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Old 13-04-2019, 14:11   #16
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Re: Hot water tank construction

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Originally Posted by alex49 View Post
One remark !!

You are talking of a plastic tank. Bare in mind that coolant water that will heat up your tank in diesel engines usually runs at almost boiling temp. 90 degree celsius. In my boat tank is made of pure copper.

Alex
Good point Alex, but I am not using my engine to heat this tank. I am using a Webasto Thermotop. I am planning on the same temperature as suggested by ColdEH in his original design, somewhere around 55 to 63 degrees was what he used.

Having said that, I will check if I can observe any softening of the tank with boiling water, just to be safe.

I did look at stainless, but too hard to find something in the correct size/shape. Got lucky with the plastic drum.
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Old 13-04-2019, 14:13   #17
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Re: Hot water tank construction

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If you do go forward.........think a 50 kg drum trying to move around in tune to your sailing movement........yeh suspending and pouring not such a good idea without LOTS of bracing.....otherwise over time it will pound away and crush the foam then want to visit the galley at the wrong time mid passage somewhere.....fuel and water tanks are always strapped in so they don't move.
Good point. I would incorporate plywood braces in the construction. They should not conduct too much heat, which is important.
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Old 13-04-2019, 14:15   #18
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Re: Hot water tank construction

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Originally Posted by Franziska View Post
You can try this one:

HR-AT - Vosschemie

91% closed cell....
Thank you Franziska, that looks like the sort of thing to use. That closed cell percentage seems to be a standard unit of measure with these products. I am looking forward to my favourite supplier opening on Monday to see what they have with that kind of ratio.
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Old 13-04-2019, 18:22   #19
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Re: Hot water tank construction

Hum. Not to rain on the parade, but plastic is really not a good material for hot water tank construction. It's strength is effected by temperature. For example PE or HDPE has melting points of 220 to 244 ish degrees F some a bit higher, but not enough. At 180 degrees F (typical engine heating temp) PE, PVC and CPVC would all fail at 40-50 psig water pressure.


PVC's temperature rating is lowest and is good to only maybe 100 degrees F at 40 psig. CPVC and PE (PEX) are rated to 140 degrees F. Above that temperature, the material will quickly loose strength.



This is why you don't see water heaters made of plastic.
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Old 13-04-2019, 18:47   #20
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Re: Hot water tank construction

Have you looked on Gumtree and Aussie Yachtie Buy Sell and Swap? There have been a few hot water tanks on their lately. One guy on the Aussie site advertised for ages.
Have you looked at an old 50 litre Rheem hot water tank? I know they are 240 volt but at least you have a receptacle that could be modified? Or even better how about a 50 litre keg from the scrapyard? I love this lateral thinking.
I just looked at beer kegs on Gumtree and you can get a stainless steel or aluminium one for $50-$65 here in Brisbane.
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Old 13-04-2019, 19:05   #21
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Re: Hot water tank construction

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Have you looked on Gumtree and Aussie Yachtie Buy Sell and Swap? There have been a few hot water tanks on their lately. One guy on the Aussie site advertised for ages.
Have you looked at an old 50 litre Rheem hot water tank? I know they are 240 volt but at least you have a receptacle that could be modified? Or even better how about a 50 litre keg from the scrapyard? I love this lateral thinking.
I just looked at beer kegs on Gumtree and you can get a stainless steel or aluminium one for $50-$65 here in Brisbane.
Cheers
+1 You beat me to it, I was just about to suggest a keg.
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Old 13-04-2019, 19:26   #22
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Re: Hot water tank construction

OK, the problem with kegs is that the dimensions are not all that suited to my location, also really hard to get the plumbing into the tank. I need to fit a copper coil in there, which means I need a big opening at the top. Any kind of metal tank is going to be a bugger to reweld, because my MIG will blow holes in anything thin.

Also, this tank is not under any pressure (other than that imposed by the kinetic mass of the water) and the temperature is fair way from engine temperatures. (see note above, 55 - 63 degrees C or 130 - 145 F)

I feel like the plastic tank will be ok if it is well supported... I suppose I could go back to my original thought of making a plywood and fibreglass tank... but I will see how the tank itself behaves without support, should get a chance to fire up the boiler this week.

Edit: Maybe the thing to do will be to support the tank with strapping before I pour the foam.... that and the plywood framework should support it well.
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Old 13-04-2019, 19:30   #23
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Re: Hot water tank construction

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Originally Posted by Fore and Aft View Post
Have you looked on Gumtree and Aussie Yachtie Buy Sell and Swap? There have been a few hot water tanks on their lately. One guy on the Aussie site advertised for ages.
Have you looked at an old 50 litre Rheem hot water tank? I know they are 240 volt but at least you have a receptacle that could be modified? Or even better how about a 50 litre keg from the scrapyard? I love this lateral thinking.
I just looked at beer kegs on Gumtree and you can get a stainless steel or aluminium one for $50-$65 here in Brisbane.
Cheers
Looked at all of the commercially available boat hot water tanks as my first port of call. None were remotely suitable to the location or the modifications required.

As for household units, quite frankly, the vitreous enamel crap that Rheem sell is a disgraceful bit of engineering I would never fit in my boat OR house. The glaze craps itself when statically mounted in a house after just a few years, it would not last a week in a boat.
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Old 13-04-2019, 21:06   #24
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Re: Hot water tank construction

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Unfortunately my budget is last millennium too. Sometimes innovation and applied effort have to replace cash.

And I like building things.
That's 2 very good reasons to proceed with the project.

1. If you are using the cooling circuit from the engine to heat the water you are never going to see it boiling in your storage tank so it's probably safe to asume a maximum temp of 100 degrees celcius. At this temperature the tank may soften a little buts that's all.

2. Since you are containing the plastic tank in a plywood box the softened tank is not going to be any problem. Key function "contained".

3. I would not worry to much about water take up, it's a hot water tank being insulated so any slight water take up is more than likely going to be evaporated off anyway.

4. If you are concerned about a square sectioned box being unable to contain the pressure of the foam you can turn it into an octagonal form box by gluing plywood fillets into the corners of the square section, it will then be square on the outside and octagonal on the outside and with the extra support even if the plastic tank gets really hot and goes plastic on you the octagonal form containment will hold it in shape.

5. fresh water generates a pressure of 0.43 psi per foot of depth so 3 feet of depth is going to generate 1.29 psi. Inertial forces in a boat would not exceed say three times that so your looking at about 5.2 psi max on the foam underneath the plastic container. I have not checked but I suspect that polyfoam is good for at least 5.2 psi of compressive strength.

Go to it mate, sounds like you have a good plan.
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Old 13-04-2019, 21:39   #25
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Re: Hot water tank construction

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That's 2 very good reasons to proceed with the project.

Well, sometimes. To be honest, I probably build too much.

1. If you are using the cooling circuit from the engine to heat the water you are never going to see it boiling in your storage tank so it's probably safe to asume a maximum temp of 100 degrees celcius. At this temperature the tank may soften a little buts that's all.

All good, 55 - 63 degrees, though I should double check that with ColdEH, he designed that system a few years back, he may have changed things.

2. Since you are containing the plastic tank in a plywood box the softened tank is not going to be any problem. Key function "contained".

Contained is good.

3. I would not worry to much about water take up, it's a hot water tank being insulated so any slight water take up is more than likely going to be evaporated off anyway.

Yep, a bit of reading on the whole saturation thing and I am feeling pretty relaxed on that point.

4. If you are concerned about a square sectioned box being unable to contain the pressure of the foam you can turn it into an octagonal form box by gluing plywood fillets into the corners of the square section, it will then be square on the outside and octagonal on the outside and with the extra support even if the plastic tank gets really hot and goes plastic on you the octagonal form containment will hold it in shape.

VERY good idea

5. fresh water generates a pressure of 0.43 psi per foot of depth so 3 feet of depth is going to generate 1.29 psi. Inertial forces in a boat would not exceed say three times that so your looking at about 5.2 psi max on the foam underneath the plastic container. I have not checked but I suspect that polyfoam is good for at least 5.2 psi of compressive strength.

AND some maths too. I love maths.

Go to it mate, sounds like you have a good plan.
I hope so, first shower will be the big test, then a winter in Tasmania. If they don't have to come down to the boat in spring to thaw me out of a giant ice block, I will consider the system a success.
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Old 13-04-2019, 21:47   #26
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Re: Hot water tank construction

Schematic, for those that like such things.
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Old 15-04-2019, 00:11   #27
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Re: Hot water tank construction

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Schematic, for those that like such things.
I think you have your red and blue arrows reversed.

Interesting concept.
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Old 15-04-2019, 00:42   #28
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Re: Hot water tank construction

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Schematic, for those that like such things.
Hi Matt, somewhere in Tas there is a business that produces a heating system using a diesel fired heater & engine heating and is built to provide hot water and cabin heat. Probably very exy but may be a source of inspiration.
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Old 15-04-2019, 00:50   #29
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Re: Hot water tank construction

Don't forget a pressure relief valve in case of temp sensor failure. Route overflow down to bilge.
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Old 15-04-2019, 01:23   #30
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Re: Hot water tank construction

There is closed cell pour/spray foam that doesn't absorb water. Fishing boats use it. You can fiberglass or paint it. UV light degrades the surface so coating is needed in direct sunlight. It comes in different expansions depending on need to carry weight. Amazon or ebay are good places to buy smaller amounts.
You don't have to pour all at once. Expansion is powerful and can bend/break things. Wear gloves and old clothes. It's like working with fiberglass.
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