Why does an electric water heater have two heating elements, and why does it only use one of them at a time?

Question

Why does an electric water heater have two heating elements, and why does it only use one of them at a time?

Answer 1

Both elements kick on at the same time when you first install it. Then lets say you use 15 gallons of hot water. The bottom element will then kick on because the cold water comes in and down a tube to the bottom of the water heater. If you just use a little hot water the bottom element will kick on  only because that's all that is needed. Now let's say you are taking a long shower - the bottom element is going to kick on when the thermostat says the water is now getting colder around the bottom element, and as you keep the hot shower going the top element thermostat also senses the existing hot water in the tank is now dropping temperature so it also kicks on.

It is essentially a back up system where the second element only kicks on as needed. That is why most of the time the lower element in a water heater or the thermostat to it is the one that always has the most trouble, the top element may not get activated as much as the bottom element.

On quick recovery water heaters both elements kick on.

Some people prefer gas water heaters because the heat up time is a lot faster than electric.

The lower element is also the one that maintains the water temperature even when no hot water is being used. The water in the heater will cool off a little and then the bottom element will kick on for a minute or two just to keep the water temperature at the temps set on the thermostat.

Answer 2

Some heaters are wired to run both at the same. Most heaters use the bottom element to heat the water 90% of the time, due to cold water is fed into the bottom of the tank thru a dip tube. Most cases both elements will draw too much current if run at the same time.

one 4500w element draws 18.75 amps at 240v

two 4500w elements would draw 37.50 amps at 240