Why do water heaters have two elements?

gp_wa

Stupid question day...

Why do tank water heaters have two elements? Only one kicks on at a time. What's the point in having the upper element?

ScottR

In (most) water heaters, cold water flows into the bottom of the tank (there's a pipe inside the tank that ends a few inches from the bottom). Hot water in turn flows from the top of the tank. So as you use hot water, the bottom is replenished by cold water, and the top stays hot.

If you only use a little bit of hot water (e.g. washing your hands as opposed to a long shower), the water in the bottom of the tank will be cold / the top will still be hot.

For large electric WHs, as the kind you're probably talking about: Only the bottom element would need to come on to heat the water down there. It's more efficient to just have that element come on as opposed to both.

If you use most/all of the hot water in the tank, of course both elements will come on as the entire tank will be cold/warmish.

ScottR

Attaching diagram.. Shows gas and electric. Same idea with the cold water dip tube, but obviously gas has one burner, electric has two elements (well, smaller WHs may have one element).

Attached Thumbnails

 

chris75

Both elements will never come on at the same time, only one at a time.

chris75

The top element heats water in the top one-fourth of the tank. When the water in the top of the tank reaches the correct temperature, the top element shuts off and the lower element comes on to heat the remaining water. This contributes to quicker recovery in high use situations.

ScottR

Hmm.. Thanks, didn't know that.

gp_wa

Does it produce less stratification disrupting convection currents?

chris75

Think about it, what is the wattage of the unit? 4500 watts, correct? so how can I possible wire a 30 amp breaker to a unit that draws 9000 watts?

chris75

What?

ScottR

Well, I get that, I just didn't have the wattage of the unit (hypothetical or otherwise :p) and the wattage of the elements handy...

gp_wa

Do you want me to repeat the question?



Something about convection currents in the tank messing up the stratification layer. You know, hot water on top, cold water below.

I don't see why else you wouldn't just put an element at the bottom of the tank and be done.

nap

there are no convection currents becuase the cold is alway on the bottom so it naturally stratifies and there are no convection currents to disrupt that.

gp_wa

There would be if the heating element was in the cold area, creating hot water that wants to go "up"...

Any other reason to have two elements?

nap

No, because the water on top is already hot.

If the entire tank is cold, only the top element will turn on. When it is satified, the lower element will turn on. When you run hot water, the water is tapped off the top and fresh water is fed into the bottom (still hot on top, cold on bottom). The lower thermostat will call for heat using the lower element but since there is already hot water on top, you not have convection currents.

If you run enough hot water so the top thermostat calls for heat, the lower element will be turned off (we are back at the start with the entire tank cold).

You always have hot water above the cold water or you are heating the top of the tank so you will end up with hot water on top of the cold water. Then the lower portion gets heated.

Now if you did something such as setting the lower thermostat higher than the upper t-stat, you could cause such currents but you don't set the t-stats like that just do you do not have this situation.


Understand why you will not have convection currents now?

So, now that that should be settled, lets go to why 2 elements;

you know the order in which they run. I just explained it. So, not what happens is you have a 50 gallon tank (just an arbitrary number). The heating does not split 50/50 between the upper and lower elements but for discussion, lets assume they do.

So, when you run hot water, you could theororetically run out 50 gallons and that would be it except, as you deplete the hot water, the cold water coming in makes the lower t-stat call for heat. The element heats up and starts heating the cold water. If you run the water slow enough, the lower element could nearly keep up with the flow and you would have constinuous hot water. If you run water faster, you will end up with tepid water after you have depleted the initial 50 gallons and then, since the top t-stat would be calling for heat, it would try to heat the water you are using right now and the lower element would not be on at all so when the tepid water runs out, you have just cold water.

The bigger reason for the 2 elements is it allows you to use a limited amount of hot water and due to the order of running, it allows for a faster recovery. As long as you do not run the water more than the 50 gallons and you run it slow enough for the lower element to actually heat the water to some degree, this means the top element only has to heat it the rest of the way to the setting so viola` faster recovery.

the numbers used are for examples sake. They will vary.

gp_wa

The point about faster recovery doesn't make any sense. 4500 watts puts 15000+ BTU into the tank no matter where it is.