On Demand Water Heater

Discussion in 'Boat Design' started by rjmac, Mar 9, 2005.

  1. rjmac
    Joined: Sep 2003
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    Location: Burlington, IN

    rjmac Junior Member

    Guys,

    I have been working on my boat design (8.5m) for the past year and I have gotten to a point where I am evaluating a water heater. Looking around my options have been, an electric(110v/220V), or running off the water jacket of aux drive.

    I did some calculations, for wt (4 liter), then did a little searching around the net, found a submerssion heater that runs on 12V/24V (600W). Did some more calculations, figure I can build an on demand water heater for half the price of what is out on the market and it will consume power only when in operation. Battery recharge will take the form of solar and/or wind generation.

    Just kinda wondering if I have missed something in searching for an on demand water heater for marine application?

    :)
     
  2. ABoatGuy
    Joined: Aug 2004
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    ABoatGuy Member

    Tried a 110VAC instant (under the sink) unit and it fell short of being useable. The manual suggested a valve in front of the unit to limit flow, but to get hot water it was running at a trickle. The 220VAC (under the sink) heaters work ok if you have the power on the boat.
     
  3. Willallison
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    Willallison Senior Member

    I suggest you take a look at this thread..... then consign your idea to the scrap heap :(
    To provide enough energy to heat the water will require a huge battery bank. Which will require a big recharging source. It will take a long time to heat a little bit of water up, a little bit. Sorry to crush your plans, but physics is physics... :(

    http://www.boatdesign.net/forums/showthread.php?t=745&highlight=water+heater
     
  4. dougfrolich
    Joined: Nov 2002
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    dougfrolich Senior Member

    I gotta second what will said. IMHO Propane flash heaters perform the best of all the on demand type, But I like my 10gallon heater that runs on 110vac, and the engines heat exchanger, I get consistent watertemp at variable flow rates, somthing that is a potential problem with all the on demand type heaters gas or electric.
     
  5. mattotoole
    Joined: Nov 2004
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    mattotoole Senior Member

    I second this. The tank units will keep water hot overnight if need be. The combination of engine heating and shore power works well enough for most people.
     
  6. ErikG
    Joined: Feb 2002
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    Location: Stockholm, Sweden

    ErikG Senior Member

    Well guys, some folks dont stay at marinas much so having 110/220V heaters arent that useful to them.
    My guess IF you DONT have an inboard (if you do, do it the conventional way), then you could use a propane/gas heater, I've used stationarys when in our vacation house and they work very well, dont use much electricity and heats well. Obviously it has all the normal drawbacks with having a chimney/heat exaust and using propane/gas.
    Another drawback is on a small boat like yours it would probably use to much space...

    As always...

    There's no free lunch...
     
  7. ErikG
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    ErikG Senior Member

    OTOH... What the heck do you need hot water for on a small boat?

    Take a dip in the sea or go home or borrow somone alses shower :)
     
  8. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Sorry Erik - but for me, not having a shower isn't boating, it's camping! ;)

    My 27 footer ran both 240v and engine heating - pretty standard stuff. When cruising, I spend zero time tied to a berth. So long as I ran the engine for around 20 to 30 minutes, there was enough hot water for my wife and I to have a hot shower - separately! :D This is a 27 foot boat remember... no separate shower stalls in here.... ;)
    So I think I'd agree with Erik, if this system would work for you, it's the one I'd go for. The heating source is already there - the engine - and the hw cylinders are cheap, readily available and reliable.
     
  9. yipster
    Joined: Oct 2002
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    yipster designer

    and when you decide to install a gasbottle water heater get a marinised one.
    the type that switches automatic gas of by flame out, same as for gas stoves in boats.
     
  10. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    For smaller boats its far easier to get a big kettle , and heat wash water on the stove.

    Few really tiny boats HAVE the water to waste on showers , others use "Sun Showerts" which is a plastic bag hung in the sun to heat 2 1/2 gal to a nice tenp. Then showering in the cockpit solves the space problem below too.

    FAST FRED
     
  11. rjmac
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    rjmac Junior Member

    I have been reading the treads and there is a lot to consider..... The design I am working on does have a huge battery bank, the aux is electric, the same setup that was used on a tartan28 with really good results. The aux drive system will be the same as his, 4-12v in series for 48 volts at 365Amp-Hr storage. I have calculated that if I use it twice a day that I will bring the battries down to about 50% charge.

    The basic physics starts with a very well know equation that examines the bulk exchange of energy, where;

    ΔQ= M * ΔT * Specific Heat

    ΔQ = Change in Energy State of Water
    ΔT = Change in Temperature (Tf - Ti)
    M = Mass of material
    Specific Heat = Specific Heat Capacity of Water

    The question arises, how much water do I want to heat and how long do I have to heat it? The average home shower consumes approx. 2.5gpm(9.46lpm) and a conservation showerhead will use 1.5gpm(5.68lpm). This is the target that I will use, the 1.5gpm, and how cold will the water be….? Well I do not plan on being out on the water in the winter time so I arbitrarily picked 55 Deg F(12.78 Deg C), which is cold, and for my wife she likes a shower temp of around 80 Deg F(26.67 Deg C), so turning the crank I end up with;

    5.73kW to heat 1.519gpm or 119A over 60sec, 343.81kJ

    This also takes into account that the process is 90% efficient and I have worked with heat exchangers that are running about 98% efficient. I am still working on the design but the weight of a 10-15gal(37.85-56.78Liter) tank vs 45lb(16.79kg) device I can locate right in the show or under the sink is very desirable.

    The big issue with this type of device, as eluted to, the flow has to be consistent and so a pressure regulator is a must.

    I appreciate you guys for the input, I’ll let ya know how it works out when I am done with the little heater… :D
     
  12. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "I have calculated that if I use it twice a day that I will bring the battries down to about 50% charge."

    and how do you propose to recharge to 100% full (needed to prevent sulphation) 2X a day??

    Using battery power is NEVER a problem . recharging always is

    FAST FRED
     
  13. rjmac
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    rjmac Junior Member

    Earlier, the battries are in series not parrallel....oops...
     
  14. rjmac
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    rjmac Junior Member

    Heavy Sulphation occurs when the battries are discharged below 50% and the usage has to be limited to a 50% discharge level.

    Also I mis-quoted myself, I can run the waterheater for 1.11 hrs to take the battries down to 50%.

    The boat has a 7.2kW generator to drive the motor when the battries get down to 50%. It has some nice by-products like access to 220-110 if I choose to do so. Also I plan using solar cells for trickle charging and also a wind generator.

    I am pretty much on schedule, don't plan to start building the boat for another year and a half. Getting ready to build a 1/7 scale model to get a good understanding of some of the issues...., some of you may remember the "Pear Review..." ha.... :D
     

  15. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

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