Underground Hot Water Storage Tank Question

[Nofossil]

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.

 

[wdc1160]

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

 

[EForest]

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.

 

[Bob Rohr]

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

 

[drizler]

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.

 

[EForest]

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

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:

 

[Bartman]

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.

 

[Bob Rohr]

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

 

[TCaldwell]

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?

 

[EForest]

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.

 

[TCaldwell]

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,

 

[jebatty]

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.

 

[Eric Johnson]

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.

 

[Nofossil]

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.