# Underground Hot Water Storage Tank Question



## Eric Johnson (Jan 12, 2008)

This came up in another thread, but in case they don't see it, I was wondering if some of our engineering types would care to answer a question I have about creating a pressurized, underground storage tank.

Would it be possible to pressurize an underground storage tank, say a 2,000 gallon steel or fiberglass fuel tank? Would the support of the ground, in other words, allow you to pressurize such a tank to more than 30 psi?

I'm thinking that it would be pretty easy to dig a pit, line it with plastic or some other waterproof liner, place the tank in the pit, get it all plumbed up, and then spray a thick layer of foam around it. Could you engineer it so that the foam would provide sufficient support for the pressure? That sure would simplify things and allow you to get a large amount of pressurized, virtually invisible storage for pretty cheap.


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## wdc1160 (Jan 12, 2008)

Eric I am constantly surprised what they can do with some of the fiberglasses they put in the ground.  Indiana has several companies now that put fiberglass pools directly into the ground in northern indiana.  Ground heaving and temperature variations seem to have little effect and the pool comes with a "20 year no cracks" policy.

I am not sure what the ideal setup would be, but I think the materials could withstand the pounding.


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## kuribo (Jan 12, 2008)

Stresses will be high around fittings. That would most likely be where you would see failure....no real way to strengthen those areas....


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## wdc1160 (Jan 12, 2008)

In indiana we believe that duct tape with strengthen anything....
Even a car........


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## jebatty (Jan 12, 2008)

Almost anything, including an egg, will take pressure with an equal opposing force. The issue will be ground movement as the tank expands. Also, not all fiberglass can take the temps needed for heat storage. Be sure to look at fiberglass rated for the heat of the liquid you intend to store.


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## Eric Johnson (Jan 12, 2008)

Are you talking about a plastic/fiberglass tank or a steel tank, or both? What would a failure in a steel tank look like? I was under the impression that the biggest problem with pressurizing big tanks comes across long spans of unsupported steel--like the tank bulges until it splits open or the welds fail. Would there really be enough force at the fitting at 15 or 20 psi to blow out the weld? I'm just trying to understand what we're up against here.

On the question of plastic or fiberglass taking the heat, I'm wondering if you could push the envelope a bit considering that the thing isn't going to collapse or blow out, since it can't physically do either. I'd hate to find out the hard way that I was wrong, but it's another point to consider.

And I assume that in most jurisdictions, none of this would be up to code.


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## Reggie Dunlap (Jan 12, 2008)

Sounds like a big propane tank might be a good option. I don't know how well foam would hold up in contact with wet soil and ground water. I insulate a lot of houses with the closed cell foam and it works great for that application. It might be simpler to build a shed around the tank and not put it in the ground. Another thing is that foam is expensive enough that you want to do it right the first time.


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## kuribo (Jan 12, 2008)

I assumed you were talking about a fiberglass, plastic, or other non-rated tank used as a pressure vessel....

Overheating will shorten the life of the tank, which could be drastic at high temps....


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## Eric Johnson (Jan 12, 2008)

I was, but I was also thinking about steel.

This is just idle curiosity on my part. I'm just wondering how practical it would be.

I guess what I'm really talking about now is something like an underground fuel storage tank or a stainless steel milk tank--a big old cylindrical steel tank that wouldn't hold pressure above ground, but ought to (it seems to me) if it were buried. You need the foam to insulate it, I think, but perhaps it would also create a more consistent material for the tank to push against, compared to sand or gravel or some other backfill. Maybe vermiculite or perlite could be used instead, but that stuff is probably a lot more expensive than foam.

Does that sound like a reasonable idea?


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## wdc1160 (Jan 12, 2008)

kuribo what do you mean non-rated?


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## kuribo (Jan 12, 2008)

Tanks are rated for pressure or not. A non-rated tank is one that has not been certified/tested for use as a pressure vessel and is not meant to be used as a pressure vessel. 

If a tank, plastic, fiberglass, steel, or otherwise, is not meant for pressurized usage, you are only asking for trouble if you pressurize it. As I mentioned, the areas around the fittings are stress risers and will be prone to failure. I would simply use a tank meant to be pressurized, i.e., a propane tank, or go non-pressurized....


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## wdc1160 (Jan 12, 2008)

Do you think a pressure rated firberglass tank exists?  Or, that fiberglass has a properties to even have this type of purpose?


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## kuribo (Jan 12, 2008)

Yes, fiberglass pressure tanks are made. Wel-X-trol makes well pressure tanks out of fiberglass....I think a 120 gallon unit retails around $800.....


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## atlarge54 (Jan 12, 2008)

I seriously doubt spun glass tanks could withstand the combination of high temp.  with the pressure. I don't have any data to back this up but I'd be willing wager on the side of failure.


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## kuribo (Jan 12, 2008)

You are most likely correct....If you add high temp service to the pressure requirement, it may be too much to ask for...In any case, the cost would be much greater than the old propane tank.....


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## Bartman (Jan 12, 2008)

Depends on what resin was used for the construction of the tank. Polyester resins are the cheapest and most common, epoxy resins are the strongest and most expensive. It's not so much the glass that's the problem, the resin that binds the glass has to give and take the heat. Spun fiberglass tanks are very strong because the glass is in different directions like biaxial glass mat which is considered the strongest of the fabrics. 

When it comes to local codes for underground tanks, I would guess that there are not many codes for water storage. Fuel, and septic, there would be, but water isn't really considered hazardous.


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## hkobus (Jan 12, 2008)

Eric,

Why would you want to pressurize the tank. I have still not made up my mind on the storage I need, but with the cost of copper and plate HX I am thinking more and more about circulating the same water from the boiler to the tank and trough the heat load. This would increase the water amount circulating in the system from 50 to 100 gal, to potentially 1000ths of gallons! 
I do believe I can feed the tank at the top and return to the boiler from the bottom and use water from the top and return to the bottom or the middle. I would build a diffuser to reduce the speed of the charge and load return water, much like in a DHW tank.
When using this amount of water in the system, it would make for a long time to eradicate oxygen from the system, even when presurized. The OWB dealers sell conditioners to buffer their open systems and protect them from corrosion. 
Now my thoughts are going more this direction and making an open system with the whole storage volume able to circulate through the boiler. Just for the DHW supply I would use HX, side arm or other.
If you can tell me what you use to make your great drawings, I will draw my idea and see what everyone thinks. But I think you will grasp the concept, and maybe there is a good reason this is not done much.
With all the water storage talk you can almost split the forum...  

Henk.


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## Eric Johnson (Jan 13, 2008)

So what you want to do is convert your whole system to an open one, is that right, Henk?

You might have a problem if you have to pump water up to a second story, but I'm not sure about that. I think my dad does that with his OWB, and it works OK.

I've always run pressurized system, which is why I asked about pressurizing the tank. 

I use Adobe InDesign, which is a desktop publishing program to do my drawings, then I convert it to a pdf, open it in Photoshop and convert that into a jpeg. It's not the most efficient or sophisticated way to make a drawing, but I use those programs all the time at work, so it's second nature. A more direct approach would be to use a drawing program. I have Adobe Illustrator for that, but I hate it, mainly because I've never taken the time to learn how to use it. My favorite drawing program is Corel Draw 2.0, but I cooked my operating system trying to install it on XP.

You might be able to make decent drawing in Word.


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## hkobus (Jan 13, 2008)

Eric Johnson said:
			
		

> So what you want to do is convert your whole system to an open one, is that right, Henk?
> 
> You might have a problem if you have to pump water up to a second story, but I'm not sure about that. I think my dad does that with his OWB, and it works OK.



Yes, I have been thinking about that, and the point you brought up. The second floor is heat deprived at this time, but plans are on the table for in floor now. I was thinking radiators, but with an eye on efficiency, I think in floor would be better and make better use of stored heat. I also plan to run higher temp downstairs and lower upstairs. With downstairs return in the middle and upstairs in the bottom of the storage.

I'm sure that when the return is submerged in the tank and the same with the draw end, it should not drain, as no air can enter the system. Different story when air enters from a leak in the system.. :bug: Some head space in the tank may prevent spills in the basement for an emergency.

I also plan to draw the supply for the floor heat through a Nat gas fired DHW tank as backup heater, that way when the tank is set for the min. required temp for the in floor, it will kick in on it own. This way the storage will frost protect the boiler and supply lines when fitted with a timer and frost sensor in the boiler room.

Henk.


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## antknee2 (Jan 13, 2008)

I was checking out some solar hot water equipment and stumbled on this fiberglass tank , wonder if can handle pressure .Anthony 

http://kingsolar.com/catalog/mfg/other/desertsun750g.html


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## EForest (Jan 13, 2008)

the underground storage idea has been on my mind for weeks.
What if you pour a concrete tank, skim the walls for a smooth finish, then line it with .060 epdm for ground water barrier, then 2-3" of foam insulation on walls and floor, another layer of epdm for storage, then seal the deal with an epdm lid covered with @ 5-6" of foam, finally pour concrete (or frame lid) over the tank and run it as an open system with 4 way valve ( for flow reversal) and flat plate HX with bronxe pump. Share your thoughts good or bad....
thanks'
Ed


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## trehugr (Jan 13, 2008)

gasifierwanabee said:
			
		

> the underground storage idea has been on my mind for weeks.
> What if you pour a concrete tank, skim the walls for a smooth finish, then line it with .060 epdm for ground water barrier, then 2-3" of foam insulation on walls and floor, another layer of epdm for storage, then seal the deal with an epdm lid covered with @ 5-6" of foam, finally pour concrete (or frame lid) over the tank and run it as an open system with 4 way valve ( for flow reversal) and flat plate HX with bronxe pump. Share your thoughts good or bad....
> thanks'
> Ed



Im haunted by the same questions gasifirewanabee. My latest obsession is using the existing 2 walls of the foundation, adding 2 more walls, and possibly building a tank inside the new foundation walls so I can insulate between the walls and the tank. 

My biggest problem is how to:

1. make the lid 
2. access the lid and HX
3. cover up the whole works so it looks like it did before I started.


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## kuribo (Jan 13, 2008)

excavate that area, drop in a concrete septic tank....no cold joints.....


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## trehugr (Jan 13, 2008)

kuribo said:
			
		

> excavate that area, drop in a concrete septic tank....no cold joints.....



What is a cold joint ?


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## kuribo (Jan 13, 2008)

the interface between new and old concrete......not a lot of strength......


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## Nofossil (Jan 13, 2008)

My own take on this is that pressurized storage is troublesome as the volume increases. My thoughts on pros and cons:

Advantages of pressurized storage:

 - one less heat exchanger and/or pump
 - no 'delta T' loss - you get the hottest possible water

Disadvantages of pressurized storage:

 - heavier / more expensive vessel
 - more leak possibilities
 - need BIG expansion tank
 - more dissolved oxygen to eat away at steel components

Disadvantages of unpressurized total system:

 - dramatically increased corrosion of steel components
 - need water treatment
 - harder to flush air bubbles out of zones

There are clearly many designs that work, and work well. My current thinking is that the ideal is something like this:

 - Pressurized / sealed system for boiler and zones
 - Large (1000++ gallons) unpressurized storage
 - Finned stainless HX coil(s) in storage tank, or alternately a bronze pump and plate HX

Because of my background in aerospace engineering, I like systems where any component 'fails safe' and can be isolated from the rest of the system quickly and without disabling the rest of the system. That's one of the reasons that I don't like connecting wood and oil boilers in series. By the same token, I don't want a leak in a large pressurized underground tank to prevent my heating system from working. I'd like to keep the boiler's vital working fluid in as small and simple a set of plumbing as possible.

Just my thinking today. May be different tomorrow.


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## wdc1160 (Jan 13, 2008)

No doubt- the ideal of using the already formed two walls actually complicates things.


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## EForest (Jan 13, 2008)

my tank will go inside the barn foundation (poured last November) below the slab.
the first layer of rubber will protect from ground water.
then lots of insulation.
then single piece of rubber folded at corners (dog eared)
treated plywood cap wrapped in rubber and fastened over tank (this is the tricky part).
my lid will be @ 8x12 so i'll need three sheets fastened together with treated 2x.
the 2x's will have ancor bolts with the L up toward the sky.
temp support below is a must.
then cover all that with more inul with bolts extending @ 2" above.
now a layer of 6 mil poly over the thing and I can pour my barn slab!
I almost forgot, there will be a 2'x2' manway and numerous sleeves for piping.
The tank will pump through a flat plate at boiler but on the way will go through a 4 way mixing valve
on supply and return side so the flow can be reversed depending on charge or draw.


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## Bob Rohr (Jan 13, 2008)

what a site that would be if it ever did rupture :0  

Steel tanks need to be fairly thick or have rods welded across to make them pressure vessels.  Underground LP tanks are common in my area, be sure to provide anode protection, however.

Insulating underground tanks is a big challange.  I have used concrete septic tanks for underground solar storage.  Easier to insulate a square tank, although not pressurized.

Even so, wet ground or water around even a foam lined tank would suck heat like crazy.  I also worry about termites and other bugs eating the foam insuation.  I know they go after foamboard in my area, I have a few samples of their work!

That warm insulation would seem to be a huge welcome sign for critters of all sorts.

Again, store the energy in the pile of wood, use as needed.  If you must "tank", do it inside where any loss is in the heated envelop.

 hr


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## drizler (Jan 13, 2008)

I don't know how relevant it is but a few years back I was kicking back at work and found a GSA pamphlet on buried fuel tanks and corrosion.    It was pretty comprehensive and got into the issues of electrolysis, soils, moisture and how most underground tanks fail.   A lot has to do with moisture and soil makeup.  For some reason tanks tend to fail at the bottom lowest portion away from the filler.   It acts like a crude electroplating device and it slowly pulls molecules from that distant place  until it makes a hole.    Apparently it doesn't have any effect on any other point but that one tiny spot.   Don't ask me to recite any references here I just read through it.  Anyways it gave a whole lot of reasons why you don't want your tank underground unless its your only choice.    I can see nowdays why the giant underground tanks are all coated in spun fiberglass so they don't touch the soil.      Food for thought anyways.


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## EForest (Jan 13, 2008)

master of sparks said:
			
		

> what a site that would be if it ever did rupture :0
> 
> Steel tanks need to be fairly thick or have rods welded across to make them pressure vessels.  Underground LP tanks are common in my area, be sure to provide anode protection, however.
> 
> ...



I realize termites can squeeze through tiny openings but never heard of one eating through epdm.
this tank will be non pressurized with 8" concrete walls, 2" concrete floor, and 6" concrete lid.
Then two layers of 060 EPDM with insul between layers.
How in the world could it rupture!
and how would other critters make it thru all that crete.
also my design is within the structure so heat loss is really a gain.
But i don't believe there will be much if any.
Please enlighten me. I only want to do this once.
Not trying to hack the thread but now you've got my attention. :roll:


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## Bartman (Jan 13, 2008)

Direct buried steel tanks are just painted with a heavy enamel paint, and have a sacrificial anode attached and strapped to one side. When I installed my 300 gal 22 yrs ago, it was $450, now they're about $1100. Back the a fiberglass was $750 for the equivalent size, now the're $900. When I buried mine I checked the tank carefully and any scratches in the paint were repainted with asphaltum paint. A few years ago I have to dig around the top of the tank for "non-tank" reasons, and it looked like the day it was installed.


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## Bob Rohr (Jan 13, 2008)

The tank I worked with, actually not my design I got involved part way through.

It was buried outside just behind an 8 panel solar array. The tank had 2" of EPS foam on the outside.  Then 2" of foam inside the tank with the EPDM liner inside the foam.  The copper coils were added, then the lid set in place.

I suspect, with the presence of moisture, some insects will either eat, or bore and nest in the foam.  It could be treated with borate for a safe termicide.  Borate doesn't last and needs to be re-treated occasionally, especially outdoors like that.

Your plan sounds fine to me.  Maybe you don't have insects in that neck of the woods.  Inside tanks, or below your slab makes a lot more sense.

 hr


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## TCaldwell (Jan 14, 2008)

Ed, why do you want this storage underground, you as a contractor, myself as a excavator know the consequence if  there is any failure a repair would be a embarising compromise at best. I know tank companys that will cast or core ports to your specs for piping needs, do you have enough room to insulate one inside the controlled enviornment of the barn?


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## EForest (Jan 14, 2008)

Tom,
after visiting your Garn I was ready to buy one (and I still could), then I realized how precious space becomes after a building is complete. That's why I'm considering underground storage and a smaller boiler. Maybe it's a pipe dream but I've got a couple of months to play with the idea before the ground thaws. Do you really think my design will fail?? Rubber is cheap so I could add another layer for insurance...  What would you do if a customer was hell bent on this concept. 

By the way, your set up has been an inspiration on many aspects of my overall plan. Thanks again for the tour.

Also, I'd prefer a boiler that can burn anything from big green chunks to old rafters full of nails, or seasoned cordwood.


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## TCaldwell (Jan 14, 2008)

Ed this is going to sound contradictory but i know the underground placement can work, your worst problem aside from a leak would be seasonal groundwater, with a little gravel,elgen drain and if no daylight a sump pump  you would have a dry tank exterior that you could insulate below grade. If you dont have a way to isolate the tank from ground water and have a failsafe way remove the water it wont work. IN the septic world this works., unfortunately when this time of year through the early spring (when you need the heat the most) is the time when the groundwater levels have the potential to be their highest.  Thats my concern. As far as the lid you can get one poured (a standard 4'' thick top) with for instance a 3'x4' cut out that you could easily fabricate a lid for and have plenty of access into the tank,just some thoughts,


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## jebatty (Jan 14, 2008)

This discussion just gets better. As I now have inside steel tank non-presssurized storage, and almost all of the heat for my shop is by direct radiation from the steel tank, and the boiler is in the shop so all stray heat from the boiler also stays inside, overall efficiency couldn't really be better. I'm re-thinking underground, as main reason for that was saving space inside. A wood or concrete EPDM lined tank of good size is relatively inexpensive to build and maintain, is compact, has no corrosion.

What are the down sides of EPDM? Will it hold up at 180F temps? Is there a better liner product for high temps?

Maybe keep the inside concept with a "tank room" addition, well-insulated, with the heat in the tank room circulated into the primary heat space by convection or small blower. This actually might be less expensive than burial, underground plumbing lines, etc. Plus service as needed would be easy.


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## Eric Johnson (Jan 14, 2008)

45 mil Firestone EPDM is rated for 180 degrees, but I've heard reports of it seeing temps approaching 200 on occasion with no problems, though you would probably want to keep that sort of a thing to a minimum. They say it should last 10 years or more. Think of the abuse that EPDM roofing is subjected to, and you get some idea of what it is designed to take. Trying to line a rectangular tank with a rectangular liner is an exercise in frustration, especially if you like neat. I still don't have mine in right.


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## Nofossil (Jan 14, 2008)

My original house design had essentially an inground pool under the basement floor. The idea was to have massive storage where the heat loss wouldn't be lost to the outside.

Cash flow at the time and worries about humidity control caused my to ditch the idea. I still think it would have been good.

Part of the idea was to have insulating partitions so that you could draw all the usable heat out of one section at a time, then chill it as cold as possible to serve as a heat sink during the summer for cooling purposes. Over the course of the summer, you would heat each section in turn by solar and heat from cooling the house. Maybe next house I'll try it.


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