240v hot water heater connected to 120v

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  #1  
Old 04-14-07, 04:26 PM
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Question 240v hot water heater connected to 120v

my buddy just hard wired a 240v water heater to 120v. i told him thats the wrong thing to do but he went ahead anyways. whats going to happen?
 
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Old 04-14-07, 04:30 PM
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it's going to take a lot longer for the water to get hot.

this is a resistive load and it will simply reduce the current through the element(s) and result in much less heat produced.
 
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Old 04-14-07, 04:36 PM
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What size wire did he hook this up too?
 
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Old 04-14-07, 05:08 PM
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Don't invite the buddy back to help in the future. If it was previously hooked up to a 240V breaker, I don't understand why he did not reconnect it as it was. Most electric water heaters require #10 wire on a double(not twin) 30A-240V breaker. If we are talking about a small efficieny apartment or trailer, it is possible it is 120V(if this is the case, I apologize to the buddy). There is a plate on the side of the heater that tells what is required.
 
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Old 04-14-07, 05:12 PM
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i think it was 12/2 romex
 
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Old 04-14-07, 05:22 PM
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yeah it say 240v right on the it so ..... ionno... i guess hes just gonna be looking at a high electric bill, i thought it would be worse. oh well what do i know im just a lowly carpenter lol
 
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Old 04-14-07, 05:54 PM
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I don't think his electric bill will be worse. It might even be less. However, he will NOT have as much hot water available and the recovery time of the tank will be longer. Remember that at 120 volts it will use less electricity per hour of operation than it will at 240 volts.

Is this a dual element tank, or a smaller unit with only a single element?
 
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Old 04-14-07, 06:11 PM
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I assume that the 120 volts will get the element hotter than the desired water temp...say 125. So, the water will heat up, but take MUCH longer.

Since power ( watts ) is a function of E squared, or I squared, if you cut the voltage in half, you cut the power to one fourth. So, an element which is 4500 watts on 240 will produce 1125 watts on 120V.
 
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Old 04-14-07, 06:38 PM
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As volatge increses amperage decreases. So at 120 insted of 240 you will pull more amps.
 
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Old 04-14-07, 07:01 PM
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Sorry burke, but in this particular instance that logic doesn't work.

Assuming an element rated 4500 watts at 240 volts the current flow on 240 volts supply would be about 18.75 Amperes. Operation on 120 volts would draw about 9.88 Amperes and yield 1,185 watts.

So it will be a little better than 1/4 of the output on 240 volts. For any given amount (gallons) of water used the amount of electricity used (KWH) will be the same although the time to initially heat and subsequently recover will be significantly (almost four times) longer.
 
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Old 04-14-07, 07:07 PM
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Originally Posted by burkej62 View Post
As volatge increses amperage decreases. So at 120 insted of 240 you will pull more amps.
you might want to use this:http://ourworld.compuserve.com/homepages/Bill_Bowden/ohmslaw.htm

plug in 240 volts and and 4500 watts. calculate. tke note of the current.

then clear the watts and current spaces and change the voltage to 120.

(this will use the same resistance since that is the only constant in this situation)

which way does the current go?

that will also give you a wattage rating for comparison to the 45oo originally used
 
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Old 04-14-07, 08:42 PM
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Several have pointed out that it will take a lot longer for the water to heat up. I figure he'll have enough hot water for one shower in time for next winter.

And yes, I agree with the others that burke's logic is flawed. Burke's statement would only be true if the power consumption were a constant. But the power consumption is not a constant. The only thing constant is the resistance of the element. The power consumption will reduce to 25 percent of what it would be with 240 volts.
 
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Old 04-15-07, 11:45 AM
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Your buddy may be smarter than you think.
If this is a 4500 watt water heater (most residential heaters are), and the supply wires are #12 Romex, he couldn't legally (code wise) power it with 240 Volts.
Water heaters are considered continuous loads and as such require the branch circuit conductors to be rated at 125% of the load.
4500 watts/240V = 18.75 Amps......18.75 x 1.25 = 23.44 Amps.

#12 Romex is only rated for 20 Amps maximum, so the heater would require #10 wire.

By powering it with 120V, he reduced the heater load to 9.4 Amps.

As posted, he will have to wait longer for the water to get hot...about 4 times as long, but he won't overload the wiring.

Also, he may have voided the warranty on the heater....

Just a thought
steve
 
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Old 04-15-07, 01:08 PM
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Using your same formula, unless my math is flawed, if he reduced his voltage to 120 volts, he will increase his amperage to 37.5 amps times 1.25, or 46.875 amps.
 
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Old 04-15-07, 01:13 PM
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your math must be flawed.

why not go to the calculator I linked. It will take all the thinking out of the deal.

remember; the resistance is the only constant in this deal.
 
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Old 04-15-07, 03:47 PM
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chandler, your logic is flawed in exactly the same way the burke's logic was. That element is just a dumb hunk of metal. It doesn't know that it's supposed to be a 4500-watt element. When connected to 120 volts, it's no longer a 4500-watt element, but rather a 1125-watt element.
 
  #17  
Old 04-16-07, 06:29 AM
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When you get 1/2 voltage the result is 1/4 wattage.
No reason to say this will not work, it just uses 4 times as long to heat water.

In Norway water storage heaters are quite common on lo wattage.
A wattage of approx 2300W is common on a 200 litre (~53 gal) and shold be enough for a 6 pesons residence.

(another discussion is dimentioning for teens showering, it may never be enough)

It shold do no harm to test if you have the capasity needed.
You may encrease capasity by adjusting the thermostat up, but it shold usually be more than 60 deg C (140 F) to kill legionella, and not to much more than 80 C (180 F) to be sure it its not reaching boiling temp.


dsk
 
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