Electric Water Heaters
December 2, 2009

How Electric Water Heaters Work

Electric water heaters work on a different principle then gas. There is no core in the center, since there is no burner, and no heat exchanger or venting system is required. Electric water heaters typically employ two immersion elements that are similar to the elements in a kettle.

One is near the top of the tank and the other is near the bottom. Both elements do not work at the same time. (Note: There are some water heaters that have only one element. There are also some water heaters designed to have both elements on at the same time. Life can never be simple, can it?)

Thermostat Control Both elements are thermostatically controlled, and the temperature setting is usually the same on the top and bottom. The thermostats are not normally accessible without removing an access panel, and often some insulation.

If the electric power to the water heater is on, there is a risk in removing these access panels and accessing the thermostats. There are live electrical connections adjacent to the thermostats. Unless you are comfortable and skilled working with electricity, you should leave these access panels closed.

These elements provide the heat and, for the most part, the water configuration is the same as on a gas or oil system. Cold water is introduced near the bottom of the tank and hot water is drawn off near the top. The cold pipe may enter the top of the tank or the side of the tank near the bottom.

When the tank is at rest, it will be filled with hot water. When a hot water faucet is turned on, hot water will be drawn off the top of the tank and cold water introduced at the bottom. This cold water near the bottom of the tank will activate the thermostat for the lower element.

The lower element will come on and heat the incoming cool water. The upper element will still be surrounded with hot water and will be satisfied.

If we keep drawing off hot water, the tank will be filled with cool water, because the lower heating element will not be able to warm the water up quickly enough as it comes in and flows through. Eventually, all of the hot water will be drawn out of the tank, and all the water will be cold.

At this point, the thermostats for the upper and lower heating elements will both be calling for heat. The upper element has priority and it will shut off the lower element so that it can operate. As discussed, only one element can work at a time.

The upper element has priority because after the tank has run out of hot water, we want to prepare quickly for the next use of hot water. The upper element heats the water near the top of the tank first, since it is the water that will be drawn off first.

Once it heats the water in the top part of the tank, the thermostat will be satisfied and the upper element will shut off. The lower element now gets electricity and heats up the water in the lower two-thirds of the tank.

Under normal use, the lower element works more often than the upper element, although the upper element has priority.

You can’t tell if one element is burned out without doing tests. It is common for water heaters to work, but not very well, because one element is burned out. Replacement of elements is not a big job and water heaters themselves are not terribly expensive.