Hot water tank construction

[GILow]

Originally Posted by sailorchic34

Ah, The pump will develop head probably on the order of 10-12 feet of head or 4.3 to 5.2 psig. This is required to overcome the friction loss in piping and heating coils, etc.

So if the break tank is connected on the discharge side of the pump then the water could exit the atmospheric vent (assuming said vent was less then 10 feet above the pump.

If the break tank was on the suction side of the pump, it might work, specially if a balance valve was added to eat the latent head, but might still be pressurized a pound or two.

Most land side hydronic systems are pressurized to 5-15 psig. You really don't want to operate the pump in the NPSH range (below zero gauge pressure) without doing calculations and checking pump curves to make sure it will work.

I've know professional engineers ( I was one too) who have missed that tiny detail on 30 million dollar systems, when sizing condenser water pumps that can and do operate below zero pressure gauge at the suction inlet.

They told me I did not know what I was talking about, when I pointed out that the pumps would be operating below their required NPSH. I was reviewing their plans as the owners rep. They built it and lo it operated exactly where I said it would and captivated all to hell. This was four 7500 gpm 900 rpm pumps. When I was asked how do we fix it, I said it could not be fixed, as it was a systemic error. I said they could raise the cooling towers ten feet higher (they were already mounted 8 feet above the ground. Alas the pump volute was 5 feet high, so they only had 3 feet of vertical head.. Fun times.

SC, with all due respect, I think you attributing some power figures here that don’t apply.

Typhoon remarked on the somewhat weak circulation pump on the Webasto in his thread and cautioned against using it to even attempt to drive the boat heating circuit.

Have a look on YouTube and there are plenty of videos that give an idea of the flow rate and it’s not great. The head it can produce looks to be maybe two or three feet.

I defer to your knowledge of such systems but I really cannot begin to imagine some of the figures and behaviours you describe.

So I concede, I have been well and truly warned by a pro and I proceed at my own risk.

I WILL report back and, as those that know with me and work with me will tell you (as will a trawl of my CF posts) I have no problems with admitting my mistakes.

[Simi 60]

Without reading the whole thread why not a bog standard off the shelf domestic hot water system tank without the power connected?
Already insulated and made for the job.

50 litre
https://www.bunnings.com.au/dux-prof...eater_p5104142

25 litre
https://www.bunnings.com.au/dux-prof...eater_p5104138

[Uncle Bob]

Originally Posted by sailorchic34

Sorry, the domestic hot water is the cold water coil from your fresh water tank that heats the domestic/potable water for cooking/ showering. I assumed you would add a second hydronic loop as your break tank is at atmospheric pressure and your hydronic loop will be pressurized just a bit. If nothing else from expanded water and perhaps pump head.

You really don't want to pressurize that plastic tank at all. Why. Well lets have fun. Lets assume the tank is 12" in diameter and is pressurized to 4 PSI (pounds per square inch). This either from pump head pressure or thermal expansion.

So the area of the cap is 113 square inches (6x6xpi). 113 sq inch x 4 psi equals 452 pounds of force trying to push the cap off. Plastic is weak structurally and gets weaker the more it's heated. This is why we don't make pressure vessels from plastic.

Fun fact: steel looses up to 80 percent of it's ultimate strength at 600 degrees F.

That is why I assumed there was a second loop...

Anyway, the break tank is just a wide spot in the piping with a bit of thermal mass.

Your basically heating the potable shower water in a more of less instantaneous way. That is energy intensive. So a 5.2kw heater is equal to 17,742 BTUH or 17.7MBH. (3412 btu's per KW) So your heater puts out 295 btu's per minute. That would heat 0.59 gpm or roughly 2 lpm

Now as I said the tank does offer some storage. Lets assume 9 gallons allows 2 gallons for the copper coil for heating domestic water. So 9 gallons x 8.33 lb's per gal x 30 F (delta between stored water temp at 140f and hot water design temp of 110F). So that gives 2,249 btu's.

Lets assume 4 lpm of hot water for the shower. That's 1.06 gpm. so heating 4 lpm takes 529 btuh (BTU's is a much easier way then Joules which makes huge numbers). So your buffer tank in theory can provide about 4 minutes of hot water.

I say in theory, as in reality it will be less. This as the temperature in the buffer tank approaches the water temp in the pipe the heat transfer rate slows down. LOTS. So lets say that you get 3 minutes of hot water. perhaps a bit less. Still not a bad sailor shower. This assumes you have enough copper surface area to get the heat transfer dialed in.

Now the normal marine water heater, puts the domestic water / fluid in the tank and the hydronic loop in the coil. In this way a 11 gallon tank will give about 9 gallons of hot water, before the entering cold water cools the tank too much.

That is a much better way of doing it, specially when the boiler is too small to provide instant hot water . It just can't be done in plastic as plastic makes a very poor pressure vessel. You will get a much better system overall by using a metal pressure tank.

BTW when I was designing that 1200 GPM instant domestic hot water system, it was fed with 125 lb steam from multiple 700 BHP steam boilers. That was for the Venetian in Las Vegas. That was the biggest system I have designed. I suspect it's the biggest anyone has designed that's not industrial.

SC, the heater Matt is describing is not dissimilar to the now defunct Woody's hot water system that used to be available here in Aus, albeit with the added capability provided by the webasto heater. http://www.marinedirect.com.au/infor...y/brochure.pdf
This is the HWS fitted to my boat and I can assure that it is very capable of providing hot water for multiple showers (short of course but not pittifully so) in a 24 hour period. The tank is not pressurised and is open to atmosphere, the domestic coil within is pressurised.
Cheers.

[sailorchic34]

Originally Posted by GILow

SC, with all due respect, I think you attributing some power figures here that don’t apply.

I checked the Webasto pump curves and the smallest Webasto 50hz pumps (wb350 or wb500) develop heads of 3 to 3.8 meters of head. That's 10 to 12 feet of head. Which is oddly right where I was coming from.

You really only need a small flow rate of about 1.5 gpm for the 4.2 kw heater and 10c delta, so with 13-15mm pipe, a small wb350 pump, should do it proud. I would recommend putting the break tank on the suction side of the pump, though, for reasons previously stated.

I wish you well with your experiment.

[sailorchic34]

Originally Posted by Uncle Bob

SC, the heater Matt is describing is not dissimilar to the now defunct Woody's hot water system that used to be available here in Aus, albeit with the added capability provided by the webasto heater. http://www.marinedirect.com.au/infor...y/brochure.pdf
This is the HWS fitted to my boat and I can assure that it is very capable of providing hot water for multiple showers (short of course but not pittifully so) in a 24 hour period. The tank is not pressurised and is open to atmosphere, the domestic coil within is pressurised.
Cheers.

Yes they work well. But the unit you have has a copper tank and is fed from the engine which has a much higher heat output, then the tiny 4.2 kw of the Webasto. Plus the copper tank will not soften with heat as thermal plastic will.

Yes a 4.2 kw unit can heat a tank of water, but that is with a properly sized heat exchanger as well. The Webasto unit does not have the same capacity that a engine cooling loop will have. Engine loops tend to be more in the range of 15kw (3 cylinder), more for larger engines.

It's not that the system will not make hot water. It's the amount it will make and the inherent issues with plastic weakening with temperature.

[Uncle Bob]

Originally Posted by sailorchic34

Yes they work well. But the unit you have has a copper tank and is fed from the engine which has a much higher heat output, then the tiny 4.2 kw of the Webasto. Plus the copper tank will not soften with heat as thermal plastic will.

Yes a 4.2 kw unit can heat a tank of water, but that is with a properly sized heat exchanger as well. The Webasto unit does not have the same capacity that a engine cooling loop will have. Engine loops tend to be more in the range of 15kw (3 cylinder), more for larger engines.

It's not that the system will not make hot water. It's the amount it will make and the inherent issues with plastic weakening with temperature.

Thanks, I have to agree that plastic is perhaps not the best choice, but I would have thought that once the water within was at temperature the performance would be similar, with the webasto possibly in front due to it's ongoing operation versus the onetime engine operation. Guess I have a bit to learn.

[GILow]

Originally Posted by Simi 60

Without reading the whole thread why not a bog standard off the shelf domestic hot water system tank without the power connected?
Already insulated and made for the job.

50 litre
https://www.bunnings.com.au/dux-prof...eater_p5104142

25 litre
https://www.bunnings.com.au/dux-prof...eater_p5104138

Which is kinda why you have to read the whole thread.

They are utterly and totally useless for the job.

But that's ok, because I went with your watching videos on a boat suggestion from another thread and it's a winner. So you are still ahead on points.

[GILow]

Originally Posted by sailorchic34

I checked the Webasto pump curves and the smallest Webasto 50hz pumps (wb350 or wb500) develop heads of 3 to 3.8 meters of head. That's 10 to 12 feet of head. Which is oddly right where I was coming from.

You really only need a small flow rate of about 1.5 gpm for the 4.2 kw heater and 10c delta, so with 13-15mm pipe, a small wb350 pump, should do it proud. I would recommend putting the break tank on the suction side of the pump, though, for reasons previously stated.

I wish you well with your experiment.

WTF? Why the heck are you talking about 240 volt, 50Hz AC pumps all of a sudden? This is a 12 volt DC furnace with a built in pump.

Sorry, but you seem so keen to make a point you are missing the point.

I keep giving you the credit of understanding this better than I do, but every time you post I end up wasting half an hour trying to understand what you are saying (because I respect your knowledge), only to discover you are talking about something totally unrelated.

Please, enough. I respect your professional expertise, but you simply have NOT understood what I am doing here. I don't think there is any further value in your contributions.

[GILow]

Originally Posted by Uncle Bob

Thanks, I have to agree that plastic is perhaps not the best choice, but I would have thought that once the water within was at temperature the performance would be similar, with the webasto possibly in front due to it's ongoing operation versus the onetime engine operation. Guess I have a bit to learn.

Sorry to disrespect SC here, but Uncle Bob, SC is simply NOT talking about the same system I am talking about.

What I have is a tank that will hold treated coolant that will never get hotter than 65 degrees C, that is NOT pressurised and should rarely go below 55 degrees C unless the system has been shut down. (unlikely since I expect it to be running 24/7 while I live aboard)

Plastic is NOT the best choice, but it is a pragmatic and budget friendly choice that MIGHT work ok. Time will tell. My labour comes free of charge, the tank was only a few Australian Pesos so it's a not a big risk. If the tank does not cope I will suck up my pride and go and fork out $$$ for a real tank, or build one out of fibreglass, which is probably my best bet anyway, but I am over fibreglass at the moment, having just finished replacing my entire deck.

As for the other stuff SC is posting about pumps, and pressures and double coils and whatever, it has no relevance to my (Typhoon's) design. The maths may be sound, but that have no real value as they relate to other systems. Very distracting really.

[GILow]

Originally Posted by Uncle Bob

SC, the heater Matt is describing is not dissimilar to the now defunct Woody's hot water system that used to be available here in Aus, albeit with the added capability provided by the webasto heater. http://www.marinedirect.com.au/infor...y/brochure.pdf
This is the HWS fitted to my boat and I can assure that it is very capable of providing hot water for multiple showers (short of course but not pittifully so) in a 24 hour period. The tank is not pressurised and is open to atmosphere, the domestic coil within is pressurised.
Cheers.

Thanks UB, I just had a look at that link you posted.

Yes, this is a VERY similar design, but the major difference is that I heat the buffer liquid (coolant) directly using a furnace, rather than through a heat exchanger full of engine coolant.

I assume they have added the additional heat exchange because they really cannot predict the operating parameters of the connected engine. In my case, I would be able to directly plumb my tank into the engine coolant system if I wanted to, but I would, of course, then have to cope with a bit of pressurisation.

But I am not even going to consider tapping heat from the engine for two reasons.

The first is that I hardly EVER run the engine. Mostly I use it to get out of and back into the marina, at the other end of the journey I use it to set the anchor. Since I will be leaving the marina in about 226 days, even that bit of the engine use will stop. I have been fortunate enough to be able to sail off my anchor nearly all of the time here in South Australia, not sure how practical that will be in other bits of Oz and the South Pacific.

The second is that the furnace should be a more efficient way of extracting heat from diesel, so why run a very noisy engine when I have a practically silent furnace ready to go?

If I find myself using marinas more than I anticipate, I would consider adding an electric heater element like the Woody, that seems a smart addition, and very easy to add.

Edit: I note that the electric heater elements for the Woody are available with settings from 66 degrees C to 99 degrees C. This means the Woody was designed to run at least a little to quite a bit hotter than Typhoon's design. I can imagine this would provide better showers than the lower tank temperature for a start, but I would be most keen to know more about the construction of the house hot water heat exchanger. I am guessing they would have gone for a plate construction. I have considered soldering some copper strap to my coil to see if that improves heat flow... not hard to do, and doubles as a strengthening web.

[JPA Cate]

As to what Franziska was on about the coil, imagine a giant slinky made from copper tubing. Whatever diameter you imagine it (9mm, inside diameter, maybe), that 9.01 mm hole is all it would take, but a lot of screwing, to get it in there. It would not accommodate the aforementioned webbing very well, though.

Ann

[Uncle Bob]

Originally Posted by GILow

Thanks UB, I just had a look at that link you posted.

Yes, this is a VERY similar design, but the major difference is that I heat the buffer liquid (coolant) directly using a furnace, rather than through a heat exchanger full of engine coolant.

I assume they have added the additional heat exchange because they really cannot predict the operating parameters of the connected engine. In my case, I would be able to directly plumb my tank into the engine coolant system if I wanted to, but I would, of course, then have to cope with a bit of pressurisation.

But I am not even going to consider tapping heat from the engine for two reasons.

The first is that I hardly EVER run the engine. Mostly I use it to get out of and back into the marina, at the other end of the journey I use it to set the anchor. Since I will be leaving the marina in about 226 days, even that bit of the engine use will stop. I have been fortunate enough to be able to sail off my anchor nearly all of the time here in South Australia, not sure how practical that will be in other bits of Oz and the South Pacific.

The second is that the furnace should be a more efficient way of extracting heat from diesel, so why run a very noisy engine when I have a practically silent furnace ready to go?

If I find myself using marinas more than I anticipate, I would consider adding an electric heater element like the Woody, that seems a smart addition, and very easy to add.

Edit: I note that the electric heater elements for the Woody are available with settings from 66 degrees C to 99 degrees C. This means the Woody was designed to run at least a little to quite a bit hotter than Typhoon's design. I can imagine this would provide better showers than the lower tank temperature for a start, but I would be most keen to know more about the construction of the house hot water heat exchanger. I am guessing they would have gone for a plate construction. I have considered soldering some copper strap to my coil to see if that improves heat flow... not hard to do, and doubles as a strengthening web.

Hi, the woody's is a great little hot water system and keeps the water hot for around 24 hours without further engine time. From what I understand your proposal will be similar in performance once the coolant is hot and probably will be very economical when used for cabin heating. The plastic tank would be a good candidate for wrapping in fibreglass prior to installing in the box to greatly enhance strength and possibly heat retention. It would not suffer heat induced deformation and if the plastic degrades you would still have a quality tank in place.
All the best with it, I think you have it fairly well sorted.

[GILow]

Originally Posted by Uncle Bob

... The plastic tank would be a good candidate for wrapping in fibreglass prior to installing in the box to greatly enhance strength and possibly heat retention. It would not suffer heat induced deformation and if the plastic degrades you would still have a quality tank in place.

Excellent idea! I will get over my current grudge against fiberglass and do it!

[Uncle Bob]

Originally Posted by GILow

Excellent idea! I will get over my current grudge against fiberglass and do it!

6 to 10 mm would give you a strong and resilient tank.

[GILow]

Originally Posted by Uncle Bob

6 to 10 mm would give you a strong and resilient tank.

Well, yes, I suppose then I could just remove the original plastic tank....

Actually, using the plastic tank as a mold is kinda logical...