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Old 10-23-2009, 03:16 PM   #1
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Replace three phase water heater with single phase


I'm interested in replacing an old water heater that is fed by three phase power (208). The water heater has three elements. Each element is fed from unique pair combinations from the three conductors that feed into the unit. In case I'm not being clear on that, let's say we have red, black, and blue conductors feeding into the unit. One element is fed from the red/black pair. The second element is fed from the black/blue pair. The third element is fed from the red/blue pair combination.

What are the issues with replacing this hot water heater with a single phase unit where I will only need two of those three conductors? The more you expound, the better I will understand the details. Thanks.

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Old 10-23-2009, 03:43 PM   #2
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Replace three phase water heater with single phase


assuming you have a unit with the proper voltage for the supply, you simply do not use one of the phases.

208 3 phase is 3 hots. Single phase 208 is the same thing minus on phase conductor.

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Old 10-23-2009, 04:09 PM   #3
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Replace three phase water heater with single phase


Key, what is your level of experience that you are working on 3-phase systems and equipment?
I assume this is a commercial/industrial setting?
Are you insured to work in such a setting?
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Old 10-23-2009, 04:18 PM   #4
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Replace three phase water heater with single phase


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assuming you have a unit with the proper voltage for the supply, you simply do not use one of the phases.

208 3 phase is 3 hots. Single phase 208 is the same thing minus on phase conductor.
Thanks for the reply. This leads to two questions:

1. Because it is just a resistance element, is the 208 voltage completely compatible with a single phase 220 volt element? Do they make a 208 Volt element?

2. By only using two of the three conductors and thus only using one of the three phases (if is is proper to say it that way), does this get things out of balance in some way? Does it affect the other circuits in some way? Does it only spin the meter 1/3 the speed that it would if all three were hooked up? In other words, do you not lose any efficiency by only using one of the phases?

Thanks.
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Old 10-23-2009, 04:33 PM   #5
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Replace three phase water heater with single phase


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Originally Posted by Speedy Petey View Post
Key, what is your level of experience that you are working on 3-phase systems and equipment?
I assume this is a commercial/industrial setting?
Are you insured to work in such a setting?
I am not the one who will be working on the replacement unit, if we replace it. We are considering installing a solar circulation hot water system and from the research I've done, I've not found a three phase hot water heater that has the extra ports for the solar lines onto the roof. This led me to question the issues of using single phase hot water heater in a three phase environment. This forum is a great place to get answers from more than one person so you can get a consensus answer (at least that is the hope).
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Old 10-23-2009, 04:50 PM   #6
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Replace three phase water heater with single phase


Quote:
1. Because it is just a resistance element, is the 208 voltage completely compatible with a single phase 220 volt element? Do they make a 208 Volt element?
Not necessarily. Resistance limits current. Without knowing what the actual resistance (or wattage and calculate) I do not know what the resultant current would be and if the element is capable of handling such current. Higher rated element to a lower voltage is generally no problem, just does not produce as much heat (lowers effective wattage). When you go the other way, you will have higher effective wattage. If the element is not designed to handle the higher currents, they will fail prematurely.

Quote:
2. By only using two of the three conductors and thus only using one of the three phases (if is is proper to say it that way), does this get things out of balance in some way?
Yes but it unless this is some super duty exceptional large enough to heat water for half the city, it will not be a problem.

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In other words, do you not lose any efficiency by only using one of the phases?
you are charged by KwH. You would have to calculate the consumed power for each proposed design to determine the cost.
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Old 10-23-2009, 05:04 PM   #7
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Replace three phase water heater with single phase


A popular element size seems to be 4500 W of power (= 15,000 BTU/hr). @240 V it's 19 A and 13 ohms.
Other voltages and resistances will affect recovery time. 1 BTU of energy raises 1# of water 1 degree F.

50 gals of water weighs 420# and raising it from 50 F to 120 F will take 29,000 BTU which gives a ~2 hr recovery time.

Last edited by Yoyizit; 10-24-2009 at 06:56 PM.
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Old 10-23-2009, 07:09 PM   #8
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Replace three phase water heater with single phase


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1 BTU of energy raises 1# of water 1 degree F.
from 60 to 61 F. at one atmosphere.

although I have heard various starting temps, 60 appears to be the most common and accepted starting point I could find.
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Old 10-24-2009, 11:34 AM   #9
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Replace three phase water heater with single phase


Be aware that with a 3 phase water heater, you may be operating 3 heating elements ALL at the same time.

Single phase (home) water heaters are designed to only operate one heating element at a time. This reduces the load on the electric company grid at 6 am when everybody is taking a shower at the same time.

So the ability of the water heater to heat a new batch of hot water may be drastically diminished with a single phase unit. Not good in the winter time!

As to the solar install, a better idea might be to install an additional tank PRIOR to the existing tank. And this tank would not need a heating element in it. The solar heated water would flow into the cold water inlet of your existing tank. (Pre-heated water is the idea.)

Also they make all sorts of differently electrically configured water heater tanks which are not on display at local stores. Might try contacting a manufacturer like A.O. Smith...
http://www.hotwater.com
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Old 10-24-2009, 11:47 AM   #10
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Replace three phase water heater with single phase


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from 60 to 61 F. at one atmosphere.

although I have heard various starting temps, 60 appears to be the most common and accepted starting point I could find.
Close enough for Guvmint wuk!
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Old 10-24-2009, 12:16 PM   #11
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Replace three phase water heater with single phase


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Close enough for Guvmint wuk!
absolutely. I was just being anal.
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Old 10-24-2009, 01:01 PM   #12
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Replace three phase water heater with single phase


I think your avatar is mutating from whatever it was into some new, possibly extraterrestrial, life form.
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Old 10-24-2009, 04:35 PM   #13
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Replace three phase water heater with single phase


Care to elaborate on the size of those elements? Do you have an idea of the number of gallons/hour you will be needing? Many water heaters are rated on "first hour" or the number of gallons of hot water might be available during an hour of usage.

If you are looking for a high capacity system, connecting a 3 phase unit to a single phase source might not yield satisfactory results, unless you increase the capacity of the single phase circuit to handle the entire load.

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