# Why do water heaters have two elements?



## ScottR (Oct 6, 2008)

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.


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## ScottR (Oct 6, 2008)

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).


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## chris75 (Aug 25, 2007)

ScottR said:


> 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.


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


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## chris75 (Aug 25, 2007)

gp_wa said:


> 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?


 
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*.


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## ScottR (Oct 6, 2008)

chris75 said:


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


Hmm.. Thanks, didn't know that.


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## gp_wa (Aug 19, 2008)

Does it produce less stratification disrupting convection currents?


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## chris75 (Aug 25, 2007)

ScottR said:


> Hmm.. Thanks, didn't know that.


 
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?


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## chris75 (Aug 25, 2007)

gp_wa said:


> Does it produce less stratification disrupting convection currents?


What?


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## ScottR (Oct 6, 2008)

> 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?


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


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## gp_wa (Aug 19, 2008)

chris75 said:


> What?


Do you want me to repeat the question?

:laughing:

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.


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## nap (Dec 4, 2007)

gp_wa said:


> Does it produce less stratification disrupting convection currents?


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.


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## gp_wa (Aug 19, 2008)

nap said:


> 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.


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?


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## nap (Dec 4, 2007)

gp_wa said:


> 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?


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.


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## gp_wa (Aug 19, 2008)

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


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## chris75 (Aug 25, 2007)

gp_wa said:


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


 
Are you really going to disect this? It works, does it not? Why not go out and buy a electric water heater and disable one of the elements and see what happens.


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## gp_wa (Aug 19, 2008)

chris75 said:


> Are you really going to disect this?


Of course :laughing:



chris75 said:


> It works, does it not? Why not go out and buy a electric water heater and disable one of the elements and see what happens.


I have, essentially. If your upper element goes out you won't ever notice. Been there done that.


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## ScottR (Oct 6, 2008)

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


I think he meant that the 20 (or however many) gallons at the top of the tank would recover faster, not the whole 50 gallons. From an end-use POV you'd get hot water back faster after depleting the tank, just not a full 50 gal. at your disposal.


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## chris75 (Aug 25, 2007)

gp_wa said:


> Of course :laughing:
> 
> I have, essentially. If your upper element goes out you won't ever notice. Been there done that.


IMO, you would notice.


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## nap (Dec 4, 2007)

gp_wa said:


> Of course :laughing:
> 
> I have, essentially. If your upper element goes out you won't ever notice. Been there done that.


as a matter of fact, in most water heaters you would most definately notice a difference. You would have NO hot water. The upper t-stat is designed (like I said, in most water heaters) so that it will not allow the lower element to be energized until the upper t-stat is satisfied. Since the upper t-stat would never be satisfied, it would never turn off the upper element and allow the lower element to be energized.

therefore, no hot water at all.

and yes, Scott R is correct on the faster recovery.


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## rolanddvcourt (Feb 8, 2012)

gp_wa said:


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


Because, since there is no disruption of the stratification layer, the top element and thermostat are off (top portion of water heater is hot). Bottom of the water heater see's the cold water that is coming into the tank. Bottom thermostat see's the cold water, and turns on, to heat the bottom incoming water. Top element doesn't need to be on because already has hot water. Sounds good anyway!


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## bob22 (May 28, 2008)

Re: convection: Cold water is denser than hot so it stays lower in the tank, reducing convection effect (in theory at least)


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## Pat P (Jun 13, 2021)

chris75 said:


> 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?


As was mentioned earlier in this string both elements never come on at the same time.


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## jmon (Nov 5, 2012)

Pat P said:


> As was mentioned earlier in this string both elements never come on at the same time.


Welcome to the forums Pat.

Going forward, when surfing the forums, before replying to a thread, look at the *date first*, on the very first line by the post #, in this thread it's Feb. 9, 2012, about 9 years ago. Just a suggestion.

You did nothing wrong. No harm in bringing back an old zombie thread. Some times they can be very helpful. However, if you are looking for answers or reply's to your questions you may not get very good results as members tend to avoid them. Just saying.

Enjoy the forums.


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## AllanJ (Nov 24, 2007)

The upper element shows its advantage when the entire tankful is cold. It takes a lot longer for enough water heated down below to migrate (via eddy currents) to the top of the tank while retaining enough heat on the way up, compared with having an upper element.

Unfortunately there is no inexpensive and simple way for a gas water heater to heat the water nearest the hot outlet quickly starting with a similarly all cold tankful. But the delay is not as bad because the same grade of gas heater heats water much faster than an all electric heater.

It is at least rumored that some models of commercial electric water heaters are built just like equivalent residential models but with some wiring differences to be able to operate both elements at the same time. But modifying the residential water heater "at home" to do this will void the Underwriters Labs or other safety rating.


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