Hot water tank construction

[sailorchic34]

Originally Posted by AndyEss

Boo, hiss.

Here we are merrily brainstorming solutions to the OPs question, and some engineer - a blond at that - slaps us up the head with reality..

It's a very gentle slap...



People always get annoyed when I tell them how much energy it takes to heat water instantly, well more or less instantly. The typical diesel boat heater would be more than adequate for a storage tank type water heater. Do that instantly or say within a minute takes a wee bit more input on the Boiler Side. It can be done. Just not with a typical hydronic heater on a 35' ish boat.



Of course if the o p where to take that plastic tank and put it on deck/ capin top and paint it black, the sun could heat it up. Then all you have to do is add a pump on the discharge to feed domestic hot water side.

[AndyEss]

Originally Posted by sailorchic34

Of course if the o p where to take that plastic tank and put it on deck/ capin top and paint it black, the sun could heat it up. Then all you have to do is add a pump on the discharge to feed domestic hot water side.

And you even come up with a low cost, alternative- excellent.

It might be aesthetically displeasing though. I'm a retired engineer, I no longer only have to think of a client's bottom line, I can indulge in style.

Any solar hot water boost on my boat will be limited to the portable hanging water shower. The first combined solar electric/solar coolant heat panel comes on the market, I'm in for dozens of panels.

I think the OP wants to heat his boat. I have no intention of sailing my boat anywhere it regularly goes below about 60 deg F and might need heat. I grew up in Chicago, and worked off the polar ice pack north of Alaska so I don't plan on sailing too far from the equator.
I did think Franzecka's spiral insertion method for a heat exchanger was very clever. I like solutions that aren't obvious.
Personally, for my hot water I use a tankless propane heater.

[AKA-None]

You could try a 10 gallon air tank, cheap from harbor freight or similar and easily welded

[cj88]

Originally Posted by AndyEss

The first combined solar electric/solar coolant heat panel comes on the market, I'm in for dozens of panels.

Heliatos, Green Yachting and Custom Marine Products all offer this.

[Franziska]

Personally I do favor also an instant water heater driven by Propane, but in Europe they are unfortunately not legal (don't ask me why, the rest of the world seems to have no issue with it).

Second would be something like the Heliatros panels and a circular flow of the heating medium through the standart Quickboat waterboiler we have. As we have no engine which we can hook up to this, we can use the unused connectors for this...

[Tricolor]

Originally Posted by Franziska

Personally I do favor also an instant water heater driven by Propane, but in Europe they are unfortunately not legal (don't ask me why, the rest of the world seems to have no issue with it)...............

Dutch website;
https://www.propaangeiser.nl/nl/prod...ropaan-butaan/

UK website;

https://www.propanegaswaterheaters.c...r-propane-but/
I don't know where you got this wrong info from.

Direct propane/butane water heaters are allowed in the whole of Europe, see the EU Certificates on the website, as they are a closed system, fresh air and exhaust from the oitside, they can be used safe on boats and rv's

[GILow]

Originally Posted by sailorchic34

Well, here I am being a Debby Downer again. Just fyi, I have some experience designing hot water systems to 1200 gpm or so.


Anyway, What the OP is looking at doing is using a 11 gallon tank as a buffer tank, with a copper heating coil and a copper domestic hot water coil.


Lets do a bit of math. Sorry for the imperial units... it's that inbred American thingy.



So lets assume you want to heat one gallon per minute of cold water to hot, say 110f (43c). so 1 gallon x 8.33 (pounds per gallon) X60 degrees (raises 50 degrees F water to 110 degrees F.) = 500 btu per minute. Take that 500 btu x 60 minutes = 30,000 BTU. allow for 80% efficiency in boiler and you need a boiler that produces 37.500 btuh or 10.8kw. This btw is how a formally blonde american engineer might size a heating or domestic hot water boiler.



Of course we can use some of the heat in the 11 gallons. That is the part from 140 degrees F to 110 degrees F (I'm assuming a typical anemic boat shower). So 11x8.33 x30 (140F - 110F) equals 2748 BTU's or enough to heat 5 gallons of water to 110 F ish.



Of course you have to reduce amount of stored water by the area of the two copper loops which will be lots. Probably about 1/3 to 1/2.



I'm betting the OP's unit is maybe 3-4 kw or 10mbh to 13.6 mbh (m for 1000, b for BTU and h for hour). That's about enough for maybe 0.2 gpm which is not a nice shower.



Now if you were using a pressure vessel, you could just take the return heating loop back through the water heater and use 10 degrees F delta to heat the fresh water. It's more or less how the typical marine water heater works. However a plastic pressure vessel as Egon Spengler (ghostbusters) would say, would be bad, as it will not like the 140 degree F working temp and oh say 50 psig working pressure.



So I suspect the OP's system may be less then optimal.

OK, at last we have the resident thermal engineer on board. Hooray.

A few points about your analysis:

1. You seem to be referring to TWO copper coils. "with a copper heating coil and a copper domestic hot water coil." Not the case, there's ONE copper coil to EXTRACT heat from the buffer tank, the coolant in the buffer tank is heated "directly" by the Webasto (5.2 kW for the record.) Does this change your thinking?

2. Point taken about the actual amount of stored heat in this system. Let's look at the numbers... I am nominally working over a 10 degree range between furnace on and off. 37 litres (yes, point noted about the displacement of the copper pipe itself, I have reduced the buffer figure to address this), raised 10 degrees C equates to 1548080000 Joules. A "boat shower" would be, according to my GUESS, around 10 litres of water at about 40 degrees C. Water in the boat fresh water tank is stored at about 10 degrees so I have to raise it 30 degrees which will take 1255200000 Joules. Less than the buffer range, so, in theory, the furnace would not even come on during the shower. As for delivery time for that 10 liters... well, as long as I can make it last. Seriously, this is not a land shower. No 9 litre per minute shower head for me. My shower head is super mean spirited, probably maxes out at around 5 or 6 litres per minute.

3. I concede in all of this that the previous assumption comes down to effectiveness of my copper heat coil to "pick up" heat at an adequate rate. John gives some guidance there, and I will have to pack in a LOT of copper pipe. But I am after an adequate shower, not a "Hollywood" shower.

Back to the thermal engineer... what have I missed?

Matt

[GILow]

Originally Posted by Franziska

Personally I do favor also an instant water heater driven by Propane, but in Europe they are unfortunately not legal (don't ask me why, the rest of the world seems to have no issue with it).

Second would be something like the Heliatros panels and a circular flow of the heating medium through the standart Quickboat waterboiler we have. As we have no engine which we can hook up to this, we can use the unused connectors for this...

Had a propane heater. (well, LPG here in Oz).

I put it in the scrap metal pile. I got a few $ for it. Very happy with the exchange.

I do NOT like more LPG (or propane) on the boat than I absolutely have to. Just too much risk for MY preferences.

Also, bringing diesel to the boat is MUCH easier than gas (propane/lpg). I can store 500 litres of diesel on the boat without a moment of concern, whereas two gas bottles are enough to make my hair turn grey.

My system is designed to be a boat heater as well as a fresh water heater, and depending on where I end up living, the bulk of the heat output may well be used on the heating circuit, not the shower.

[GILow]

Originally Posted by Franziska

I meant you create like a spiral before inserting it and than feed it in the keg in a screwing type motion by rotating the spiral through the hole.

I am trying to picture it... I'll be honest, I am having trouble.

[GILow]

Originally Posted by AndyEss

Instead of MIG welding a new SS or Al top to a beer keg (yep, blowing holes through the thin metal is a problem) why not glue in a plastic or plywood cap?
The adhesive would have to have some compliance to accommodate differing expansion rates of the metal keg and cap, but that shouldn't be too tough to spec. Your buffer tank isn't pressurized, so this bond won't be loaded excessively.

Noted, I will look into this idea if the plastic tank proves problematic, thank you.

[GILow]

Originally Posted by sailorchic34

Well, here I am being a Debby Downer again. Just fyi, I have some experience designing hot water systems to 1200 gpm or so.


Anyway, What the OP is looking at doing is using a 11 gallon tank as a buffer tank, with a copper heating coil and a copper domestic hot water coil.

....

Now if you were using a pressure vessel, you could just take the return heating loop back through the water heater and use 10 degrees F delta to heat the fresh water. It's more or less how the typical marine water heater works. However a plastic pressure vessel as Egon Spengler (ghostbusters) would say, would be bad, as it will not like the 140 degree F working temp and oh say 50 psig working pressure.



So I suspect the OP's system may be less then optimal.

Sorry SC, but I really don't think you are describing my system at all. I have read your post a number of times now, and nothing tallies with what I have drawn.

Did you see my diagram? Where is this second domestic hot water coil? How does the heating loop have anything to do with the water heater?

I am VERY confused now.

[Tricolor]

Originally Posted by GILow

Had a propane heater. (well, LPG here in Oz).

I put it in the scrap metal pile. I got a few $ for it. Very happy with the exchange.

I do NOT like more LPG (or propane) on the boat than I absolutely have to. Just too much risk for MY preferences.

Also, bringing diesel to the boat is MUCH easier than gas (propane/lpg). I can store 500 litres of diesel on the boat without a moment of concern, whereas two gas bottles are enough to make my hair turn grey.

My system is designed to be a boat heater as well as a fresh water heater, and depending on where I end up living, the bulk of the heat output may well be used on the heating circuit, not the shower.

As a marine engineer for over 40 years I hate the use of propane/butane on ships, seen some serious accidents with that. My problem with Franzika's post is that she dumps wrong information here. For producers difficult to fight against, see the Bavaria bashing.

[GILow]

Originally Posted by AKA-None

You could try a 10 gallon air tank, cheap from harbor freight or similar and easily welded

Well, "easily" depends on skill and equipment. But yes, an air tank is not too hard on the budget. One more for the backup options if the plastic tank does not cope.

[GILow]

Originally Posted by Tricolor

As a marine engineer for over 40 years I hate the use of propane/butane on ships, seen some serious accidents with that. My problem with Franzika's post is that she dumps wrong information here. For producers difficult to fight against, see the Bavaria bashing.

Yes, important that we stick to facts where possible. Sometimes we get it wrong, but that's why we have Cruisers Forum, where there are lots of people to help point out our mistakes.

The subjective stuff, like risk aversion, is down to personal taste. I decided I would sleep better with less gas on the boat. I've still got a gas powered stove but it is coupled with two independent gas detectors, each with two sensors, a remote control shutoff valve connected to one of the detectors, and a timer circuit that turns the gas on for a maximum of two hours while I am cooking before the solenoid shuts off again.

As you can imagine, such a setup is NOT suitable for boat heating. Who wants to get up every two hours on a cold night to reset the gas circuit?

[GILow]

Originally Posted by sailorchic34

Of course if the o p where to take that plastic tank and put it on deck/ capin top and paint it black, the sun could heat it up. Then all you have to do is add a pump on the discharge to feed domestic hot water side.

Try that in Tasmania in the middle of winter and let me know how it works for you.