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Old 21-03-2015, 11:47   #31
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Re: Running 220v water heater on 110v supply

Thanks a lot for the detailed explanation sailski. I do understand the charging regime is not ideal but as I stated above, all on board plugs and appliances are running on either 12v from the batteries ( fridges, lights, pumps) or 220V from the inverter, (power sockets) for computer charging etc. The battery chargers automatically run on 220V or 110V ( christec 40A x 2 ) so this leaves the water heater which can be switched on or off at the panel.
From the information here I understand it's fine to run the water heater and given my experience with the water being very hot and steamy and discoloured, I'm guessing it was more to do with some air being in the tank or possibly the thermostat needs adjusting. On two previous occasions when the water heater was heated by the engine, the pressure relief valve opened and sent a few litres of water into the bilges. From the link above I see that the temperature can be adjusted using a gate valve so I will look into that if it continues to open the relief valve. Thanks again for all the input.
Btw, some people would suggest having an expert diagnose systems and wiring etc, but from my experience the information found on CF is usually a lot more professional and accurate than the many technicians and experts we have come across in our travels. So short of being located somewhere where I would have absolute faith in the technician, such as mainsail or goatboatingnow amount others, CF is the best I can do. Occasional reading between the posts and clarifying answers is required and the good thing is we all learn from the experts in their various fields, so, thanks again !
Monte


Oh btw, I did also look at connecting to a 220V outlet with a 220V, 50A adaptor and wiring the 2 terminals together as suggested, which sounds fine for my purposes, however it was mentioned that the circuits may not trip as they're supposed to, as well as it being illegal to run it that way. The next option would be a step down transformer which I guess is around $1000 plus installation. I will consider that option if the need arises and if we spend more time in 110V waters...
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Old 21-03-2015, 17:44   #32
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Re: Running 220v water heater on 110v supply

I did exactly this on mine. 230v/50hz genset and (sometimes) 110v/60hz shorepower. Made up switches as per the pic. Works fine. Takes longer to heat up on shore power, but you're docked so who cares.

hmmm. Pic was right side up when I imported it. Don't know why it inverted itself.
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Old 21-03-2015, 18:28   #33
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Re: Running 220v water heater on 110v supply

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Originally Posted by Sailski View Post
This is a classic misunderstanding of the physics in play



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Old 22-03-2015, 08:40   #34
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Re: Running 220v water heater on 110v supply

Hmm. On 240 volts the resistor is 48 ohms and draws 240/48 = 5 amps. So the wattage is amps x volts = 5 * 240 = 1,200 watts.

It is current that heats it up, not voltage, all the voltage does is force the current to flow through the heating element.

If you have half the voltage you have half the force for current so you have half the current. So the element now has 2.5 amps flowing to produce heating = 2.5 amps. Since current produces heating it should be half the amount of heating = 600 watts.

BUT the volts x amps = 120 x 2.5 = 300 watts. Where did the other 300 watts go? Its the same piece of heating element with half the current. How does the voltage across the heating element produce heating watts, the heating element is insulated electrically from the water?
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Old 22-03-2015, 09:28   #35
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Re: Running 220v water heater on 110v supply

All those who understand this have explained it, yet confusion still remains, so here is one more attempt.

Ohm's Law states V=I*R or I=V/R or R=V/I, where V is voltage expressed in volts, I is current expressed in amperes and R is resistance expressed in ohms.

Electrical power is the product of current and voltage, assuming a resistive load, like this water heater, expressed as P=V*I, where P is power expressed in Watts.

Because of Ohm's Law, by substituting for V or for I, the power equation can also be rewritten as: P=V≤/R or P=I≤*R.

So, because power is proportional to the square of the current and Ĺ squared is ľ, then half the current is one quarter the power.
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Old 22-03-2015, 12:40   #36
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Re: Running 220v water heater on 110v supply

Ohms law is a good example of how a little knowledge can be a dangerous thing. I have seen it misinterpreted to defy reality many times on web forums by people misusing it.
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Old 22-03-2015, 12:48   #37
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Re: Running 220v water heater on 110v supply

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I gotta quit typing w/ booze in hand.


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Since I put my foot in my mouth yesterday. Had anyone looked at a step up transformer?
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Old 22-03-2015, 13:08   #38
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Re: Running 220v water heater on 110v supply

This has been most entertaining....
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Old 22-03-2015, 13:11   #39
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Re: Running 220v water heater on 110v supply

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Since I put my foot in my mouth yesterday. Had anyone looked at a step up transformer?
Yes. Post #12
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Old 22-03-2015, 13:15   #40
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Re: Running 220v water heater on 110v supply

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This has been most entertaining....
More than once I have seen people use ohms law in a way that would have reducing the voltage in a circuit to zero result in an infinite current.

You don't have to be an electrician to know that's not possible.
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Old 22-03-2015, 13:31   #41
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Re: Running 220v water heater on 110v supply

Andina is correct half the wattage as simply put in ohms law


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Old 22-03-2015, 13:37   #42
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Re: Running 220v water heater on 110v supply

Adina is not correct. Power is I squared times R. If current is halved then power goes down to 1/4.
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Old 22-03-2015, 14:16   #43
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Re: Running 220v water heater on 110v supply

You need o get back to basic ohms law - better still setup an experiment

I = V / R
You know that I = 5 as you calculated
You know R - 48 as you calculated.
We know V = 110

P = V * I
110 x 5 = 550 watts which is half.
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Old 22-03-2015, 14:27   #44
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Re: Running 220v water heater on 110v supply

Ben, If at 220 V the current is 10A that is 2200 watts. Then at 110V the current will be 5A. That is 550 watts. That is precisely 1/4 of what it was.
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Old 22-03-2015, 14:38   #45
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Re: Running 220v water heater on 110v supply

I equals 5 amps only when voltage is 240. At 120 volts I = 2.5 amps.

2.5 amps x 120 volts = 300 watts. All based on the assumption of 48 ohms of resistance in the element.
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