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Swimming pool as a storage tank?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by BrownianHeatingTech, Dec 11, 2007.

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  1. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    The suggestion was recently made to use an aboveground swimming pool frame for a hot water storage tank. I think that's an interesting idea, except that I can't seem to find any pools in the 500-1000 gallon range, which is all that most people want in their basement.

    Any help?

    Thanks,
    Joe

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  2. webbie

    webbie Seasoned Moderator Staff Member

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    That is not a bad idea - it is capable of holding the water, etc. - even an old hot tub is 400 gallons (mid-size).

    A swimming pool would have to be insulated and/or lined, as the standard plastic is probably not designed for the high temps...

    How much does even a new "soft tub" sell for? That is a hot tub with air insulated sides.
  3. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    I don't think I'd trust the soft tub with the temp, even with insulation. If it did boil over, the small mess would become a large mess once it melted a hole in the air bladder...

    And yes, it would need a new liner. EPDM seems to work well for folks. Insulation wouldn't be required, but not insulating a heat storage tank would be pretty silly.

    Joe
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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  5. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    The smallest I've found was 48" high by 10' diameter. 262 cubic feet, or 1960 gallons.

    For 1000 gallons, something 7.5' diameter (at 48" high) would work well. That's just under 1100 gallons (figured for a 40" water depth, to give some room above the water line), leaving some room to add some insulation.

    Joe
  6. hkobus

    hkobus Member

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    With the sizes pools come in I would be a bit concerned about losing the stratification in the storage. Pools are designed to have large surface area, for my storage tank I would rather have the same volume in a tall structure to get the benefit of stratification. In my mind with large surface, I would need more HX area in the top of the tank too.
    I would like to go for 5 or 6 ft tall tank, any comments are welcome.

    Henk.
  7. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    I would, too. But the height becomes an issue in a typical basement (getting the heat exchangers in - and out again, if they need service), and the structural integrity of the tank becomes an issue as the pressure mounts due to depth.

    I think most customers will see the cost/benefit ratio working out best at a 4-foot depth.

    Those who want the taller tank, which may offer a couple percent better performance, will likely end either with steel tanks or cast concrete cisterns.

    Joe
  8. Jags

    Jags Moderate Moderator Staff Member

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    Once again, probably speaking out of line, but......
    If someone was looking for a few hundred gallons of storage, with all the oil burners out east, why couldn't a several hundred gallon oil tank be used? Its designed to fit through doors for basement installs, designed for fluid storage (round corners and all that), structurally sound, and could be insulated by a spray foam application. Comes with pipe fittings and usually its own legs. Once cleaned of old fuel oil, if mods needed to be made to the tank, simply weld or braise your new fittings to the tank.

    Not sure of the practicality of the idea, but if you needed more storage, add another tank and run pipes between them. I know that there has to be a down side, cuz I can't be the first person to think of it. Just an idea.
  9. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    I've considered it. Problem is the cleaning the old oil part. Start to subject old oil to 200-degree water temps, and it will out-gas a lot of nasty chemicals.

    The tanks also aren't all that rust-proof, unfortunately. Rust-out is the typical reason a tank is replaced. I could get a properly-designed heat exchanger through the top ports, but it does limit the heat exchanger choices. The legs would help with keeping it off the floor, for reduced heat loss.

    Tipping it on-end and cutting the one end (now the top) off might work, although I expect you'd need a brace or two to keep it from spreading.

    I'm thinking that the horizontal tanks might be the best choice. You could cut a round hole in the flat top, without weakening the structure much, and that would allow a larger heat exchanger to be installed. Of course, stratification would be about zero. However, one could stack the tanks on a frame and use the top tank for recovery and the bottom tank for charging, with a decent-sized pipe connecting them so the water can convect as it heats/cools (need to weld a bung into the bottom of the top tank). That mandates a sealed penetration for the lower tank's heat exchanger, as it will be under the water line.

    All sorts of possibilities. I think that the swimming pool idea may be the "best bang for your buck" and the most attractive to my customers for that reason.

    Joe
  10. Eric Johnson

    Eric Johnson Mod Emeritus

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    Well, new 250-gallon oil tanks go for about $250 at Home Depot, as I recall, so you're right at $1 a gallon, which seems to be what the DIY alternatives all go for, more or less, from swimming pools to propane tanks to DIY sheet metal and wood affairs. I think corrosion might be a problem in a nonpressurized tank, though I bet you could treat the water. Might be perfect if all you wanted was 250 gallons, but I expect that trying to tie two to four of them together wouldn't yield the same results as a single, 500 or 1,000-gallon tank. But as with everything else, I could be wrong on that, too.
  11. Jags

    Jags Moderate Moderator Staff Member

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    Ahhhhh.....the rust factor. See, I knew it was too simple. And I can also see that stacking tanks would limit the natural stratification of the water column. With out even asking, I am going to assume that you guys have exhausted the poly tank options. I would imagine that cost would be a major factor in those (maybe heat as well).
  12. hkobus

    hkobus Member

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    Jags,

    I looked at poly tanks, as we use them for storage of fertilizer and such, but as you suggested the temp is the issue. They recomend less than 120 F and peak temp at 140 F (less than 10 Min). The price is right, at below the $ 1 per gallon here, and many used shapes and sizes available.

    Henk.
  13. Jags

    Jags Moderate Moderator Staff Member

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    Ever see this one? http://www.rainkeepertanks.com.au/polytanks_rainhaven.php (grand daddy slim)

    Not making any suggestions, and I am not sure of details, but they make those things in all sorts of shapes that I have never seen.
  14. Tarmsolo60

    Tarmsolo60 Feeling the Heat

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    have any of you priced an stss tank? when I bought mine from tarm I think it was $1700(without hx), it came in a pretty easy "snap together" form. It had the raceways to get the pipes into the tank and had all the silicone and screws to assemble and seal everything. So, I was under $2 per gallon for insulated storage that had a warranty. I seems like it would be hard to beat if your paying $1 per gallon, then insulating it, plus all the messing around getting it figured out and hoping its going to work ok. I know sometimes half the fun is figuring things out and building them yourself and saving money, but, by the time your done you might not be as far ahead as you think DIY. If a nice used tank like nofossil has presents itself at the right time I would jump on it for sure and use it. Maybe STSS knows what it would cost to assemble a new similar tank and has theirs priced so its not really worth it to DIY.

    also, if your trying an old oil tank don't use it in a pressurized system it's not strong enough, use Hx's
  15. Eric Johnson

    Eric Johnson Mod Emeritus

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    That's what I thought, too. The quote I got from STSS was $1,945 for the 957-gallon tank. The price for a tank plus hx for my system netted out at just under $5,500 including freight.

    I'm sure it's worth it, but I knew I could do it for a lot less. Maybe not as good, but probably good enough for my needs.

    That's probably why most gasifier manufacturers tend to downplay the need for a tank.
  16. Tarmsolo60

    Tarmsolo60 Feeling the Heat

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    I bought mine a couple years ago thats probably why it is $200 more now. So with the prices you just stated it seems like trying to save money on Hx would be a much bigger benefit than what you would save on the untested tank. I didn't buy my coils from them, they wanted $500/180' coil at the time. I figured I could make them cheaper, and I could have if I would have done them then. I waited and copper didn't get cheaper. I ended up buying soft L and making them.

    So knowing you can buy a 957 STSS for $2000, how much cheaper can you construct a insulated tank around 1000 gallons?
  17. Beno

    Beno New Member

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    How come a 120 gal Indirect costs as much as 900 gal tank with hx ? What does the Indirect have that the sorage tank doesn't?
  18. Eric Johnson

    Eric Johnson Mod Emeritus

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    Tarmsolo60:

    In my case, using an existing poured concrete cistern and some cinderblocks, surface bonding cement, etc., about $500. That's with 2" foam insulation all around and a EPDM pond liner. And a lot of time and effort, which I'm more than happy to spend in lieu of cash.

    I probably have another $500 worth of materials in the heat exchangers, but some of that was materials that I had onhand.

    Bear in mind that none of this is actually working yet, so I'm not crowing about it just yet. Hopefully this weekend.

    Another guy here has a thread in which he describes how to build a similar tank to the commercial versions for around $700 in materials from the big box store.
  19. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Ability to hold pressure.

    The STSS (and other) storage tanks are open - no pressure other than the weight of the water.

    The indirect is a sealed pressure vessel, and can handle at least 100PSI, usually more (most seem to be tested to 300PSI).

    Joe
  20. Beno

    Beno New Member

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    Sorry, but what is the pressure needed for? And if it's needed, how come a large storage tank doesn't need it?
  21. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    The pressure in the indirect is the pressure created by your well, which pushes the drinking water up into your house.

    The large storage tanks use a copper coil to heat the water. The water in the tank never mixes with the water inside the coil - it just sits there, so it doesn't need pressure to move it around.

    Joe
  22. Tarmsolo60

    Tarmsolo60 Feeling the Heat

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    Eric,
    I saw the pics of your cistern and would be doing the same thing as you if that was in my basement. I was talking about someone who had to go out and buy a tank. It looks like you could build a duplicate of my tank cheaper than $2000 but as for using a different tank or swimming pool new and making it work I don't think you would be that far ahead of the game.
  23. Eric Johnson

    Eric Johnson Mod Emeritus

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    As I understand your question, Beno, it's one of life's big trade-offs:

    Pressurized Storage: No heat exchanger needed, because the tank is part of the heating system and the water it contains flows through the boiler and into the house zones, etc. Because the system water becomes oxygen depleted, corrosion is not a problem, so you can use a steel tank, fittings, etc. But the bigger the tank, the more substantial it has to be in order to hold the pressure. That much additional water in your system also requires a pretty substantial expansion tank--not something an Extrol 60 can handle. So what you save on the hx is offset to a large degree by the cost of the tank and expansion capacity.

    Non-Pressurized Storage: The water in the tank is not part of the system, so it is not under pressure. It contains some oxygen, which can be a concern. And you need an expensive heat exchanger to move the heat around. You can use any tank that can take the heat, though you probably want to stay away from steel unless special precautions are taken with water treatment.

    There are issues of stratification, etc. that I don't understand well enough to describe, other than to say that you have more flexibility with nonpressurized storage, depending on how you pipe and position the heat exchanger(s) in the tank.
  24. Eric Johnson

    Eric Johnson Mod Emeritus

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    I totally agree. Other than saving money, one of my big motivators is (hopefully) the satisfaction of designing something and making it work. It's all part of the decision process, but it's nice to see people thinking creatively. Here's a pic of my homemade hx--half of it, anyway, sans fittings.

    Attached Files:

  25. verne

    verne Member

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    with the price of hx I will definitely go with pressurized tank .any thought if burrying one would be better than just insulating it in the barn . I know it frees up space for wood storage. maybe the heat from the boiler would help it more?
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