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Old 01-03-2018, 21:36   #1
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Using low-voltage water heating elements as a diversion load.

Has anyone tried retrofitting a 120/240 VAC hot water tank with a DC element? I'm re-designing my solar system and adding a wind generator. Rather than shunting excess I think a better diversion would be heating water. Just curious if anyone else has tried it..
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Old 02-03-2018, 04:49   #2
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Re: Using low-voltage water heating elements as a diversion load.

You can dump your excess solar panel current into a 120/240 heater element.
Voltage rating of element doesn't matter.
However! You can't tie your solar panel DC into the same element that is being fed 120/240,without using a "transfer" switch/relay.

Len

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Old 02-03-2018, 05:30   #3
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Re: Using low-voltage water heating elements as a diversion load.

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Originally Posted by deblen View Post
You can dump your excess solar panel current into a 120/240 heater element. Voltage rating of element doesn't matter.
[...]
Except that the 120 vac heating element won't draw much current at 12 volts, and won't heat much.

I would replace with 12 volt elements; but remember that all electrical heating takes lots of energy.
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Old 02-03-2018, 05:31   #4
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Re: Using low-voltage water heating elements as a diversion load.

There's a dual AC/DC hot water heater element available to do just what you are thinking about and it was installed in my water heater when I bought my current boat. Unless you're located someplace like the Caribbean I doubt that you'll spend much time with your batteries fully charged so any current is available to for your dump load. Even then, not too many cruisers report having amps to spare very often. Also, the AC element that's included in the dual element I had was such low power that it took at least an hour of running the generator just to get my water heater up to temp and now that I have a "normal" AC element it takes about 20 minutes. The only way I'd go with such an arrangement again would be if the hot water tank had two separate places to install heating elements so my ability to use AC power would not be degraded like it was with the dual element.
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Old 02-03-2018, 05:48   #5
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Re: Using low-voltage water heating elements as a diversion load.

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Originally Posted by TreblePlink View Post
Except that the 120 vac heating element won't draw much current at 12 volts, and won't heat much.

I would replace with 12 volt elements; but remember that all electrical heating takes lots of energy.
You are correct that the 120vac element "won't draw much current".

In fact it will only draw 1/10 the amps current as a 12V element would draw.

But due to ohm's law,both elements will produce the same wattage,& therefore,the same heat into the water.

Len
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Old 02-03-2018, 06:15   #6
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Re: Using low-voltage water heating elements as a diversion load.

Quote:
Originally Posted by deblen View Post
You are correct that the 120vac element "won't draw much current".

In fact it will only draw 1/10 the amps current as a 12V element would draw.

But due to ohm's law,both elements will produce the same wattage,& therefore,the same heat into the water.

Len
Huh? I realize that heating elements change resistance a bit with temperature, but not THAT much. Here is the simple math:

10 Ohm element: with 120 volts RMS: 12 amperes. times 120 volts,
= 1440 watts.


10 Ohm element: with 12.6 volts DC 1.26 amperes. times 12.6 volts
= 15 watts.

Chris
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Old 02-03-2018, 07:13   #7
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Re: Using low-voltage water heating elements as a diversion load.

Somewhere along the way,

"You can dump your excess solar panel current into a 120/240 heater element. Voltage rating of element doesn't matter."

(using the original 120 volt element at 12 volts)


Morphed into:

"both elements" (another, different element?)

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Old 02-03-2018, 07:40   #8
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Re: Using low-voltage water heating elements as a diversion load.

Quote:
Originally Posted by deblen View Post
You are correct that the 120vac element "won't draw much current".

In fact it will only draw 1/10 the amps current as a 12V element would draw.

But due to ohm's law,both elements will produce the same wattage,& therefore,the same heat into the water.

Len
That's interesting, less current and less votage, yet the same power ... Can you show your calculation ?
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Old 02-03-2018, 08:18   #9
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Re: Using low-voltage water heating elements as a diversion load.

I dumped excess solar into a water maker. Same idea
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Old 02-03-2018, 08:27   #10
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Re: Using low-voltage water heating elements as a diversion load.

Quote:
Originally Posted by TreblePlink View Post
Somewhere along the way,

"You can dump your excess solar panel current into a 120/240 heater element. Voltage rating of element doesn't matter."

(using the original 120 volt element at 12 volts)


Morphed into:

"both elements" (another, different element?)

I was the one who mentioned the 2 separate elements. The reason being that I think it's a VERY bad idea to connect AC and DC systems through a hot water heater, even if you have a switch to only have one connection live at a time. And if you don't have switches to keep only one of the 2 connections live at one time, the first time you turn on 110V AC power and it feeds backwards into the DC system, there's going to be a noticeable smell of smoke and a boat full of things that don't work any more. If you can feed the AC power from your wind generator to the hot water element before it gets converted to DC, that would be safer but it means that the AC power from your boat could also electrify your wind generator and that's a bad idea too. So, in order to keep your AC and DC systems separate you need 2 separate elements. To do otherwise would be too expensive to do safely for very little gain and would be an invitation to disaster.
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Old 02-03-2018, 08:27   #11
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Re: Using low-voltage water heating elements as a diversion load.

Quote:
Originally Posted by SV FlyingSpirit View Post
Has anyone tried retrofitting a 120/240 VAC hot water tank with a DC element? I'm re-designing my solar system and adding a wind generator. Rather than shunting excess I think a better diversion would be heating water. Just curious if anyone else has tried it..
I am about to do this on my cat - although using 48v traction LiFePo4 banks.
Not very efficient to heat water; but the best way to use excess solar.
Missouri Wind & Solar (??) have both 12v. and 48v. replacement heater elements
that should fit most water tanks. They have some very informative how-to videos also.
Graham
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Old 02-03-2018, 08:39   #12
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Re: Using low-voltage water heating elements as a diversion load.

Dual (meaning you don't lose the ability to do AC heating) elements can work if the configuration of the tank allows. My current water heater does not, so I couldn't use the one I had in my Raritan, which did.

My vendor for that part was SVHotwire...

Divert Load FAQ – svHotWire
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Old 02-03-2018, 08:41   #13
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Re: Using low-voltage water heating elements as a diversion load.

I think what appeals to me is just having the 12 volt element hooked directly to the batteries through the breaker panel. Turn it on when you have excess power available or if we absolutely need to run it at the same time as my honda generator is charging the batteries. It'll run fine off shore power obviously. No need to complicate the system by running more wires and worrying about diversion loads.
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Old 02-03-2018, 08:43   #14
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Re: Using low-voltage water heating elements as a diversion load.

Quote:
Originally Posted by rom View Post
That's interesting, less current and less votage, yet the same power ... Can you show your calculation ?
You use I to the 5th power.
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Old 02-03-2018, 08:48   #15
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Re: Using low-voltage water heating elements as a diversion load.

Dual (meaning you don't lose the ability to do AC heating) elements can work if the configuration of the tank allows. My current water heater does not, so I couldn't use the one I had in my Raritan, which did.

My vendor for that part was SVHotwire...

Divert Load FAQ – svHotWire
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