Dual Element Water Heaters

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bcorbin

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I'm looking at a Lochinvar water heater cutsheet, which says upper and lower elements are both 4.5 KW. Without knowing any better, I would size the feeder to this for 9 KW / 240V = 37.5 amps. I also get spotty information that both elements do not work at the same time, which would yield only 18.75 amps. Is this typical?
 
They generally can't run at the same time. So size it for 4500 watts. Unless you have a very unusual model.
 
Most 4.5KW dual element tank water heaters that I have ever worked with just operate one element at a time. I have a 50gal Lochnivar water heater in my house that runs that way. It should say on the spec sheet though.
 
If it is a typical household water heater only one element will run at any time.

10/2 with a 30 amp breaker and your all set.
 
Thanks for the information. Any idea why they have two elements then? I can't think of a reason you would want the top one on and the bottom one off, unless the water heater somehow knows it isn't going to need much hot water. (assuming hot water flows out the top, and cold water comes in the bottom...I guess that's how they're plumbed)
 
bcorbin said:
Thanks for the information. Any idea why they have two elements then? I can't think of a reason you would want the top one on and the bottom one off, unless the water heater somehow knows it isn't going to need much hot water. (assuming hot water flows out the top, and cold water comes in the bottom...I guess that's how they're plumbed)
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Heats up faster. You move the fire from the bottom of the pot to close to the top of the pot.
 
Hot water tank

Hot water tank

The HW tank has a dip tube that runs to the bottom of the tank when filling. And the elements that i have seen DO NOT operate at the same time.
 
Got a model number?

Is this resi or commercial? I ask mainly because Lochinvar is a brand favored in commercial installs, and they may well both operate at the same time if this is the case.
 
Generally the way these things work is that the upper control sends the power to the upper element to heat the water near the top of the tank fairly quickly. Once that t-stat is satisfied the controller switches the power to the lower element to heat the rest of the water. When we change out burned out elements, if it's the lower one that's failed we always replace it with the super duper high quality ones because they last much longer in the nasty sediment at the bottom of the tank. If it's the upper element that's failed it gets the el cheapo element because they usually burn out because some brainiac turn on the water heater before it was full of water.
 
bcorbin said:
Any idea why they have two elements then? I can't think of a reason you would want the top one on and the bottom one off, unless the water heater somehow knows it isn't going to need much hot water. (assuming hot water flows out the top, and cold water comes in the bottom...I guess that's how they're plumbed)
Click to expand...

The top element has a thermostat that either sends power to the top element or to the bottom thermostat. The bottom thermostat is either sending power (when the top thermostat sends power to the bottom thermostat) to the bottom element or off.

Theory is if you use a lot of hot water and the cold/hot level line moves up past the top thermostat, the top element heats the water trying to keep up with demand (less water to heat), and once the top thermostat is satisfied then the bottom thermostat has power to do with as necessary.

The bottom thermostat does more of the work since it comes on whenver you pull enough hot water to raise the cold/hot line to its level. The top one only works when you raise the cold/hot line to its level.
 
while on the topic of water heaters, does anyone know of one that works on a 20a 12/2 240 circuit? They all seem to be 18 to 19 amps and with continuous load it needs a 30 amp breaker.
 
danickstr said:
while on the topic of water heaters, does anyone know of one that works on a 20a 12/2 240 circuit? They all seem to be 18 to 19 amps and with continuous load it needs a 30 amp breaker.
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Sure. All the old water heaters were served with #12, since they weren't "quick recovery". You can still get one (with a full sized tank) that only needs a #12 feed but you have to go to a plumbing supply house. Obviously, the shorties often are lower wattage too.
 
iwire said:
I have never seen a whole house electric WH that was served with 12 AWG.

Must be a local thing? :-?
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I have. Like Marc said the older ones could be wired with #12.
And I've rewired a lot of newer ones that were changed out by plumbers
but not rewired with #10..... Guess they couldn't be bothered by such
things as proper sized wire.
 
Ahhh.....I see. So the first element keeps the water immediately around it hotter than the rest of the tank. When enough hot water flows out, the thermocline (I guess that would be the right word for it) rises past a thermostat, and the heater switches to the other element. Add one more "non-electrical thing I had to learn in order to do the electrical design correctly" to the list. :D Thanks for the very good explanations.
 
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