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    Thread: Greenhouse pond build

    1. #1
      Wlantry's Avatar
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      Greenhouse pond build

      Hey, folks,

      I hope all is well in your world. I'm building a greenhouse, and I want it to be mostly passive solar for heat. That means having a large heat sink, and water's a good choice. Besides, the garden plan still calls for two large ponds, and I'll need a place to raise water plants and grow on koi and goldfish. So a raised pond, 20 feet long by 40 inches wide by 30 inches high seems like a good option.

      A word of warning: this is a very low end build. The sides are going to be wood, and reclaimed wood at that. Some of the timbers come from a second story redwood deck that used to be on the back of the house - it was built in 1963, so that wood was cut over half a century ago. Others come from the rebuild of the deck: scraps and rejects. Some of the 4x4s used to be fenceposts, etc.

      At first, I thought I'd just screw one side of the pond frame onto the greenhouse wall. Then I realized I was looking at 1200 gallons of water - c. 10,000 pounds - and while most of that weight would push down, some of it would push *out*, and it could risk wracking the whole structure. So now the plan is to sink 4x4s upright as posts - one every five feet. Digging in my rock-root-clay ground is a bit of a night mare, so I'm only sinking the posts about a foot, but that should be enough to keep the sides from pushing out. And I've dealt with enough concrete dust for one lifetime - especially in enclosed spaces - so I'm using that newfangled post setting foam.

      I'm going to line the inside of the 'box' with thick weed block - I have an irrational aversion to old carpet (don't ask). The liner will be three layers of six mil black plastic. There will be cross-bracing (top bracing?) a 2x4 tying together each set of posts at the top. I've left about 30 inches of room at the east end for a filter. Also, I need to think about the most efficient means of heat exchange. First thought? A substantial air pump, so that during the day the warm greenhouse air gets pumped into the pond, heating the water, and during the night that heat is released into the greenhouse via the air being pumped though the water. All that oxygen should also make the fish pretty happy.


      Here's a pic to give you a general idea of the structure:


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      A look inside:

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      The first posts going in:

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      Bonus pic: first snow on the greenhouse. This is zone 7b, and a few winters ago, we got 40 inches of snow in a single storm. "When in doubt, build it stout," as the saying goes:

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      Thanks,

      Bill

    2. #2
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      Bill,

      I have some thoughts, but don't want to ruin your dream, but I believe you should research the other pond in a greenhouse projects using the search function on this forum.

      Solid dense substances like stone and concrete (and sealed water tanks) absorb solar energy and release it slowly. Open water tanks with large surface areas, can release its solar energy quickly, that is the reason for swimming pool covers. Evaporation is your main problem.

      I would suggest you look into rock or concrete floor within your greenhouse and placing individual Rubbermaid stock tanks atop that for your plants and goldfish. Your tank design is fraught with problems: use of multiple layers of thin pond liner, provisions for sealing the piping and seams of the plastic sheeting, stability at the bottom of the wood walls from outward pressure, differential settlement of the soil, etc.

      Just my opinion.

    3. #3
      Wlantry's Avatar
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      Grumpy,

      Thanks for your thoughts, and for your civilized means of expression. Someone else put it less gently: she said "That's the silliest idea I've ever heard!" and went on to explain that the outward pressure at the bottom would be nearly equal to the entire weight of the water. Who knew?

      But a fool and his folly are not easily parted, and my therapist is always saying I'm stubborn as a mule. So I'm determined, even if I'm starting to think this may end up like Zorba the Greek's "beautiful failure." Based on some of the warnings I've received, I've changed the design. I'll find some other use for that recycled deck wood, and only use fresh 2x10s. I've shortened the pond a bit, and I'm going to try to dig it deeper, so it'll make a better heatsink. That'll be good exercise for my sexagenarian body, or what's left of it!

      I did a lot of digging yesterday:

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      And got the North wall up.

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      Here's the foam:

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      and here's what it does:

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      I'm far enough along in the greenhouse build that it's warm enough in the evening - in the 50s when it's in the 30s outside - for me to keep working, and I've rigged up some temporary lights. So I was out there digging last night for much of the evening:

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      Perhaps my main role in life is to serve as a warning example to others? It's certainly starting to feel that way!

      Thanks,

      Bill

    4. #4
      kdh is offline Senior Member
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      Quote Originally Posted by Wlantry View Post
      Grumpy,

      Thanks for your thoughts, and for your civilized means of expression. Someone else put it less gently: she said "That's the silliest idea I've ever heard!" and went on to explain that the outward pressure at the bottom would be nearly equal to the entire weight of the water. Who knew?

      But a fool and his folly are not easily parted, and my therapist is always saying I'm stubborn as a mule. So I'm determined, even if I'm starting to think this may end up like Zorba the Greek's "beautiful failure." Based on some of the warnings I've received, I've changed the design. I'll find some other use for that recycled deck wood, and only use fresh 2x10s. I've shortened the pond a bit, and I'm going to try to dig it deeper, so it'll make a better heatsink. That'll be good exercise for my sexagenarian body, or what's left of it!

      I did a lot of digging yesterday:

      Name:  PXL_20201230_200910241.jpg
Views: 364
Size:  239.3 KB

      And got the North wall up.

      Name:  PXL_20201230_200819325.jpg
Views: 364
Size:  241.5 KB

      Here's the foam:

      Name:  PXL_20201230_201247397.jpg
Views: 365
Size:  154.5 KB

      and here's what it does:

      Name:  PXL_20201230_204012812.jpg
Views: 364
Size:  279.8 KB

      I'm far enough along in the greenhouse build that it's warm enough in the evening - in the 50s when it's in the 30s outside - for me to keep working, and I've rigged up some temporary lights. So I was out there digging last night for much of the evening:

      Name:  PXL_20201230_233833928.jpg
Views: 362
Size:  236.8 KB

      Perhaps my main role in life is to serve as a warning example to others? It's certainly starting to feel that way!

      Thanks,

      Bill
      lol

    5. #5
      cottagefog is offline Senior Member
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      It looks great. A propane tank, with a tank top heater would warm that green house up really quick if you wanted it to be more comfortable in there.

      Looks like a great build so far. May I suggest where you have all three of your 2x10s butting up. Not that it is weak, but stagger those joints in the future for added strength.

      I also am old school. Concrete vs foam for the 4x4s. Is that foam ridged enough to hold that push-out pressure from the pond water? I dont know.

      One more thing, then I will shut up. Any venting for the greenhouse when spring and summer comes?

      Good luck and keep the photos coming.

    6. #6
      Wlantry's Avatar
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      Quote Originally Posted by cottagefog View Post
      It looks great. A propane tank, with a tank top heater would warm that green house up really quick if you wanted it to be more comfortable in there..
      Thanks. I looked at heaters like this: https://www.amazon.com/Mr-Heater-F23.../dp/B01DD6C4MY but they all have a low oxygen sensor, and if the greenhouse is sealed tight it would automatically shut off?

      >Looks like a great build so far. May I suggest where you have all three of your 2x10s butting up. Not that it is weak, but stagger those joints in the future for added strength.

      I did think of that... after I'd dug the holes! It would have been much stronger!

      >I also am old school. Concrete vs foam for the 4x4s. Is that foam ridged enough to hold that push-out pressure from the pond water? I dont know.

      You're right. I know you're right. It seemed like a good idea at the time. The only one who's happy with this part of the plan is likely my pulmonologist. But it's true: her vote counts double!

      >One more thing, then I will shut up. Any venting for the greenhouse when spring and summer comes?

      Yes, I'm going to have 12" auto-start fans, up near the ridgeline, above the two doors. Won't need those until Spring, though, so they're not part of the build yet.

      >Good luck and keep the photos coming.

      Thanks for the encouragement. And no worries: this disaster will be well documented!

      Thanks,

      Bill

    7. #7
      Wlantry's Avatar
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      Quote Originally Posted by Grumpy View Post
      Open water tanks with large surface areas, can release its solar energy quickly, that is the reason for swimming pool covers. Evaporation is your main problem.
      So I keep asking myself why every greenhouse builder doesn't do this. Instead, they opt for large rocks along the north wall, or sealed barrels filled with water and painted black. And many complain they do almost no good, because there's not much active heat exchange.

      Hence the idea to have open water, with pumped air through the pond providing the exchange mechanism. I always thought water made a very good heat storage medium. But the thought that it easily transfers that heat makes me wonder about this: I need some plan for emergency heat: it rarely gets desperately cold around these parts, but once every few winters we get into the single digits. I'm not sure propane will work, so I may be stuck with electricity. But assuming the water will release its energy quickly, wouldn't it be more effective (or efficient) to heat the water, rather than to heat the air? I could put something like this in the pond, keep the filter and the air pump running, and keep the air temperature above my target low of 38 degrees?

      https://www.amazon.com/hygger-Submer.../dp/B08GG45YGX

      Or an I simply entertaining yet another foolish idea? Maybe I should make Wile E. Coyote my icon!

      Thanks,

      Bill

    8. #8
      cottagefog is offline Senior Member
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      Those heaters that use the small green tanks are not cost effective, unless you have a pile of old ones and refill them yourself.

      Not sure your budget but this small heater would be nice, and they do make a double headed Mr. Heater, and even a larger one yet!
      This single head Mr. Heater is on sale from time to time for $35. Double head for around $70 and the large 80k BTU for $99. (bigger the better)

      https://www.menards.com/main/heating...207124&ipos=15

      I would not be concerned of running out of oxygen in that large of an area running it for a few hours. You could always open the door now and then if you get paranoid, or a headache, dizzy or loss of consciousness (my lawyer said I take no responsibility of any injury, or death).

      I have used propane heaters in closed areas, like garages and sheds for years and never came close to thinking I was running out of oxygen though my wife might dissagree ;- )

      Great on the fan/venting, ans as long as the successes outnumber the disasters you got this!

    9. #9
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      High density foam sheets are great for insulating the inside of tanks such as this. Personally I'd go with 2 to 4 inches on the sides. Since the bottom is almost at ground level you'll loose more heat than gained from ground contact. Putting the foam on the bottom like on heated cement slabs will cut this loss. Covering the pond on very cold nights will help greatly. Based on cold weather performance I've seen on a friend's unheated greenhouse it might be possible with minimal water heating in extremely cold weather. Heating the water a little will be many times more efficient than heating the greenhouse air .
      The real Batman wears polyester! Don't be fooled by the plastic imposter.

    10. #10
      cottagefog is offline Senior Member
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      Just wanted to be clear, my suggestion for the heater was for human comfort during the construction, NOT to maintain the temp of the pond water, or greenhouse after the build is over with.

    11. #11
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      Hi Bill:

      My wife abd I built a greenhouse from a kit a few years ago. I would never do it that way again (the kit construction details suck, Farmtek is basically upcharging for cheap materials that are used contrary to the individual manufacturer recommendations...), but that part is done. I shot some video for the koi club this summer, apologies for the amateurish narration:

      https://www.youtube.com/watch?v=BBBJrghu0yw

      I'll just make two points:

      My greenhouse is 1500sf in WA state (where a hot day is 80oF), yet it still gets over 120oF in the summer. You'll want active cooling - opening the doors is not enough.

      The insides of my greenhouse drip water steadily all Fall, Winter and Spring. It is always humid and damp. We can get away with it because it's all steel and plastic, but if I forget a steel tool (wrench, screwdriver) for more than a day it will be covered in rust. Wood rots quickly, and yes, mold will grow on treated lumber! I'll get a gas line out to it next Spring, and we'll be installing a heater mostly to raise the dew point.

      Good luck,

      -t

    12. #12
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      To run cooler in summer cover the greenhouse with a shade cloth and open doors and vents on the ends to keep temps lower. Common method for commercial setups. Design ends with plenty of vent area.
      The real Batman wears polyester! Don't be fooled by the plastic imposter.

    13. #13
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      Bill, don't let a few arrogant, know it alls deture you!!!! I've had my wild ideas called 'horrible' and 'terrible'.......(Ya, I can see mistakes they've made...IMHO,,,, ) But,,,,,I've also done a crap load of research, and have found plenty too know that I'm on the right track!!! Even,,,,,a couple fine folks have PMed me just too encourage me and give me more proof!!!!!

      Lots of wayzzz too skin a cat (or coon!).......If you've put the thought in, and feel confident, go for it!!!

      Cheers,
      KTK
      Last edited by KTownKoinut; 12-31-2020 at 07:35 PM.

    14. #14
      delbert is offline Supporting Member
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      It won’t take much to keep it warm in 7b




    15. #15
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      Quote Originally Posted by Wlantry View Post
      Grumpy,

      Thanks for your thoughts, and for your civilized means of expression. Someone else put it less gently: she said "That's the silliest idea I've ever heard!" and went on to explain that the outward pressure at the bottom would be nearly equal to the entire weight of the water. Who knew?... But a fool and his folly are not easily parted, and my therapist is always saying I'm stubborn as a mule. ...
      Who knew? Anyone who has taken physics...

      Once filled, the pressure pushing outward is approximately 0.5psi per foot of depth. Your pond appears that it'll have maybe two feet of water in it, so the walls will see about 1psi at ground level. The problem is that it's pounds per square inch, so the base will see roughly 145 pounds per square foot at the base, decreasing for every inch above that, reaching zero at the top. The worst part is that the force never lets up; it's there from the moment it's filled, and if the walls bend outward some, it's still pushing outward at 145 lbs, further distorting the walls.

      Given the design, I'm fairly sure that over time (hours, days, weeks, maybe months), the walls will gradually deform outward further and further. How long the clay soil will work depends how much moisture is in it, so things may get interesting a few days after rains have softened the soil around the posts.

      As an aside, on another forum, I posted that someone's car design would likely fail and explained why. He got all upset, not because I'd pointed out a problem, but that I had rained on his parade. He "showed me" and built it the way he wanted, and one day, went off a mountain road backwards because of the poor design. Physics doesn't care about belief, hopes, and dreams - it's just numbers.
      Last edited by kimini; 12-31-2020 at 09:02 PM.

    16. #16
      KTownKoinut is offline Senior Member
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      Bill, I just looked through your ideas carefully, and kinda like Kimini says,,,,,,there's a freack'n lot of pressure gonna be in there, Dude!!!! I've bin in home construction since the mid 80's, including some massive concrete forming,,,if ya want some pointers???? Otherwizzze, have at it..(with all due respect, I just like how those words sound together!!! ) Buuuut,,, I'm think'n of some fairly easy mods you could do to 'beef' things up a lot!!! ....But only if you wanna hear my thoughts? I don't wanna just hammer your ideas!!!!
      Last edited by KTownKoinut; 12-31-2020 at 09:56 PM.

    17. #17
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      Regarding the pressure on the sides- I am reminded of trailers on the farm that would have the sides bow out. Even sides on a short 8 foot trailer would bow out under the pressure of corn or oats (obviously much lighter material). What a farmer would do is have a chain across from one side to another to hold them from bowing out. A light chain with a turn buckle.
      I think in your situation you would be wise to have crossmembers across the top to hold the sides in. One every four feet? Use the cross members so that the walls pull against each other.
      I think if you look at some of the indoor wood pond builds featured on this site they use a cross member at the midsection.

      edit to add, it would be pretty easy to put cross members down at ground level, I was thinking primarily of the pressure up near the top, but dig down 3.5 inches and attach 2x4's across from one post to the one opposite of it, and you will have the bowing at the bottom solved.
      Last edited by mplskoi; 12-31-2020 at 10:26 PM.

    18. #18
      mplskoi is offline Supporting Member
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      Karl has done some excellent indoor pond builds. Notice how much wood he uses to keep the sides from bowing out.
      https://www.koiphen.com/forums/showt...oor+pond+build

      Someone correct me if I am wrong, but the width of a pond has nothing to do with the pressure put upon the wall to bow. So a 3 foot wide pond will exert the same pressure on the wall as an 8 foot wide pond?

    19. #19
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      Total pressure on a side wall will be proportional to the size. For the same depth a 20 ft long side will see double the pressure of a 10 foot long wall. If the walls are supported only on the ends larger beams are needed to prevent bowing on a longer span. PSI remains the same at any given depth but double the area and double the total pressure exerted on the side.

      Typically on above ground pond displays constructed with in-ground posts the tops of the wall usually bow out and the bottom of the post remains in place. Seen some with a cable or two across the top to prevent bowing out. Same principe as post #17 and reference to supporting the sides of grain trailers and truck beds. If no cable is desired then a larger support beam on the top of the wall will be needed.
      Last edited by batman; 12-31-2020 at 11:52 PM.
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    20. #20
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      Batman- can you (or someone else) confirm my statement- is it correct that the width of a pond does not impact the concern of bowing or side pressure, only the length? As in, if a pond is 3 by 20 the pressure at the midpoint of the 20 side would be equal to the pressure at the midpoint of a 20 foot side on a pond that is 6 by 20?


      Looking at what Wiantry is trying to do I think putting a crossmember every 4 feet would solve his bowing issue. Of course it might also be nice to set plants in pots up on those boards too. Could work out nicely.

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