that make funny noises as they churn out gallon after gallon of hot water for bathing, laundry, cleaning and other purposes. Even more interesting is that most gas water heaters do not require any electricity, yet they can turn the gas valve off and on as needed.

Gas Water Heaters

Natural gas or propane is burned at a burner at the bottom of a steel water tank to heat water. The cold water pipe inside the tank flows into the bottom where it is the hottest while the burner is on. The hot water outlet pipe draws off water from the top of the tank when hot water is being used.

Most gas hot water heaters do not require any electrical connections to control the flow of gas to the burner. The on and off control of gas flow is mechanical and powered by the temperature of the water itself. The burner control valve is the gray square box on the side of the hot water tank that has a temperature control knob and a knob to enable relighting a pilot light.

A pilot light fixed next to the burner continuously stays lit with a small flame. This flame ignites the gas sent to the burner when the gas control valve turns on. A small copper tube called a thermocouple is close to the pilot light. Heat from the flame creates a small electrical current that keeps an electromagnet open that permits gas to flow. If the pilot light goes out, the thermocouple cools off, and the ability for gas to flow to the pilot light or burner is shut off.

Protruding from the back of the gray box is a thermostat and safety device that is threaded directly into the steel tank. They are in contact with the water in the tank. As the water cools, contraction causes a lever in the thermostat to let gas flow to the burner where it is ignited by the pilot light. As the water heats up, expansion closes the valve. The safety device is a redundant shut off lever in case the thermostat fails and the water begins to overheat.

Another safety device is the pressure relief valve on the side of the tank. It will pop open to let hot water out if the tank should begin to overheat. Before this safety feature was invented, gas water heaters could rupture with enough force to shoot like a rocket up through the floor.

Electric Hot Water Heaters

Electric water heaters have two sealed electric heating elements that are threaded directly into the tank. The steel tank, pressure relief valve, cold water inlet and hot water outlet are basically the same. The control box on an electric water heater controls the flow of electricity to the heating elements.

The flowing electrons of electricity in the heating elements causes corrosion. A sacrificial anode made of magnesium is inside the tank, helping lengthen the life of the heating elements. It is called “sacrificial” because it corrodes instead of the heating elements and tank. The same concept is used on large ships to protect steel hulls.

Typically, an electric water heater costs more to operate, but it does not require a flue pipe to vent hot gases from the burning propane or natural gas to the outside. Electric water heaters may need heating elements replaced every few years, but it is not uncommon for both gas and electric water heaters to last a couple of decades or longer. Tankless water heaters are much more complex, using both electricity and gas and also having circuit boards and fans. They create hot water on demand but have many more parts that can fail.