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Old 20-05-2013, 10:13   #1
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Designing a hot water storage tank

Having successfully worked out how to build my refrigerator box using vacuum insulated panels (VIPs), and still having a balance left in the supplier's account, I'm considering building a custom water heater storage tank using VIPs to improve the insulation of it. I'm considering making the tank rectangular to maximize interior volume relative to the location of the tank and to provide better insulation cover with the VIPs. Cylinders and other shapes aren't as effective with the demands of the VIPs. I'd like to include a heat exchanger coil, a 110voltAC heating element for at the dock, and I'm considering a 12 volt heating element should I install a wind generator that shunts the excess power to the heater when the batteries are topped out. Has anyone actually constructed their own hot water storage tank system from scratch? Any tips?
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Old 20-05-2013, 10:29   #2
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Re: Designing a hot water storage tank

My only thought about rectangular is that the shape doesnt do well under pressure. They are usually round with domed ends to not fail from the pressure. A few psi will make a flat walled tank oil can pretty bad. 40psi could be a bigger problem. I suppose you could wrap a cylindrical tank with normal insulation and then surround it with a rectangular vacuum panel setup...?
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Old 20-05-2013, 10:35   #3
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Re: Designing a hot water storage tank

Good intentions and a nice idea for dual elements.

It would seem, however, that even with your "available" insulation, you might want to consider the ROI and cost benefit comparison of what you're suggesting compared to what's available on the marketplace: 1) an Isotemp well insulated heater; 2) a more standard, say Seaward el cheapo with your insulation added outside.

Downside is that neither would have dual elements, but you'd have to have a great battery bank to support even the 12V element. Depending on where you sail, a Sunshower could avoid the need for the 12V element.

Good luck.
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Old 20-05-2013, 10:39   #4
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Re: Designing a hot water storage tank

I stripped my Ardic tank's original plastic cover with fiberglass insulation down to the SS, and wrapped it with Aerogel.
There are pics and a description here on CF.

Run water heater via inverter?
Post #19.

It was easy to do and worked out great.
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Old 20-05-2013, 10:44   #5
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Re: Designing a hot water storage tank

Its all about PSI. As Cheechako indicated square sides would bulge out quite nicely. 40 psig which the average boat pressure water system develops would generate 5760 pounds of pressure per square foot on the side walls. Round is the only way to go with pressure water.

Now that said, you could make an atmospheric hot water tank and just put a second pressure pump on the discharge of the hot water tank. You would use a float valve to fill it and have an air vent. Its how the British did it for quite a while. Avoids side wall pressure and "BLEV"seam failures.

You could also build a square box and put a round water heater inside of it too. Though not quite an efficient use of space.
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Old 20-05-2013, 10:45   #6
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Re: Designing a hot water storage tank

I'm not really qualified to comment, but a few things come to mind. If you are living aboard, your banks will not likely ever be "topped off" unless you are on shore power then you dont need aux. power. If you are not living aboard, you dont need hot water.
I built rectangular tanks like you are designing for fuel and water. I love how much they hold without taking up much space. I will be using the engines heat exchanger.
A few years ago I was trekking in nepal and every roof top had a black water tank for heating water (not for black water). I'm thinking your excess solar or wind power would be well served running a hot water circulating pump thru a solar heater of some sort. Thats my plan anyway....just thinking out loud
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Old 20-05-2013, 14:40   #7
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Re: Designing a hot water storage tank

If you were building out a new deck you could embed tubing in the deck and circulate water through it. Or build tubing into a bimini.

Someone should make solar panels that are cooled using water. It would increase their efficiency as well I believe.
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Old 20-05-2013, 15:26   #8
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Re: Designing a hot water storage tank

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Originally Posted by zboss View Post
If you were building out a new deck you could embed tubing in the deck and circulate water through it. Or build tubing into a bimini.

Someone should make solar panels that are cooled using water. It would increase their efficiency as well I believe.
nice idea
this could be adapted for sailboats as well
RV SOLAR HOT WATER KIT - BHAsolar
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Old 20-05-2013, 16:28   #9
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Re: Designing a hot water storage tank

The problem with ingratiating solar hot water with PV panels is as the hot water temp rises the PV voltage will fall.

For solar hot water on a boat you need roughly 5 SF of solar hot water panel per 6 gallons of hot water, Less closer to the equator. 1/2" header, 1/4" tubes on 6" centers, black pained aluminum flashing, a box with clear poly top, a small 12v pump and a differential controller and your good.

However, I find that a 2 gallon garden sprayer works well for heating shower water during the summer. At $12 at the big box store and a small can of black paint its cheap, easy to fill and lasts many times longer then a hanging solar shower.

Sorry just a little segway...
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Old 20-05-2013, 16:30   #10
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Re: Designing a hot water storage tank

are you talking about building a pressurized tank or just a holding tank?

i am considering building a "warm water" holding tank to sit between my cold water tank and my calorifier. if i raise the ambient temperature of the warm tank to 20 degrees celsius, and then feed that into my calorifier, i can increase my available hot water very considerably.

the downside is that my "cold water" will always be warm.

i constructed a cold water tank out of 3mm steel by building a frame out of angle and sheet metal, painting it with ameron epoxy and then a potable water epoxy, and the sealing it with ct1 plus stainless bolts. but it isnt pressurized.
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Old 20-05-2013, 18:55   #11
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Re: Designing a hot water storage tank

We found a copper cylindrical tank in a junk yard, plumbed it and insulated it and covered it in fiberglass, and mounted it above our diesel heater with a water heating coils in it, it heated enought water to heat bus heaters thur out our 42 ft boat !! worked well in Alaska, just used a low pressure relief valve to keep things safe! the main thing no matter how ya heat the water! I tryed to figure a way to heat bathing water also but needed heat in the cabins more then the xtra bathing water so just went with the heat set up ! It can be done on the cheap if ya can find a used pressure tank, and ya don't mind a little fab work ! Just my 2 cents
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Old 22-05-2013, 09:27   #12
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Re: Designing a hot water storage tank

Craig (CHEECHAKO), thanks for the reminder of pressure, as I will be going that route. I suspect I will simply purchase a standard unit with stainless tank, junk the exterior and reinsulate with the VIPs. I can simply switch out the heating elements, depending on the availability of shore power, and reconnect the power wiring to the wind charger, should I go in that direction.

I also appreciate sailorchic34's details of designing an integral solar heater into the mix. Hot water is really nice, sometimes, even in the tropics. Thanks to you all.
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Old 22-05-2013, 09:50   #13
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Re: Designing a hot water storage tank

Nothing like a warm water rinse on the stern after a snorkel!
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Old 22-05-2013, 10:20   #14
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Re: Designing a hot water storage tank

Quote:
Originally Posted by smaarch View Post
nice idea
this could be adapted for sailboats as well
RV SOLAR HOT WATER KIT - BHAsolar
I basically did the bimini thing on my previous boat.
It worked well, but being at the high point it needed periodic burping of air.
I prefer the system I have on my tri.
Enough solar to heat water via the inverter.

Here's a link to the previous boat:

Only Solar

Post # 68.
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