Heating Water

Discussion in 'Electrical Systems' started by messman, Jul 31, 2010.

  1. messman
    Joined: Nov 2008
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    messman Junior Member

    It has been some time since I have been here because of personal issues, but I am back and moving forward on planning my boat build. I have come across a couple of systems out there for heating water, propane based and I even found a 12v water heating system. I really dont like the idea of propane because of the potential dangers of it, so I am leaning to the 12v concept, but I can see a huge drain on my battery power. So I decided to ask the question, How do you all heat water for shower's etc. on your boats? How does your system work? If you have a 12 volt system how long does it take to heat up your water (I am only looking at heating 10-12 gal at 1 time)? and Does your system place a drain on your house batteries (assuming you are using a 12 volt system)? Thanks in advance

    Chris
     
  2. hoytedow
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    hoytedow Senior Member

  3. messman
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    messman Junior Member

    That is actually on my list of possibilities, the draw back is when I want/need to take that late nite or early morning shower.
     
  4. BTPost
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    BTPost Junior Member

    Really depends on what powers you vessel thru the water. If Wind Power, then Propane is your friend, If your vessel has an ICE, for power, use the Cooling System BTU's to heat your Domestic Water. 12 Vdc is about the LAST thing you would want to heat domestic water with, and just how would you recharge your 12 Vdc batteries for the next time?
     
  5. hoytedow
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    hoytedow Senior Member

    One question would be: How long will the water stay warm after the sun goes down?

    I don't know the answer to that one.

    The next question is: How hot does it have to be?
     
  6. hoytedow
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    hoytedow Senior Member

    Once it has been heated by the sun, it will hold the heat longer if the entire apparatus is tucked into a 5 gallon styrofoam cooler.
     
  7. messman
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    messman Junior Member

    Hoyt,

    I am guessing around 90 or so would be the temp I would look for.

    Bruce,

    Batteries would be recharged by boat motor. I am also going to have a small gen set for emergency reasons.

    My main goal with my questions, is to decide how I am going to go about heating my water, so I can lay things out for the best weight distro, efficency, and space saving. I already know the general layout of my deck and cabin. But I still have a lot of decisions to make about detailed layouts and this is the first of many.
     
  8. hoytedow
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    hoytedow Senior Member

  9. jonr
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    jonr Senior Member

    Engine, solar, propane, heat pump, anything but 12V resistance heat.
     
  10. messman
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    messman Junior Member

    jonr,

    Respect the opinion, but why do you say what you say?

    Hoyt,

    Thanks for the information, but that is only designed to raise water temp a couple of degrees, would not work in an application such as this.
     
  11. CatBuilder

    CatBuilder Previous Member

    12VDC hot water heating:

    You have 10 gallons you want heat to 90 degrees. Let's assume your water starts out at about 70 degrees.

    We can immediately say it takes 8.34 BTUs of thermal energy to raise 1 gallon of water 1 degree F (it come from the definition of the BTU).

    So, if you are heating up 10 gallons, it will take 83.4 BTUs to raise your pot of water up 1 degree F.

    Next, we have to account for you heating from 70F to 90F, we we multiply the 83.4 BTUs by the 20 degrees rise in temp, giving a total of 1,668 BTUs you need. This will bring 10 gallons of water from 70F to 90F. If the water starts out colder, now you know how to account for that. You'll need more power.

    Let's see how this applies to a 12VDC system...

    Since BTUs and Watts are both units of energy, we can convert them directly. 1668 BTUs = 488 Watts.

    Using Ohm's Law, Watts=Volts x Amps...

    we see that 488 Watts = 12 Volts x A, solve for A....

    You'll need to supply about 40 AH to your heating element to do this heating job using 12VDC. Now, this all assumes no losses. I'd round that to 50AH. Now don't forget that you can only drain a batter to 50% discharge, so you will need 100AH of battery capacity to do this heating job.

    WOW!!

    I was "doing the math" here to show that using 12VDC wasn't a good idea for hot water. Guess that depends on what you mean by "hot water!"

    This was a real eye opener. If you have a very large solar array and are not looking for extremely hot water, it is quite possible to build a 12VDC hot water heater into your system. I'm shocked, actually!

    I was going to say "go propane" or "go AC heater from your genset", but the numbers show you could easily integrate this into a good sized house DC system.

    Does anyone know a good source for a 12VDC heating element that pulls .5 to 1 KW?
     
  12. jonr
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    jonr Senior Member

    I'd also look at how long it takes to deliver these watt hrs and the wire size needed. And the effect on battery life.

    A heat pump would be about 2.5x more efficient which helps some. Since you are using an engine to charge the battery, why not heat the water then (with engine heat) and store it? I've used propane and it works fine even when heating on the fly. Solar is free.

    If you want to do it, you could use a large water heater element(s) designed for 120 or 240V and use it at 12V.
     
  13. messman
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    messman Junior Member

    CatBuilder,

    I found some elements designed for a 12v system that are rated at 600 watts. I will have to find the link again, but I also found a 12v / 220 water heater that is made Down Under I believe it is..As soon as I dig up those links I will post them here. I am not looking for really hot water, just hot enough to take a comfy shower without having to mix hot and cold water.

    Now what do you mean by a good sized House system?

    I was thinking going with 3 or 4 100ah for my house power, too much, not enough? I will also have a solar system (Not a large one) to help maintain battery power.
     

  14. messman
    Joined: Nov 2008
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    messman Junior Member

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