Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Plumbing

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 10-14-2008, 07:43 PM   #1
DIY
 
Join Date: Aug 2008
Location: Seattle, WA
Posts: 162
Rewards Points: 0
Share |
Default

Why do water heaters have two elements?


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?

gp_wa is offline   Reply With Quote
Old 10-14-2008, 08:42 PM   #2
DIY'r
 
ScottR's Avatar
 
Join Date: Oct 2008
Location: Hicksville, NY (Yes, it's an actual town).
Posts: 520
Rewards Points: 0
Default

Why do water heaters have two elements?


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 is offline   Reply With Quote
Old 10-14-2008, 08:48 PM   #3
DIY'r
 
ScottR's Avatar
 
Join Date: Oct 2008
Location: Hicksville, NY (Yes, it's an actual town).
Posts: 520
Rewards Points: 0
Default

Why do water heaters have two elements?


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 Images
 
ScottR is offline   Reply With Quote
Old 10-14-2008, 10:00 PM   #4
Electrician
 
Join Date: Aug 2007
Location: Connecticut, Litchfield
Posts: 2,015
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by ScottR View Post

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.

Last edited by chris75; 10-14-2008 at 10:06 PM.
chris75 is offline   Reply With Quote
Old 10-14-2008, 10:05 PM   #5
Electrician
 
Join Date: Aug 2007
Location: Connecticut, Litchfield
Posts: 2,015
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by gp_wa View Post
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.
chris75 is offline   Reply With Quote
Old 10-14-2008, 10:33 PM   #6
DIY'r
 
ScottR's Avatar
 
Join Date: Oct 2008
Location: Hicksville, NY (Yes, it's an actual town).
Posts: 520
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by chris75 View Post
Both elements will never come on at the same time, only one at a time.
Hmm.. Thanks, didn't know that.
ScottR is offline   Reply With Quote
Old 10-14-2008, 10:39 PM   #7
DIY
 
Join Date: Aug 2008
Location: Seattle, WA
Posts: 162
Rewards Points: 0
Default

Why do water heaters have two elements?


Does it produce less stratification disrupting convection currents?
gp_wa is offline   Reply With Quote
Old 10-14-2008, 10:39 PM   #8
Electrician
 
Join Date: Aug 2007
Location: Connecticut, Litchfield
Posts: 2,015
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by ScottR View Post
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?
chris75 is offline   Reply With Quote
Old 10-14-2008, 10:40 PM   #9
Electrician
 
Join Date: Aug 2007
Location: Connecticut, Litchfield
Posts: 2,015
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by gp_wa View Post
Does it produce less stratification disrupting convection currents?
What?
chris75 is offline   Reply With Quote
Old 10-14-2008, 10:42 PM   #10
DIY'r
 
ScottR's Avatar
 
Join Date: Oct 2008
Location: Hicksville, NY (Yes, it's an actual town).
Posts: 520
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
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 :p) and the wattage of the elements handy...
ScottR is offline   Reply With Quote
Old 10-14-2008, 11:16 PM   #11
DIY
 
Join Date: Aug 2008
Location: Seattle, WA
Posts: 162
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by chris75 View Post
What?
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.
gp_wa is offline   Reply With Quote
Old 10-14-2008, 11:53 PM   #12
nap
You talking to me?
 
nap's Avatar
 
Join Date: Dec 2007
Location: sw mi
Posts: 5,407
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by gp_wa View Post
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.
nap is offline   Reply With Quote
Old 10-14-2008, 11:57 PM   #13
DIY
 
Join Date: Aug 2008
Location: Seattle, WA
Posts: 162
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by nap View Post
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?
gp_wa is offline   Reply With Quote
Old 10-15-2008, 04:21 PM   #14
nap
You talking to me?
 
nap's Avatar
 
Join Date: Dec 2007
Location: sw mi
Posts: 5,407
Rewards Points: 0
Default

Why do water heaters have two elements?


Quote:
Originally Posted by gp_wa View Post
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.

Last edited by nap; 10-15-2008 at 04:31 PM.
nap is offline   Reply With Quote
Old 10-15-2008, 04:40 PM   #15
DIY
 
Join Date: Aug 2008
Location: Seattle, WA
Posts: 162
Rewards Points: 0
Default

Why do water heaters have two elements?


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

gp_wa is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
2 water heaters - saving energy rjamesh Plumbing 16 10-18-2008 10:33 AM
(Water Heater MYSTERY) Hot water out from dead tank with cold water in it? Inayity Plumbing 6 07-14-2008 08:36 PM
Is 1/2 Pex more restrictive than 1/2 copper? twilightcall Plumbing 19 06-24-2008 07:32 AM
Advice - Tankless Water Heaters sandy2982 Plumbing 13 05-09-2008 10:27 PM
Tankless Water Heaters Recalled for Repair Due to Carbon Monoxide shimp Plumbing 1 01-22-2008 06:52 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.