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Old 07-11-2015, 11:30   #1
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12V Water heater?

Ok, so I've been searching for a 12V water heater.
When I say 12V I mean no other fuel but 12V, I'm trying to cut out any flammable fuel except for my engine.
I know they make heater elements which are 12V but a whole system is what I'm looking for.


Or, just a 120AC water heater that I can run a generator to heat the water when needed. If I go this route it would need to be low amps, I could also use it from shore power but, I only have 30 amp service.

Does anyone know if they make one?
Thanks for any help!
Ken
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Old 07-11-2015, 11:50   #2
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Re: 12V Water heater?

A reasonable water heater will draw 2000 watts


Basic calc for a 12 volt supply = 167 amps draw so for a 5 metre run cable size will need to be 150mm sq - about the size of your thumb.


Most likely the reason you cannot find one.
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Old 07-11-2015, 11:52   #3
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Re: 12V Water heater?

Quote:
Originally Posted by KenH View Post
Ok, so I've been searching for a 12V water heater.
When I say 12V I mean no other fuel but 12V, I'm trying to cut out any flammable fuel except for my engine.
I know they make heater elements which are 12V but a whole system is what I'm looking for.


Or, just a 120AC water heater that I can run a generator to heat the water when needed.

Does anyone know if they make one?
Thanks for any help!
Ken
Ken,

I suspect you will discover- given your stated parameters- that using waste heat from your engine's coolant loop will be your most efficient source for heating water (if you use a tank type water heater.)

Most water heating elements are AC- requiring shore power or a generator.

There are 12 VDC elements available as supplemental elements in a water heater for dumping 'overflow' from solar/wind/etc. 12VDC charging systems, but if you do the math, it is doubtful a 12VDC element using surplus charging amps would ever impart enough energy to heat water to a 'reasonable' shower temperature.

And to use a 12VDC heating element from a battery bank would require you to run your engine [to charge your batteries] far longer than it would take to heat water from its waste heat...

Depending upon where you sail, you may be better off with solar water heating, or consider a propane powered on-demand unit.

We currently sail in cooler climates, and mostly rely on our engine [supplemented by the generator] to produce hot water in a very well insulated tank water heater that retains a usable temperature for 24-48 hours after the last engine run.

If you want more details about the approach we took, here is our blog post on that topic.

Have fun with your project.

Cheers!

Bill
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Old 07-11-2015, 12:00   #4
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Re: 12V Water heater?

Quote:
Originally Posted by paulajayne View Post
A reasonable water heater will draw 2000 watts


Basic calc for a 12 volt supply = 167 amps draw so for a 5 metre run cable size will need to be 150mm sq - about the size of your thumb.


Most likely the reason you cannot find one.
Understood with the size of cable.. So, the 120AC method?
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Old 07-11-2015, 12:04   #5
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Re: 12V Water heater?

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Originally Posted by wrwakefield View Post
Ken,

I suspect you will discover- given your stated parameters- that using waste heat from your engine's coolant loop will be your most efficient source for heating water (if you use a tank type water heater.)

Most water heating elements are AC- requiring shore power or a generator.

There are 12 VDC elements available as supplemental elements in a water heater for dumping 'overflow' from solar/wind/etc. 12VDC charging systems, but if you do the math, it is doubtful a 12VDC element using surplus charging amps would ever impart enough energy to heat water to a 'reasonable' shower temperature.

And to use a 12VDC heating element from a battery bank would require you to run your engine [to charge your batteries] far longer than it would take to heat water from its waste heat...

Depending upon where you sail, you may be better off with solar water heating, or consider a propane powered on-demand unit.

We currently sail in cooler climates, and mostly rely on our engine [supplemented by the generator] to produce hot water in a very well insulated tank water heater that retains a usable temperature for 24-48 hours after the last engine run.

If you want more details about the approach we took, here is our blog post on that topic.

Have fun with your project.

Cheers!

Bill
Thanks Bill..
I live in the PNW So, solar is out for me, specially during winter months.
I have been looking at a newer Propane models and they do seem better then what I had.
Do you know anything about a lower consumption 120AC heater?
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Old 07-11-2015, 12:32   #6
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Re: 12V Water heater?

Ken,

On your 30A service you can nominally get 3600W, if you can devote half of that to water heating you could probably find an 1500W or 1800W 120V heating element pretty easily, either as a complete heater unit or as a replacement element to put into a tank.

Usually the smallest 120V element you can find is around 1000W (~8A), if this is still too large then you can use a small 240V element. This may seem counter-intuitive, but if you purchase a reasonably small 240V AC heater it will make a low power 120V AC heater. The reason for that is that a 240V resistive heater element will run at ~1/4 power when run on 120V. Sometimes finding a 240V element is easier than finding a small 120V element.

So, if you can pick up a 2400W 240V AC heating element, if you run it at 120V it will use 600W (5A). It won't be the fastest thing in the world, but it works. A 40 liter (~10 gallon) tank at 20C (68F) will take about 1.5 hours to get up to 40C (104F) with a 600W element (assuming a well-insulated tank).

If you care at all about the math, E=IR. R is the resistance and is fixed (within reason, it will change slightly with the temperature of the element), so if you cut E (voltage) in half then I will also be cut in half (because R is fixed) and P = EI, so 0.5E*0.5I = 0.25P
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Old 07-11-2015, 12:56   #7
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Re: 12V Water heater?

Quote:
Originally Posted by KenH View Post
Thanks Bill..
I live in the PNW So, solar is out for me, specially during winter months.
I have been looking at a newer Propane models and they do seem better then what I had.
Do you know anything about a lower consumption 120AC heater?
Hey Ken,

The water heater we decided to use [IsoTemp Spa model] offers a 750 watt 115 VAC option, and that is what we went with.

Because...
While it admittedly slows the AC recovery rate [compared to the usual ~1500 watt elements...] it also reduces the demand on shore/generator power and allows you to better balance your shore power loads if you are limited to a 30A 115VAC service. I haven't tried it, but I'll bet you could easily use a 2kw suitcase generator to make hot water with the 750 watt element... Our 10kw generator doesn't even notice the 750 watt [~7A 115VAC load... ]

We describe our thinking [for better or for worse...] in our blog post linked in our earlier response in this thread- if that is helpful.

Cheers!

-Bill
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Old 08-11-2015, 08:08   #8
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Re: 12V Water heater?

Isotemp Spa is the one you need.
Isotherm SPA Water Heaters
In addition to the reasons Bill gives, the Spa comes with a mixing valve that allows you to set the temp very hot but not get burned. The mixing valve adds cold water to the hot as it comes out of the tank to bring it down to a safe temperature. This effectively increases the amount of hot water you can get from the tank.
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Old 21-01-2016, 08:46   #9
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Re: 12V Water heater?

Quote:
Originally Posted by KenH View Post
Ok, so I've been searching for a 12V water heater.
When I say 12V I mean no other fuel but 12V, I'm trying to cut out any flammable fuel except for my engine.
I know they make heater elements which are 12V but a whole system is what I'm looking for.


Or, just a 120AC water heater that I can run a generator to heat the water when needed. If I go this route it would need to be low amps, I could also use it from shore power but, I only have 30 amp service.

Does anyone know if they make one?
Thanks for any help!
Ken
Ken,

If you are still looking for a 12VDC water heater, I just this morning received an email 'flyer' about one from Whale. [3 Gallons, 12VDC, costs almost as much as the 11 gallon IsoTemp I wrote about earlier in this thread...]

See: Whale 3 Gallon Hot Water Heater White Epoxy at AhoyCaptain.com - Always your best source.

FYI

Bill
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Old 23-01-2016, 21:23   #10
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Re: 12V Water heater?

Do a search. There are several marine water heaters out there. Probably some on Ebay or Amazon. I had a Raritan on a commercial fishing boat. Most marine water heaters also have a coil that heats with your engine water. Mine worked fine. I rarely used the 120v side. A house water heater will rust.
One disadvantage of running a generator under light loads is glazed cylinders. They start burning oil and oil shows up in your exhaust.
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