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Subterranean Heating/Cooling System

March 22, 2008

In my internet wanderings, I came across this site:


Sounds like an interesting and fairly inexpensive method. And they actually listed a greenhouse that is implementing this process in my little old town of Durango!


I'm just curious if any of you have installed this or have any ideas about it.

Comments (150)

  • waterstar

    I also found a less expensive way to buy the 4" perforated ADS pipe. I am getting it for 44 cents a foot instead of the 62 cents charged by home depot and Lowes. The rep sells it cheaper than the company can quote you. Maybe there is one near you?


  • annalog_gw

    Hi Waterstar,
    Thanks for the additional information. I will think of 200 cfm as a bare minimum but look for higher cfm fans. Also, since I was including the volume in the central shed, I can almost double the greenhouse air exchange rate by closing the doors between the greenhouses and the shed. ;-)

    Was that ADS pipe price with or without a sock?

    Thanks, Anna

  • waterstar

    Hi Anna,

    The ADS was without the sock. My understanding is that you only need a sock if you have sandy soil. I have heavy clay.

    Shutting the door to the shed will help, but remember, you are trying to heat the soil around the roots too. So I am going to put in ADS every 1' horizontally and 2' vertically. No closer horizontally or the airflow may just go from one pipe to another and not warm the soil. So, I will have a bit more tubing than required, but I also have the prefab HFGH that I thinks needs more. ( :
    I think both sunnyjohn and ropers sites have links to sources for fans. I read that john died, so that is why his info is not updated.
    Bless you, keep working on this! We are not the only ones out there with affordable HFGH. ( :

  • annalog_gw

    Hi Waterstar,
    My soil is very dry sandy clay. It is OK when undisturbed but turns to powder if disturbed when dry. If disturbed when wet, the resulting mud dries with a concrete-like consistency. So I am only concerned about dirt infiltration during installation. I suspect that I will use ADS without the sock but cover it with strips of landscape fabric that I have onhand before I fill with the dry subsoil. My plans are for ADS every 1' vertically and 1.5' - 2' horizontally with the exception of near the plenum and possibly through the doorway. I am considering putting at least some the underground cement blocks sideways on each side of the doorway so that the tubes have more space to go between the greenhouse area and the central shed area. I agree that the smaller greenhouses (especially HFGH ;-) ) may need proportionally more tubing.

    That is interesting about the horizontal airflow. That has me thinking about adding small vertical barriers where the tubes need to be closer together. I have some scraps of vinyl siding material that could work perfectly. (This is left over from when we had to replace the siding with a block wall. We have been using it for various projects, mostly for chicken furniture such as nest boxes, but we have lots of short pieces left.)

    I found a link to a blog entry remembering John Cruickshank from the SOPI Permaculture Blog.

  • waterstar

    Hi Anna,

    I think you may want to go ahead and get the sock covered ADS. Did you check Roper's site to see what he says? And, oh my! your soil sounds much worse than mine! ( :

    I don't think I'd add any vinyl in-between the ADS pipes. It might interfere with the roots sneaking around and getting places. ( : Besides, I don't think you will need it, but that is just my opinion.

    How soon will you start digging? I hope to have mine dug this coming week if the rain holds off. I'd really like to get my seedlings in there this spring....we shall see.

  • annalog_gw

    Waterstar, I will start digging as soon as it is not so windy here; we are getting 30 mph wind forecasts. Digging with a shovel is not as much fun in the wind. :-) My soil is great once I add lots of compost. I am very glad we do not have the underground caliche we had in Tucson.

    Did the rain hold off; did you start digging?

    I posted some images of my greenhouse structure plans in the other thread. I will post one here after I model the pipe layout for the SHCS. Since it is too windy to dig, I have been planning instead.


  • waterstar

    Hi Anna,

    I was going to dig by hand, but Curlygirl talked me out of it...thank heavens! Maybe you could barter, trade or rent or something. We found a local who will dig the hole for about $250. Less than for which we could rent equipment.

    I am soooo fortunate to be in 7b. We had three days of 75 degree weather. Worked my head off outdoors. Before that it was in the 30's or less for a bit. Rain and cool today, so no digging yet. But the truth is that we are still gathering our other supplies. ( :

    Start seeds next week. Have a sneaking suspicion the GH won't be ready and my windows and rooms will be packed!


  • waterstar

    Curlygirl and Steve333,

    This is what I thought I'd use on the walls for insulation in the heat sink and on the N side of the greenhouse wall.


  • steve333_gw


    Extruded styrofoam is the way to go, but typically one uses a 2" thickness around foundations. On my smaller GH (~12 yrs old now), I used styrofoam forms for the foundation concrete, stripped the inside layer off after it cured, and put it all on the outside, for 4" of thickness total.

    It will up the price some if you go to thicker insulation, but it's one of those things that you only can do (easily) at this point in the construction.

    Lowes, Home Depot and all the building supply places should have the 2" thick version of that same stuff too.

  • waterstar


    THANKS for the help. I will get the 2".

    The only styrofoam forms I found are for horizontal foundations. Do they make one for a vertical hole (at the bottom we will put a sump pump if it turns out we need one.)?

  • annalog_gw

    When insulating underground concrete walls around the SHCS heat sink, is it better to put the insulation on the inside or the outside of the walls? Can the walls act as part of the heat sink?
    Thanks, Anna

  • waterstar

    Hi Anna,

    You don't need a concrete foundation with the HFGH. My hubby is using very tall chain link fence posts set in concrete on the four coners. He will anchor the inside of the GH to the posts. We will also use these posts to hold wire so I can espalier dwarf fruit trees.

    Trex type boards will be placed on top of the post concrete and used as a base for the GH to sit on. (Note: we may also bury it a little in the ground so I can wheel things in and out...flush with brick walkway on outside that continues on the inside....but we may not need to bury it.).

    If he thinks he wants more security, he will drill a hole in the Trex type board and use a large screw (foundation anchors? don't know the name...about a foot long) to anchor into the soil (put at an angle so it does not hit the heat sink).

    The insulation will go right next to the dirt. The nice thing is, we won't have concrete in the heat sink to syphon off heat/cool air. ( :

  • annalog_gw

    Hi Carolyn, the 6'x8' HFGH is much shorter than the 10'x12' HFGH. The door is only 5'4" tall and my husband would need to duck to avoid hitting his head. (I could walk in OK as long as I am not wearing heels. ;-) ) To avoid problems with headroom and for other reasons, we are putting the greenhouses on kneewalls about 32" high (four rows of 8"x8"x16" cement blocks.

    Also, we have severe rodent problems here (gophers, field mice, and rats). Cement block walls and hardware cloth are the only solutions we have found to work to keep out unwanted rodents. I am planning on layers of hardware cloth anchored in the concrete walls above and below the heat sink. I am concerned that if we do not wall off the SHCH heat sink, then rodents will eat their way through the ADS tubing and use it as their climate controlled home.

    The attached image shows the current plan for the two 6'x8' greenhouses with their doors opening into a central 8'x9' shed. The front wall of the central shed is 8' tall with a standard door shown. The rectangle in the roof that looks like a window is meant to represent a solar electric panel. The ground is represented with a transparent brown so that it is possible to see the bottom two rows of the four underground rows of blocks. If the ground was level then I would be digging down 34" but it will be less on one end and more on the other. The outside raised beds will take care of the difference in ground level as well as providing some extra insulation for the knee walls.

    There are some reasons other than cost why we will be digging the heat sink hole manually. These are:

    -- There is a small yucca growing where the central shed will be. I want to try to save it so that I can relocate it to my mother's house. These have deep tap roots and I should be able to get it out while digging the heat sink area.

    -- I do not want to disturb the yuccas that are to the south and north of the central shed. Also there is an existing shed to the north as well as trees to the east and west that impede easy access with digging machinery.

    -- I can certainly afford to burn calories and build muscles digging. My back and knees are in good shape. I may be crazy but I find digging in this soil a good meditative activity. ;-)


  • waterstar

    Hi Anna,

    Your design looks absolutely BEAUTIFUL!!! And what a great job drawing it!!!!

    The only thing I can think of (and it is probably already planned) is to change the entrance into the shed from steps to a ramp so you can wheel plants/soil in and out easily.

    I would copy your design in a flash, but we have fire ants here, so that will be my problem. They LOVE raised beds. I am planning on instant grits for the GH. They are not active in the winter, so I can then, but in the summer only the fruit trees will be in there and I'll be watching it like a hawk for any signs of them.


  • annalog_gw

    Hi Carolyn, there are no steps. The entry into the shed is level. Those brown blocks are actually directly under the shed door two to three feet underground. It is an optical illusion due to me selecting a transparent color for the ground in the modeling program that I used. I cannot draw but I can get a computer program to draw for me. ;-)

    We have some fire ants here but mostly other kinds. Fortunately we also have horned lizards that love to eat ants. ;-D


  • annalog_gw

    Carolyn, while thinking of ants, have you tried sprinkling ground cinnamon on the ant trails and where you don't want the ants to be? Both my daughter and I have had success driving them away with cinnamon.

  • waterstar

    Hi Anna,

    Did you try it on fire ants or sugar ants. I know sugar ants need to smell their trail, so that will work. ( : Fire ants , I don't know how they work..... Sounds like I need to import Lizards. ( :

    Such a super plan you have. Be sure and post pictures as you go!

  • annalog_gw

    It was fire ants at my daughter's house. I had first heard about using cinnamon to get bees to leave. I used it when a swarm was investigating our shed. I had an old can and tossed the contents up in the air of the shed so that it fell all over. The swarm left.

    My daughter had problems with fire ants in the raised bed along the back of her yard. She did not want to use poison because of her two small dogs. I suggested trying cinnamon. She did and the ants did not like it. She had to keep reapplying it after watering her lawn. The newer ant outposts disappeared fairly quickly while the main nest took several applications. Now she rarely needs to sprinkle cinnamon outside. Large economical boxes can be found if it works on your ants; it would be too expensive using those tiny cans that are in the spice section. Asian and Mexican groceries, CostCo, and Sam's Club are where I find them.

    This post was edited by Annalog on Thu, Mar 13, 14 at 11:40

  • grandwheatgrass

    @Steve333 and @waterstar,

    It may be the way that the image looks based upon the perspective, but both beds in the GH are going to be the same height (18 in.). I plan on using the reserved space that is not beds for overwintering potted plants/trees, germination stations, and a mobile workbench.

    I am still learning and exploring my options. @Steve333 said that 4' may be too deep. My initial calculations were at 3', but I wanted to make sure this system was effective so I thought I might just go a little deeper. Maybe I am just overplanning. I am concerned with the need to cool in the summer and thought that deeper would help in storing and converting heat underground. I saw on Ropers' site that temperatures still got 140+ úF even with SCHS working.

    @Steve333, what is the temperature range that you have seen in your SCHS greenhouse during December - January? My main goal is to overwinter plants that can't handle freezing and secondary possibly grow citrus or similar plants more tender plants.

    I love your advice!Hopefully this new picture can help give you an idea for what I have in mind. We recently got some bids from landscapers for the other parts of our yard, I bought our fruit trees and have potted them until we are ready for them and I will be gathering the materials to do the digging and underground work myself. It's starting to come together! I can't wait to get gardening!


  • steve333_gw


    I can certainly understand your efforts to exclude rodents. We have gophers as well as the other varieties (rats, mice, voles, rabbits). From what I can tell, I did not have any move into my underground tubes, so far at least. And those tubes were un-enclosed for 4-5 months. But it's hard to say how long this will last.


    There are several foam based concrete forms systems out there. Typically for pouring the walls of a house foundation. These days they even make ones that are designed to make it easy to take the inside foam off. So yes you can get them to make a sump pump well.

    In general, you want to keep the concrete/block thermal mass inside the insulation. It just adds extra thermal mass, you paid for it may as well use it is my take. Also exposed foam on the inside does not stand up very well to gardening activities.


    My memory is a bit fuzzy on this, but I think I recall a thread on digging the SHCS deeper on one of John C's forums. The gist of that was that it was a waste of effort, and that it worked better at the shallower 3' depth. I suspect that it actually will depend on the details of your project: temp range you're working with, type of soil etc. It would be nice to have some good simulation program to run this by, but I don't know of any, so we just have to take our best guess. As I said though, I can't be sure on this.

    I've seen a fairly wide range in the GH this winter. It got down to 20F or so on nights I did not run the SHCS fan. With the fan it stayed ~45F warmer than outside on the really cold nights. During the day, it got up to 120F on very sunny days with no cooling or venting. SHCS helped lower the temps some, but it was very clear that I will need shade cloth or additional ventilation to keep temps in check for the summer (as expected).

    I expect to be able to use the GH all winter for greens and the like, and I am hoping to keep a couple of hardy citrus in there too. I will use aux heat when needed, but my initial tests lead me to believe that my heating bills should be fairly low. Heating on only maybe 5-10% of the nights.

  • waterstar


    Wow, 120F on sunny days is amazing. This summer I will plant some sacrificial heat tolerant plants in there to see how the temp problem goes.

    If I cannot keep it cool enough this summer, I will plant clematis and/or pole beans outside on some of the south side of the GH as well as the west. I will have hubby sink extra pipes outside the GH, about 1-2 feet away?...and these can be used as the trellis supports.

    Thanks for the advise on the forms. I really appreciate your help. I have another question now. I am going to use 12" ADS as my plenums (Roper said that size would be ok). Is that what you (and Curlygirl?) used? If so, I hope your experience will help me. The adaptors between the 4" tubes and the plenum tube are really expensive. My hubby says if he can't find the right size bit for the drilling then he is going to use hot glue for a temporary hold while silicone dries and is the permanent hold. I just want to double check the reason for the adaptors. If he can get a good fit and a good seal it should be good, right?


    I will try experiments with the fire ants and cinnamon this summer. Goodie.


    Are your intake and outlet pipes on the North Side and your planters on the South side? ( :

  • annalog_gw

    @steve333, thanks for confirming that the insulation should go outside the underground concrete block wall around the heat sink. Now I am considering adding a fifth row of underground blocks so that the foundation is 40" instead of 32". Then, if I use 4'x 8' insulation panels, only 8" will be inside the raised beds. On the other hand, even if 16" is at the lower back of a raised bed 24" high, my normal gardening activity should not disturb it. I would prefer to stay with four underground rows of blocks. Now, I am beginning to think that I am going to need a water barrier between the outside raised beds and the greenhouse foundation walls. If so, then it is the water barrier that will need the most protection from gardening activity. Off the top of my head, I can think of Tyvek or something that is painted on. Painting the outside of an underground wall means digging a bigger hole. More to research! I did not think I would need to worry about waterproofing foundations in the desert. :-) Surface water is not a problem for the greenhouse site; it is already channeled elsewhere.

    Does the wall above the heat sink need to be insulated or should the outside raised beds be sufficient, especially if the beds are covered in the winter?

    Rodents are persistent. Field mice ate holes in the plastic plumbing lines under our house requiring plumbing and insulation replacement, as well as adding a block wall under our manufactured house. We put rat poison inside that space as no other wildlife should go there. That area is also treated to prevent termites. We will not deliberately use poison elsewhere on our property. Gophers seem to become more of a problem as more neighbors cut down and pull out native desert plants and do not replace them with anything else. (Maybe because the gophers ate what they planted before moving to our acre.) I am certain that our gopher population density is at least triple the normal for this area. Fortunately, we have not had severe problems with the rabbits and rats that live here. I suspect that is due to us leaving their natural food supply alone and that the actions taken for mice and gophers works for them also. In addition, the snake population seems well fed.

  • steve333_gw


    I would think that one of the paint on foundation waterproofing materials would be the way to go. If you get a water based one, it should be safe for the foam, and can even be used to coat any foam that is above the dirt to prevent the sun from degrading it. There are co's that spray these coatings on, so you only need a 1' of clearance (as long as the dirt isn't falling in).

    Typically I have insulated the wall all the way up. But I am in a much colder climate and don't have the outside beds.


    I looked into larger ADS for plenums, but backed off once I got some pricing. I would have ended up spending more on the plenum ADS and fittings than I did on the ADS for all the tubes. I ended up using those blue plastic 55g drums; like you see in some of the youtube vids. I found it was pretty easy to fit them to the drums, I drilled a hole with a 4-1/8" hole saw and just inserted the ADS. Key thing is to do this in warm weather or heat the ADS with a heat gun/hair dryer. If the ADS is warm it becomes very flexible. When it was at 35F (the outside temps when I was doing this) it was impossible to get it into the holes. Experiment on the exact size of the hole. You want it so that it is sized for the indents in the ADS. That way once the ADS is in there the taller ridges will hold it in place. No glue needed.

  • waterstar


    THANKS for the tips about the heat. I'll give your tips to my hubby ( :

  • annalog_gw

    @steve333, thanks for the tips for connecting the ADS with the plenum. I found a locally available water-based waterproofing coating that looks good and that we can paint on. We will decide when it gets closer to time to use it. I will probably have the insulation go all the way up. If the total foundation wall is under 6 feet (half above and half below ground), then that will be manageable. We might paint the below grade section before building the above grade section.Then we would be able to fill in the outside sooner.

  • myfrozenlittlepond

    Annalog, I know this is off topic, but using rat poison under your foundation does not prevent other wildlife from exposure to it. Rat poisons do not act immediately, they take hours to days to kill an animal. So the targeted animal ingests it, then runs off, often times filling their cheeks with vast quantities that they move to other areas to cache it away for later consumption, and then go back for more. the product works by inhibiting blood clotting, so they bleed to death. any predator (including domestic pets) who then consume the dead aniimal or their caches of rat poison that were moved around, will suffer the same bleeding problems. Hawks, owls, cats and dogs and any other exposed animals will suffer the same effects. Please don't use it! There are sticky traps, snap traps, and other effective means of eliminating these unwanted visitors that don't kill other animals. I am not a "bunny-hugger", I am a veterinarian and wildlife rehabilitator who sees the end effects of these poisons often. I wish the companies who make these poisons were required to tell consumers what is actually going on! I hope this helps you make a more informed decision. You probably had no idea!

  • annalog_gw

    Myfrozenlittlepond, it was almost four years ago that we put the one package of rat poison bars under our house inside the block wall enclosure. We have not added any since and I am not sure if any was eaten. If it is still there, should I bury it in place or dispose it another way?
    Thanks, Anna

  • waterstar

    Life is full of surprises! YIPES! We got our heat sink dug today. I was so excited. And then we hit a pipe.....a sewer pipe, YUCK!

    Turns out there is an unexpected drain field from the septic tank here. Good grief. All of the drain field was supposed to be 40 ft. away from this area.

    We replaced the pipe...it hooks onto a perforated ADS on each end....guess it a "y" just outside the hole.

    It only had a little bit of moisture in it, so I know it is not the main line. I am going to run some washing and observe what happens. Also, it is going to rain tomorrow. We are going to leave the hole open to the rain. I want to see how the drainage is.

    Fortunately, the pipe is right at the 4 foot level. This means we can put 1 foot of gravel on top of it, plus one foot of clay soil on top of that and our deepest tube will go on the level. By observing the water levels I can tell if this will be a problem.

    If need be we can always put a sump pump in place and pump it underground a long ways away. Oh my.

  • myfrozenlittlepond

    Annalog, that is a great question. Disposal in a landfill risks critters accessing it if it is not buried well. Burying it yourself risks the same - I am not a chemist and am not sure how stable these chemicals are in soil. They are very stable in the body. I guess I would go the landfill route, with hopes that they properly bury waste and that it will degrade with the garbage over time. try to put it in a container that would at least temporarily keep it safe.

  • hex2006

    Hi Guys
    Good to see some shcs digging action going on, its the best part..not ;)
    It seems that everyone is destined to run into "something" while digging their trenches. I hit an old clay land drain about 3ft down, it looked just like a clay sewer pipe until i dug under it and found it was only half a pipe.

    I have an odd shaped plenum, nicknamed "the birdhouse", made from 4x1 pressure treated timber and punched galvanised metal strip. Its been in the ground for about 5 years and hasnt fallen apart so far. Its a good option if you`re struggling to find something suitable.

  • grandwheatgrass


    Regarding the 3' vs 4' hole for the SCHS would 3' provide more warming for the winter, but 4' more cooling for the summer? I am just trying to understand if in zone 6a if I will have more problems with heat or cold.

  • grandwheatgrass


    What kind of performance to you get from your SCHS in the summer for keeping temperatures down and the in the winter for keeping temperatures above freezing? Did you follow the calculations provided by Sunny John? I will be digging in about 2 months and I want to see if following the recommendations provided by Sunny John exactly will give the best results.


  • hex2006

    The system has performed well for moderating summer greenhouse temps and keeping it frostfree over winter without any additional heating.
    In the 4 seasons its been running i cant say we`ve had two summers or winters that were the same but i havent needed to resort to anything extreme like forced summer venting or running the system at night to ditch excess heat from the mass to increase the following days cooling capacity. A cold summer followed by a harsh winter would be the ultimate test but it hasnt happened yet.
    I used John`s calculations for the overall tube length vs greenhouse size but the layout is non standard. I use a much higher cfm than the recommended 5 airchanges per hour, i find the most efficient heat transfer in summer peaks at 20-25 air changes per hour with diminishing returns at 45 air changes. A lot depends on your local conditions, both inside and outside the greenhouse but it really doesnt have to be 100% perfect in order to work well.

  • steve333_gw


    I can't really say precisely what the effects would be (it was 4-5 years back when I was reading John C's blogs on this). I recall (vaguely) that the consensus was that it wasn't worth the extra expense to go down further. But if for other reasons you had to go down deeper than 3' (deep frostline, or whatever), then I don't know that it would be bad.

    My guess would be that a deeper (and bigger) SHCS storage volume would store more heat, and perhaps have a slower transfer of that heat up to the topsoil level. Depending upon the situation, that could be good or bad.

  • hex2006

    If you get plenty of sun it could be worth going to 4ft if its not too much extra effort (back hoe vs manual spade). The added depth gives you more mass and buffers against direct heat gains from the surface which can reduce the cooling capacity later in the season.

  • annalog_gw

    It seems to me that the payback calculations focus on winter heating. My primary concern is summer cooling. I'm going to dig with a shovel but I hope to get to 4 feet before I need to stop.

    I am now thinking that maybe it will be better to have two systems for each 6x8 GH, one for tubes under the greenhouse and one for tubes under the central shed. The plenums would be in the same place as in the image in an earlier post but side by side. I could then use the systems together or separately. Does that make sense or is it overkill. The total tube length will be about the same but I could have two different lengths for individual tubes.

  • hex2006

    I`d be concerned about the coupling between systems,particularly when the shed doors are open to the greenhouses. The intakes will favor taking air from the shed as its the path of least resistance, much closer than the far end of the greenhouse.
    Its worth investing in a handheld windspeed meter/anometer for balancing the tubing, calculating the actual cfm in the system and checking for blocked tubes..The skywatch xplorer 1 is cheap but effective. Small smoke bombs normally used to check chimney flues are ideal for highlighting the airflow path, short circuits and dead spots,

  • annalog_gw

    @hex2006, good point about the system coupling. My intention is to have both greenhouse doors closed most of the time but it would be good if the system worked nearly as well with the doors open. I could move the intakes to the far ends of the greenhouses for the lines that go under the greenhouses and just leave the intakes for the shared shed space in the ends near the shed. I could also add airblock curtains (overlapping strips of vinyl) to the inside of the greenhouse door openings to limit airflow when the doors are open. I will keep thinking for a while and then post some other ideas.

    I will definitely invest in some testing equipment. Thanks for the suggestion.

    It will be a while before we are close to the installation phase as we will be excavating the entire space in order to install the gopher exclusion system (hardware cloth/metal mesh with 1/2 inch openings under the space and a block foundation wall around the space). We will add moisture barrier and insulation outside that. If we are speedy, it will might be this fall but more likely next spring. ;-)

  • hex2006

    The bonus of having a system in each greenhouse is you can make direct performance comparisons between the two by using different settings.
    The shape of the plot makes for an interesting tube layout, if summer cooling is the primary goal i`d be tempted to incorporate the subsoil under the raised beds (increased mass) to make 2 square (ish) areas for the tubing layout rather than the 3 rectangles. Foundation block walls being in the way of the tubing is the tricky part ;)

  • annalog_gw

    @hex2006, tubes through the walls could be done by inserting 8x8x8 blocks turned on their sides. It is digging the 12'x24'x3' hole and buying nearly twice as many blocks that makes me cringe. :O ;-) :-)

  • hex2006

    It probably wouldnt be worth the extra expense and digging :) We dont have destructive critters llike gophers to consider which makes the job much easier. With limited space for bends you may find 3" tubing easier to install than 4". The 80mm can handle a 12" minimum radius bend if that helps any.

  • annalog_gw

    @hex2006, I like the idea of running some of the tubing in the outside raised beds. Each greenhouse could have two systems, one to store heat under the greenhouse and shared shed and one using the outside beds for cooling. I might not dig as deep for the outside raised beds (maybe just a foot or so. That would save on blocks and digging. The inside heat sink would still be insulated but the outside wouldn't.

  • grandwheatgrass

    Questions on Questions and Calculations:

    You can use different tube lengths with different quantities of tube sections and have the same overall tube length. How do you decide how long to make the tubes and how many tubes? Sunny John says anything between 30 and 70 ft individual lengths for the tubing. Why within those constraints? Would it not depend on the size of greenhouse you have? I am planning 16 ft lengths. If I follow the underground design of Roper's matrix then 16 ft. lengths should be great for a 16.5 ft long greenhouse.

    I have seen different estimates for Tubing Air Speed FT/SEC and Time in each Tube. Sunny John says that the tubing air speed should be between 8 and 16 ft/sec. Has anyone found results that would favor this estimate or would you suggest other values?

    My greenhouse will have a base of 192.5 sq/ft. According to Sunny John's calculator I should have 1/3 more length of tubes as compared with the square footage of the greenhouse (257 ft).

    Would this be an appropriate combination of individual tubing quantity and length?

    • Average Length of tubing = 16 ft.
    • # of tubes in each plenum = 15
    • Total Feet of Tubing = 240

    Referring to "style 1" below, I plan to use (2) 12" ADS for the plenums. I will have 3 vertical layers (1' on center) of 4" tubes with 5-6 tubes horizontally (2' on center).

    Finally, I have seen many different ways of doing the underground part of the SCHS. Which methods have you used and why? What are the pros/cons?

    Style 1

    Style 2

    Style 3

  • hex2006

    The air volume of the greenhouse is more important than the floor area. The shcs has to move the air volume to keep pace with the constant solar gain that would otherwise cause the greenhouse to overheat, For effective cooling, you should aim for a relatively small temperature differential between the inlet and outlet of the tubing and maximise the flowrate.
    Taking the air down to the soil temp may seem like the best idea but accepting less of a drop and moving more air is a better approach.

    If the air drops to the soil temp halfway along the tube, the other half isnt doing anything for cooling, or to put it another way, you`re only cooling half the amount of air you could be. If you accept 50% or 25% of the temperature difference between the soil and air as your shcs outlet temperature you can move 4x-8x the volume of air through the same tube.
    Be aware that doubling the cfm in the tube increases the velocity which can result in a 3 fold increase in losses. To minimise losses its better to use two tubes half the length (they have the same surface area as a tube twice the length) to move the same amount of air without any of added loss. If the shorter tube length can maintain the desired in/out temperature differential for cooling purposes, it doesnt need to be any longer.
    Getting even coverage of the mass, balancing the flow and positioning the shcs inlet and outlets can be more of a challenge with multiple short tubes.

  • waterstar

    Hi All,

    Hex2006, I simply do not understand the cfm as well as you. I was thinking of getting something like a 677cfm inline fan (based on the person I quoted in my post of Thu, Mar 6, 14 at 15:31). Your point of cooling only half of the soil is very helpful. Now, I am confused again by what size fan to get. (As a reminder I have a 10x12 HFGH with 120 sq. ft. of floor space and 1,044 cu ft. I am putting in the standard 4" tubes with the usual spacing...three layers. This will accomodate 21 tubes about 9.25 feet long (layed in a serpentine fashion) for a total of 194 feet that feed into two 12" ADS pipes as the plenum. Do you think my fan of 677cfm is good?

    Also, good news on the pipe we found on the dig. It turned out to be a connecting pipe, not a drainage pipe...so no drain field under the GH. WHEW! It rained several days, and I also ran the washer lots and no problems except considerable standing rain water (pic 1). After I put a fill of about 2 inches over the pipe it rained a lot more. Very little water standing (pic 2..on next post because I can't figure out how to put 2 pics in one post). We will still put a pipe in that can hold a sump pump in case we find we need it later...but don't think we will have to do so. What a RELIEF!!!

  • waterstar

    Here is the pic of rainwater after only little fill.

  • hex2006

    The 677cfm info came from testing different airflows with my shcs :) My gh specs are 154sqft floor, 900cubic feet volume which translates into 75cfm for 5 airchanges per hour , I have 200ft of 80mm perf pipe arranged as a fifteen armed octopus in a radial pattern (like a flower). Each arm consists of a13ft tube which loops out and back from/to a central plenum. My floor is circular so it seemed like the most logical layout.
    When i tested the heat transfer with different fan cfm it became very clear that at 5 airchanges per hour only a small part of the 13ft tubes were in use, even though i saw a 27degF drop between inlet and outlet.
    At 200cfm (13.34 airchanges/hr) the difference was still 27F, 400cfm (27ac/hr) it was... 27F.
    When i ramped the fan upto 677cfm (45ac/hr) the in/out temperature difference dropped to 23.4F which indicated the full 13ft was being used. At that point i ran out of fan power so i couldnt tell how much cfm it would take to get the inlet/outlet temps to match (ie; airflow needed to outrun the shcs) but it would probably have been 1000`s of cfm ;)
    You`ll get more efficient cooling by moving a lot of air with a 10F drop in temperature than moving a small amount of air with a 30F drop.
    Your greenhouse isnt a million miles away in terms of floor area and volume but rectangles dont lend themselves to radial tube layouts and not many would want the plenum in the middle of the floor ;)
    Looking at the numbers i`d say you could use a fan upto 1000cfm, a speed controller is a good idea as it allows you to alter the cfm through the season. A big fan running on a low speed is far quieter than a smaller fan running flat out, both will move the same amount of air.

    Note: anyone with a running shcs system can test this theory by comparing in/out air temperatures at different air flowrates. A system with a fixed cfm fan can be tested by blocking half the tubes in the intake plenum to increase the cfm and velocity in the other half. Be aware that the top tubes may be in warmer soil than the bottom tubes in a standard barrel plenum, which can affect the results.

    This post was edited by hex2006 on Tue, Apr 1, 14 at 4:25

  • waterstar

    Thanks! I'll get the larger fan!!!

    I was thinking of this type of layout, except I will put mine in a serpentine pattern to increase the air turbulence. I am confused as to how I would put this in the middle and still keep the tubes the same length. Duh.

  • hex2006

    The corrugations in the drainage pipe will create plenty of turbulance, The bends in a serpentine layout tend to increase pressure losses and make balancing the tubes more interesting, Wavy trenches take a bit more digging than straight ones :)
    I would use a layout that fits in with how you plan to use the greenhouse, no sense putting a plenum right in the middle if its going to be in the way :)
    In my case, the central plenum was the best choice. The highest point of the greenhouse is right in the middle of the roof. The central location takes advantage of the curved walls which act like a chimney so the warm air travels up to the top where its drawn into the inlet duct.

  • annalog_gw

    NOTE: We hit the limit for number of posts in a single thread. Continued in Subterranean Heating/Cooling System - Continued.

    Waterstar, good news about the water pipe.

    I have been trying different layouts on paper. Below is my latest draft showing a single level of pipes. There are two systems for each greenhouse, one for the internal heat sink and one for the outside. The outside system is intended for greenhouse cooling when the inside heat sink has reached capacity. Both return the air to the original greenhouse. The intake plenums are now in the north corners farthest from the doors.

    The outside systems each have a single exit from the greenhouse through the lowest level of the greenhouse foundation wall and two return entries. There will be underground plenums on each side of the holes in the foundation wall to facilitate tubing attachments at the different levels. We are planning on using plastic totes for the underground plenums. Estimated tube lengths is 16 feet. 6x16=96 feet.

    The inside systems will have three tubes on each level with a tube under each growing bed and one under the walkway. These will continue into the central shed area and loop back to exit on the north side of the greenhouse near the door. Since the foundation wall under the greenhouse doors does not need to support much, multiple levels can have sideways blocks. Estimated tube lengths is 22 feet. 9x22=198 feet.

    96 + 198 = 294 feet of tubing per greenhouse. The estimate from the SHCS calculator was 120 feet of 4" tubing for each greenhouse. I will use 4" for the outside systems and maybe 3" for the inside since the space is limited and the length is exceeded.

    The pipe levels for the east and west greenhouses will probably be offset by 6 inches in the central shed area. However, there are three places where the pipes for a single greenhouse cross. Should I put an air block between the pipes where the pipes are too close? How close is too close?

    Thanks in advance for your feedback.
    NOTE: We hit the limit for number of posts in a single thread. Continued in Subterranean Heating/Cooling System - Continued.

    This post was edited by Annalog on Thu, Apr 3, 14 at 7:27

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