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SHCS and Greenhouse Design FAQ's

Initiated on May 06, 2004 | Last updated 11/29/2007 | Problems?

Terms and Definitions

How

Operation

Sizing

Elements

Misc Issues

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Back to the Subterranean Heating and Cooling System page 



Terms and Definitions

How

Operation

Sizing

Elements

Misc Issues


Subterranean Heating and Cooling System (for solar heated greenhouses)
Underground Air Circulation Tubing (for a SHCS - 4" thin wall, corrugated, perforated polyethylene drainage line - commonly called ADS or Advanced Drainage System)
To put it in a nutshell, hot moist daytime air of the greenhouse is circulated through underground tubing and then back into the greenhouse cooled and dryer. The result is a greenhouse cooled in the day by soil that is heating up, and a nighttime greenhouse that is much, much warmer because of the high soil temperatures. It makes perfect sense, but we never discovered how great it would work until we did it in a number of projects and noticed that because the air is so moist and dropped in temperature, that the phenomenon of dewpoint occurs. Because it reaches dewpoint it "acts" as a refrigerator using the phase change of water rather than the phase change of freon.

First up, how does one cool air? What is  actually going on? From what I can see here, it's entirely based on the fact that greenhouses are our own living, breathing mini-micro rain forest that we custom tune to time shift the daily and the seasonal extremes. In other words, we are looking at the perfect model of a solar powered cooling machine - a mini rain forest we control - that can absorb the daily solar gains above ground, and induce them into cooling 'afternoon rains' underground.

In the solar greenhouse, the plants and soil give up their water as vapor, and to do that they must have some heat (BTU's - British Thermal Units) to do it. Some BTU's are used for directly evaporating water from the soil and plant surfaces, some BTU's for powering plant transpiration (the plants breath in the air, then breath it out as moister air - just as we do - chemical photosynthesis AND physical transpiration create vapor from solar gain - both processes consume BTU's). So, we have this air that is using up most of the BTU's from the sun by creating vapor. If we don't remove those BTU's from the air, then we will have to remove the air from the greenhouse. Old school says remove the air. Now, if its 90 degs F outside, and your house is hovering around 100/110, old school logic doesn't pay off much - the most cooling that is physically possible is to get it is the 90 degree temperature of the air you replace it with. Not much good, considering that if you're trying to  cool a 100 degree greenhouse with 90 degree air, it will take huge energy sucking fans if it's enclosed or you have to build a house that is virtually open to the outside. Anyway, maybe that is what has worked best for cooling. That would depend on whether a totally open greenhouse works for your production plans or not. If it doesn't then we are looking at huge fans if we go old school. Throw in the added expense of some fan$y evaporative cooling pads, and you might just squeak by.

So lets look at "new school" approach. First, the golden rule when your are moving heat through something. If there is no difference in temperatures within the model that you can take advantage of, there will be no heat movement. Heat only moves about if there is a "hot going to cold" pathway. In other words, to move heat you need to have a heat "sink". Old school knew that, so everybody started cramming cold massive things (heat 'sinks') into the space, using up expensive floor space in the process - trombe walls, concrete floors, rock storage, water barrels, etc, etc... with very little consideration of the fact that most houses are sitting on literally tons of heat sink in the form of the 3 feet of subsoil under and IN the building already.

Problem was, how to interface the sub-soil as an effective heat sink.  One method is embodied in the construction details of the SHCS - the subject of this consideration. When the initial thought to use the soil as a heat sink was floating around in the industry, most engineering types were thinking of conducting the heat of just the air into the soil. They were just expanding on the original concept of water barrels, rocks and mass in the space. 'let's just let them heat up in the sun, or by us moving hot air around them and that would suck up the heat...' 

So we have this thing called conduction as the old school starting point. Choose a good heat sink, and move hot air to it, and therefore conduct the heat to it somehow... Now, what was missing in this picture was that we are not looking at the ACTUAL heat gain engine in a hot, moist, for real greenhouse - the vapor cycle. The air itself doesn't actually absorb many BTU's, it is the water heating up and vaporizing, and the BTU's to do that are absorbed into the system as "heat of evaporation" contained within the actual vapor component of the air. So, looking at hot air conduction transfers we're missing the real point. There was some use in including conductive transfers, but not enough to ever justify taking up space in the greenhouse. What we actually need to do here is get back those BTU's used in the "heat of evaporation". 

Now, the interesting point here is that physics law tells us that it takes 500 times more BTU's to vaporize water then it does to raise it's temperature one deg F. And it tells us that if you condense water out of the air, you are getting back the "heat of evaporation" as "warm water". So we are looking at a potential cycle of heat transfer that is 500 times more efficient than conduction transfers alone! Of course, that number is never actually met in a system that can be practically set up - that's just the 'theory', but even if there is 10 times more heat transfer, it's still money in the bank. So.... after that bit of long winded pre-amble, let's move on.

If we are to zero in on using the "heat of evaporation" gains and take advantage of them, we have to somehow condense the water out of the greenhouse air. So we get into another concept - dewpoint. When water vapor reaches dewpoint, it condenses into a liquid, and in the process releases all the energy it took to cross over the evaporation barrier and become liquid water again. 

So what does it take for the air to reach dewpoint? It must be cooled to the dewpoint temperature.  To do that we need to introduce an opportunity for conductive heat losses that lower the temperature of the air to dewpoint, the point at which the air saturates, goes to the highest level of humidity it can physically maintain. Then we take it just 1 deg further. Bam! the vapor condenses, and the heat of evaporation is released, and it rains. Now look at this - the dewpoint is dependent on the humidity of the air AND it's temperature. That's why we call it 'Relative Humidity - RH'.  The higher the humidity, the less conductive loss is needed to get to dewpoint - i.e. the higher the dewpoint temperature.  That sounds like a greenhouse full of plants doesn't it - hot and steamy - meaning the conductive loss we need in a greenhouse to bring the air to dewpoint is minimal. 

Now for some rules of thumb - dropping normal ambient air temperature by 30 deg F will often bring it to dewpoint (the cold sweaty glass of water experience.) In a greenhouse that is much higher in humidity, that figure likely drops to 20 or 10 or even as little as 5 deg F. So how do you drop the temperature of vapor saturated greenhouse air by that much.? Well you are going to need something at least a few degree's lower than the dewpoint temperature. And what is the most logical source of this kind of heat sink? Of course it's the soil - soil seldom gets above 60 deg's F on its own. If it's at 60 deg F., then 90 deg F. saturated air should be able to get to dewpoint without too much trouble. So that is the "mark" to work with - max soil temp of 60 - 70 deg F., saturated air temps in the 80 to 90 deg. F. range. Sounds like a normal working greenhouse to me! Now, how do we get all that air intimate with the soil so that conductive loss does bring all the air in the greenhouse to dewpoint?

Enter the SHCS. All of the UACT installed at close proximity to each other and laced throughout the sub-soil gives us good intimate contact with the mass.  How much soil to interface is the next question. So some more physics... It takes one BTU to raise the temp of one pound of water one deg. F (close to the same for soil mass). Loose, moist soil weighs in at about 76 lbs/cu.ft., so a yard of it is about 2052 lbs. So, it takes about 2000 BTU's to raise one yard of soil one deg. In other words, for every square yard of surface, the current SHCS specs (3 layer matrix interfacing with about 3 feet of soil) can suck up 2000 BTU's or about 222.22 BTU's per square foot for EACH single degree change. A 20x30 foot greenhouse (600 sqft) would have to absorb 133,332 BTU's before the soil temp raises one degree! Just in case you need a gauge, a normal household water heater uses 30k BTU's per hour - this represents the same heat transfer as you'd see in a gas fired water heater running for about full tilt for 4 1/2 hours. Now, experience has shown that the soil temp raises at least 20 deg. above normal in a summers season using the SHCS. That represents 2.67 million BTU's of accumulated heat tied up in the soil, the equivalent of a hot water heater steadily running full tilt for about 89 hours (. Looking at cooling only, that translates to 2.67 million BTU's of cooling effect. (not to mention the obvious perk of having all that water vapor recycled back into the soil as warm sub-irrigation for the plants!)

If the daily solar gain adds more than 1 deg of change in the soil temperature, there still could be losses that same night.  The important thing to remember is that EVERY day there is an addition as long as there is a potential of solar gain but the loss each night is not necessarily as large as the gains every day.  Each day that there is a 1 degree change in the soil temperature, you have gained this hypothetical amount of BTU's.  Every day that you do that represents money  in the bank as cooling benefit, and each night that the losses are less represents money in the bank as heating benefit.  I believe that these systems can easily do that 1 degree shift in soil temperature every day for at least half the year on average.  In this example that represents 180 days x133,332 BTU's = 24 Million BTU's of heat transfer.  At the end of the summer season you will be going into the winter with most of those gains stored as elevated soil temps.  For every cold winter day that the sun shines brightly into the greenhouse, you store enough heat that day to make up for that night's losses and therefore don't draw on these reserves.  You only draw from the summer's stored gains when the winter sun's gains are less than the night's losses.

Let's look at some cash flow statistics for a 600 square foot solar heated greenhouse.  In 2004 in Colorado, 1 million BTU's of Natural Gas costs about $8.50. For every degree of soil temperature increase, we are pumping the gas equivalent of $1.13 underground. Do that for 180 days and that represents $204 in accumulated gains.  A single 20F rise in total soil temp represents $22.60 in Nat Gas potential energy stored from the sun. This is not an aggregate total though. The energy is being added and subtracted on a daily basis. The 20F increase is easily achieved in 3 months, so, taking in account the available gain for the year, you can be sure that the system has moved around 2 to 3 times that amount of energy (up to $612 per year in gas heating stored in the soil) and made it available for night heating and better soil temperatures. From these numbers, we can see the significance of raising subsoil temperatures. This is not theory, I regularly see soil temps elevated 20F in SHCS greenhouses. If you build a simple hoop double inflated greenhouse for $4/sqft, say a 20x30 600sqft GH ($1500 delivered for frame and skin), the gas energy absorbed each year represents almost 1/2 of the cost of the greenhouse that is being "banked" in the soil as heat you'd otherwise be paying cash for to get the same high quality growing environment.

So, the numbers so far:

  1. - keep the soil temperature and saturated air temperature differential high - in the 20-30 deg F range
  2. - interface with the subsoil for about 3 feet deep using a 2' OC horizontal/1' OC vertical UACT matrix
  3. - move all the air underground at least 5 times per hour and dump it back into the space AND keep the air speed in the tubing at least 4 feet per sec. 

If you can do this, then you can expect dewpoint, and if you can achieve dewpoint, then the system pays for itself quickly. 

If you just barely get to those kinds of figure's in your design, then you still benefit - because the soil and the plant roots will warm up from conductive transfers, a solar interface to the sub-soil remains a huge bonus even without a perfectly optimized magical vapor phase changer - your plants will still grow much faster AND you still get a longer growing season than if you'd done nothing.

Overheating of the soil? Not an issue. Remember, heat only moves from "hot" to "cold". In order to heat the soil to undesirable temperatures (>90F.) you'd have to have the air temps in the 110-120F. range. At which point, it is the air temperatures that will be your problem!  But because of the dynamic we are working with here, the hotter it gets, the more efficient the phase change action works.  Only when the soil reaches a null point of about 75 deg F does the effectiveness drop off.  But that only happens when there is little need for night heat, so you don't need to store it anyway... just vent it out for that period anyway.
Venting? Yes, you will have to vent, but only to manage your humidity and temperatures when they start to go beyond the numbers that work for a SHCS and the plants. Vent to prevent temp >95/100F. or humidity >95/100RH. Modulate the venting to keep the humidity and temps high, but use the SHCS to cool as much as it can before you vent. Too much premature venting will dry out the space, impact the performance of an optimized SHCS cycle and stress the plants.

It should be clear that there will be control for some portion of the summer. If your summer net gain period is beyond 3 months and your winter net loss period is less than 3 months, then the system will be performing well for you but you will have very little mid-day late summer cooling potential. You will have to vent to more to get by.  It's usually not so hot by then, so it's not normally an issue. But, and it is a BIG but, do not fall for the shortsighted conclusion that the SHCS solves all your heating and cooling issues. Conventional, controlled and moderate venting will still be necessary.  Besides, we've been discovering that it is the air conditioning effect on the space AND the soil that is incredibly useful all on it's own. The plants simply thrive in magical ways because of the warm moist air at the root zone. That is a factor for you to keep well enough in mind. Of course, with plants in pots or on benches, that effect is less, but the warmer 'floor' still has value to you as radiant heat affecting your production positively.

On another tact, say all you want do is cool all year, you have virtually no heating needs. How about adding UACT outside the perimeter of the greenhouse. Whether the cost is going to work, I don't know yet, but I've some projects on the go in New Zealand where we're testing the installation of extra tubing outside the perimeter of the greenhouse. Once the soil matrix in the building is up to temp, the circulating air is going to be directed to the matrix outside, permitting a longer period of the year that cooling can be achieved with out the daily gains accumulating inside the building. Combine this with careful use of shade compound to limit gains and using open structured greenhouse design for maximum venting, you could have a winning combination for super cooling without needing to vent excessively.

So, you're trying to optimizing for cooling, not heating. [see section just before this one] Any cooling emphasis in the design will, of course, show up as effecting the heating performance of the space - that is just the design - transfer the heat as condensate underground and the space cools, but the soil also heats up. So, that being the case, I am going to say here, right up front, that this is new territory for me. My designs have always focused on keeping greenhouses from freezing. I was a Canuck hobbit in another life after all. It just so happens that the cooling effect was a critical bonus, but not a design focus. That being said, we are going to have to step back from the concepts I've focused on for heating emphasis, and see if there are any factors that are in the picture that we can dance with that have a cooling emphasis. The first one is to open the greenhouse completely. There is no use fighting the heat gain if effective design can allow you to easily open and close your house, preferably automatically. The second thing you could do is add a small and simple automatic misting system to absorb every little wee bit of the solar energy - especially when the greenhouse is completely closed. The extra humidity can be vented or condensed as stored heat with the SHCS. Some are now experimenting with extra Underground Air Circulation Tubing (UACT) outside the perimeter of the GH, installed and used only when pure cooling is required. That way they do not have all of their heat sink (and heat storage) inside the GH.

The ultimate answer for enclosed greenhouses looking for cooling efficiencies is to control the glazing exposures. By limiting the summer light only to strong enough levels for growing, most of the heat gain can be avoided. This is being accomplished with:

  • painted on shade compounds
  • shade cloth
  • planting arrangements that shade the house deciduously in the summer
  • mechanical shutters
  • adjustable parabolic light accumulators
  • bubble filled double skinned glazing

For extra cooling effect, combine interior shade cloth with an automatic mist cooling system.  Be sure to direct the mist cooling to the zone above the shade cloth and try to vent directly from this zone as well.  Think of it as cooling clouds passing by the canopy of the forest below.   

You need to move all the air in the greenhouse through the UACT at least 5 times an hour.

Measure your space, calculate the total volume (length times width times height), and multiply that number by 5. That gives you the total volume of air your fan has to move every hour. Divide that number by 60 to get the volume of air to move in a minute. That is the standard measurement used to size any fan and your fan in particular - Cubit Feet per Minute - or cfm... or you can divide the total volume by 12 to get the same number because you are also moving all the air underground once every 12 minutes. When you look at the performance chart for the fan you have in mind, it will indicate the cfm at the different air pressures it can generate. Choose the second highest cfm figure in the chart, it indicates the performance if generating only a wee bit of back pressure. That should be what the system will be like if built as we design them. 

We recommend using 'in-line' duct booster fans exclusively now. The common low volume 'squirrel cage' fans we used to specify used shaded pole motors, different than straight in-line fans.  It turned out that when you close the vents on a tight house with the SHCS running, that the squirrel cage blowers style fans feed air pressure back onto themselves from the exhaust outlets of the SHCS.  In the process, the oversped their normal running speed and by design, also drew more current.  The extra current resulted in high motor temperatures in an already high temperature environment and we were getting fast motor failtures.  If you happen to be using a direct drive squirrel cage blower with a shaded pole motor, you should have a qualified electrician test the current draw of the fan with all the vents, windows and doors closed. Operating this way, the fan is essentially back pressuring itself and will tend to overspeed. With shaded pole motors, this can lead to over drawing of current. You or your electrician can control excessive current draw by restricting the intake on the blower cage till it is just below the name plate rating on the motor. If this tuning procedure is not performed, you could burn out your blower motor prematurely.

The system works by moving a lot of air underground at relatively low speeds inside relatively small tubing. Contact time and surface area is the secret. It doesn't matter that you can move a lot of air with bigger fans and bigger pipe. The consumer 'more is better concept' does not apply here at all. It is the amount of air that has long contact times with the soil that works. Experience has shown that when you do the numbers, small fans reach that limit easily, bigger fans are never needed. It relates to just how much tubing can you practically cram into one barrel plenum or culvert plenum. Remember, all the air is circulated at VERY slow speeds through MANY short tubes. The end result is that a lot of air gets moved but not the kinds of movements that are required for your typical fan powered household ventilation systems. We are talking here of only moving the volume of the greenhouse underground once every 10 to 12 minutes with virtually no back pressure because the speeds are so low. Larger fans simply don't work out well. A 750 cfm fan is just about all the air one plenum can handle if it is completely crammed with tubing - works out to about 10 cfm for each tube. With a bigger fan the tubing air velocity gets so high the air wouldn't be in contact with the soil long enough to go to dewpoint.  

The only practical way to use bigger fans for massive greenhouse projects is to use Rock Beds and no UACT at all.... Those kinds of designs are just now being considered by a few engineers, builders and architects.  Hopefully I will be able to share the specifications soon for managing such designs.  To date, we've designed systems for managing 60,000 square foot greenhouses in the west using rock beds only.  They are working out with 5,000 cfm fans... but I feel we can do it by using the existing ventilation fans already standard practice for venting in the west.  I'd like to see my designs for using the existing fans to drive the venting when needed, but also cycle them in one at a time to run the SHCS when it's appropriate.  It's all just a matte of placing them properly and using damper control to direct the air into and through the house or under and then back into the house.  Email me if you are interested in more up front on the progress with these kinds of considerations.

Thermodynamic losses would seem to point to entirely insulating the bottom and around the sides of the SHCS soil zone. However, the practical costs of doing so may not be warranted.

Insulating under the SHCS soil zone would give some more control of the heat entrained in the soil after a summer season of charging it up. Insulation around all of the soil mass would slow down the losses to the deeper subterranean soil. The heat loss to that zone would be a factor when the SHCS soil temperature zone was getting higher than the ambient temperature below it and on the perimeter at the sides. But keeping in mind that for the movement of heat to be significant, the temperature differential has to be large as well or the energy doesn't move. The soil temperature in a typical Western Zone 4 SHCS system will not get much higher than 75 degrees and the ambient soil under the tubing zone is typically 55 deg F. Therefore I'd say that the migration below the SHCS zone will be minimal - there is only a 20 deg F. differential to drive the process through solid mass. However, the sides of the soil mass can have upwards of 80 to 90 degrees differential! Therefore the likely place to use insulation to best advantage is the sides. Using it under the soil mass would do something, about 1/4 the effect of doing the sides, but typically 6 times more money to accomplish  - that cash would be best spent on insulating the sides only. As always, the biggest exposures - the glazing, foundations and walls - should get your attention and capital investment for minimizing heat loss. Use double skinned glazing and standard wall and foundation insulating methods to control your looses there.  And watch your construction detailing... an air-tight greenhouse is a winner greenhouse.

The depth question.... I am always reminded that telling folks what it should be in their specific case seldom comes to advantage considering the ease of misinterpretation that's so prevalent in email. Perhaps I would be better to describe what is required first, then see what the final verdict is after we have the tubing drawn into a design. After all, the design must work, and the numbers are only guidelines.

We are going to try to put in three layers of tubing. First, we have to start by keeping the top layer of tubing free of hindrance from the gardener's activity - measure your pitch fork and garden tools and make sure any planter located tubes are safely buried below that depth. Secondly, the purpose of the tubes is to chill the humid greenhouse air to dewpoint - so keep them surrounded with enough thermal mass so that they are likely to be cool all day. That means the depth of the top pipe depends also on the density of the soil. Peaty lighter soil would have to be deeper as it can heat up quickly from the system and the sun - for clay sandy soils, the pipe could be higher. Our experience shows that heavier subsoils continue to act as good heat sinks if the tubing has 6" of subsoil above it (8" OC from the surface) You should go deeper if your soil mass is lower. Then there is the relative distance between the tubing in the soil matrix... That too can be affected by the soil type, but generally we are getting good results with normal density subsoil by placing the tubing 1' On Center (OC) vertically and 2' OC horizontally. If the available installation zones in the house is insufficient, then you can decrease the 2' OC horizontal spacing to as low as 1' OC and still maintain your heat sink cool enough. Increase or decrease those numbers depending on the relative density of the subsoil (I'd consider road base the densest soil type that is dig-able, and rich pure organic and peaty the lightest soils.) Using these guidelines, you can make a good guess at just what the deepest tubing layer will be.

more on depth....

How deep? It makes sense that we would keep the pipes as close to the surface as we can but still influence as much tonnage of soil as is practical. A reasonable depth to influence below the root zone is 2 feet. Therefore, the deepest we would ever need to go is 3 feet, one foot to stay clear of the root zone so that gardening tools are not going to make a mess of the tubing and two feet of subsoil to act as the heat sink. You could go much more and put in much more pipe to extract much more heat, but there is a ROI that is a factor. In Zone 4, we've found that 2 feet of effected subsoil works well enough for our investment in tubing and other equipment.

The 3 foot deep guideline represents about a ratio of space with the rest of the greenhouse of about .33 parts soil, 1 part air for a normal average greenhouse height of say, 9 feet.  Lower that height to something like 7 feet, and we have about .43 of a cubic foot of soil for every cubic foot of air we are trying to influence.  The higher the ratio of soil to air, the better cooling and heat stabilization we can achieve.  Picking about 1:3 seems to work just fine for now.  We can move the ratio in the right direction by lowering the roof or putting in more tubing deeper.  At some point we'd run out of growing space or money for more UACT, so for now, it's 1:3  to as low as 1:2.  

To install the tubing, I usually just rent a trencher for half a day, rough cut the trenches to the projected depth and bring it back. That only takes a half day rental ($120/day here). Make sure you get a 6" model that can cut as deep as you want. Then excavate the trenches by hand with trenching shovels and backfill each layer as it is placed. 

If your soil can't be trenched or is too heavy or useless to use for planting, you can excavate the entire mass and then backfill it with a better choice for soil. This approach almost always requires that you have equipment and an operator on site for the duration (or a neighbour who can come over for many sessions with a front end loader.) You can then lay in each layer in a pattern that allows them to be backfilled with the excavating equipment. Plan this part well - you can't drive on the tubing until it is sufficiently backfilled. It is best to never drive on top of any of it. Try to backfill from the edges, reaching in as best you can with your equipment.  

Use underground 18" ADS culverts as as plenums for each fan system - one at each end of the house or system, exhausting at one end of the culvert, and blowing air into the opposite end on the culvert for the other end of the system.  You can cut just one hole in a barrel plenum and put it over the end of the culvert for your exhaust plenum and fan plenum.  Then cut holes for the UACT to fit  into the culvert soil-tight.  Cut all of your tubing exactly the same length and lay them into each trench or at each back fill level as you go along.

The UACT can be below the surface in the paths and then come up a little higher into the planters if your plan indicates where they are. The last one I did like this, I put in all the tubing right to the plenum and under the paths and as deep there as I could manage. Where they were under planters, I trenched a little higher for the bottom layer and left the top two layers loose on the surface. Then I created ferrocement planter walls 24" up from the floor, surrounding the loose tubing. When the walls were done, I could backfill the planters with amended subsoil, leaving the second layer of tubes on the undisturbed grade of the floor. Then with better growing soil, I filled the planter with the top tubes suspended about the middle depth of the planter. Take a look here.

If the existing soil is heavy and has enough aggregate for drainage you can use it safely. Too much clay content is the worst choose, it will not allow for proper drainage. Choosing to do an install by excavation and then backfilling with a different material of choice best suited as a heat sink, I'd go with "road crush" as your backfill (a mix of all the grades of aggregate from clay to gravel). It is the best mineralized material for perennials to benefit from, is very massive, and if you have no heavy traffic on it and do not compact it, is has good heat and balanced water holding and drainage performance. We have done this in the past and been very happy with the results as far as cost, availability and performance for the SHCS and the deep rooted perennial plantings. Pea gravel is also a good choose. When backfilling planting zones, just make sure you use materials suitable for plants and do make sure the soil has good drainage performance.
This is a misunderstanding, for which I would appreciate your indicating, if you can recall, where it was you read this section. Under no circumstances have we suggested to or did fill the pipes with anything!... I am sure that it is a poorly crafted language issue at work here. Be assured that your suspicions and concerns are warranted. Air tubing moves air and indeed is not going to do that if they are full of aggregate... or anything other than air for that matter.
The design goal I work with for trencher installs is to develop a three level matrix of tubing that is 2 foot on center horizontally, and 1 foot on center vertically. More space between the tubing would mean you'd not be influencing all the soil in the heatsink. Keep in mind that soil conducts heat poorly. The tubing has to be close together to influence all the soil as a mass. Spreading it out is OK, but you will not be raising the average soil temperature much. It's a balance - if you go too close, the soil temp raises too high too quick to be effective in the latter part of a charging cycle (high soil temperature reduces the cooling effect). If you go too far apart, the charging cycle gains do not raise the soil temp enough for night time benefit. Zone 4 works best with the three levels -  2'OCH x 1'OCV matrix. Go here for a free online calculator for your system.
Here are the numbers we've been designing to for most of our systems in the western states at Ag Zone 4:

1. You have to size the fans to move all the air of the greenhouse underground at least 5 times an hour (the fans have to move the entire calculated volume of the greenhouse in Cubic Feet per Minute every 12 minutes - divide the entire volume by 12 to get the minimum cfm figure)

2. There has to be enough individual tubes so that the back pressure on the fan is as close to zero as is possible. The starting design goal is to have the air moving down each tube at a minimum speed of 4 feet per second. You can go 2-4 times that speed and still extract effectively. The total time all that air spends underground in a charging cycle is more important than anything. Obviously it can't be underground all the time or it would never heat up... There is a balance between how much time it is above and below. Start with at least 4 f/s and increment upwards while monitoring the performance and your fan power needs. Somewhere you will find a sweet spot where the gains are not negated by the cost of extra electricity.

3. The tubing has to interface with as much of the subsoil fill as is possible. A general layout to design for with a trencher is to put in three layers of tubing. Space the trencher 2 feet On Center horizontally and then place three tubes 1 foot On Center vertically. Note that you need to adjust this figure according to item 5. below.

4. The individual tube lengths should all be as close as possible to the same length.

5. The combined total length of all the tubing should be approximately 1/3 longer in feet than the total square footage of the greenhouse (i.e.: 6 feet of tubing for every 4 square foot of floor space.) If this number is smaller than the initial layout in 3. above, leave it the same, if not, try to add more tubes to get close to that amount of tubing.

6. Stress the system with heat and humidity, watch you plants for the signs of stress. Vent only to prevent plant stress (temp >95/100F. or humidity >95/100RH.) Modulate the venting to keep the humidity and temps high, use the SHCS for your cooling as much as possible (store heat) as much as it can... then vent. 

 Go here for a free online system calculator that is based on this design guide.

Re: soil sock for ADS (Advanced Drainage System) tubing and other similar perforated corrugated thin wall poly tubing)

This material is available two ways: with a sock already on the tubing or the sock as a separate item that you need to apply yourself. The latter is a real pain, so if you can, get it already on the tubing.

The sock material is supposed to be an option available at the tubing supply house. If not in stock that way at the suppliers, then ask them about a special order for the sock or tubing with the sock pre-installed.

BTW, you only need the sock if your soil is VERY dry AND sandy during the install. The SHCS will keep the soil structure intact by reason of the high levels of condensed water vapor that it introduces. The key is that when working, the SHCS will saturate the soil enough to keep it from crumbling through the perforations. As such, there should be little need for the sock being used to keep out the soil EXCEPT possibly during installation.

Infiltration of soil and debris into the tubing through the perforations isn't normally a problem BUT proper screening at the blower plenum and exhaust bunkers after the installation is still very, very important. Turns out that the tubing is perfect breeding for pack rats and such. And they WILL mess up the system. And don't forget - the tubing arrays must be tuned after installation and periodically through their lifetime (to be sure, vermin still find their way in, and occasionally tuning them will indicate that kind of situation clearly). Each tube should be handling roughly the same amount of air, any that are using significantly more should be crushed closed enough to encourage more air movement in the slower tubes.

The tubing ends are exits for the conditioned air AND burrow entrances for every varmint that comes along too. So.... of course, they must be screened individually or collectively if you use a bunker. No smaller than 1/4" screen though... and the same for the fan end.
Don't even think about putting in your own holes. There are way too many required. Try to find another supplier for the pre-perforated option. And the perforations are very important to prevent any and ALL puddling. Puddles could lead to a breeding vector for nasty organisms that moist soil alone cannot support.

Using 3/4" or larger crushed clean rock is currently not recommended EXCEPT in the bottom of the foundation drainage lines to keep the water levels below the bottom of the deepest air circulation tubing. The current thinking is that using it to backfill the tubing will limit the most important function - heat transfer. A crushed rock fill could limit the intimate contact of the mass to the tubing. The amount of air entrained in rock fills could also act as insulation as well as prevent it from entrapping moisture for the use of the plants in the greenhouse.

On the other hand, if you did use such a material in the trenches, it could be of advantage. Because the tubing is heavily perforated, the air will be escaping into the spaces between the stones. If it does that it may cool better and condense out the moisture better. Currently we don't know if that is a benefit in the overall performance. It would take a pioneering experiment, doing some trenches this way and checking the air coming out those tubes. Comparing the measurements with a control set of regularly backfilled tubing would tell the tale.

Another option for backfilling is pit run screened to no bigger than 1" rock.  This should infill denser than crushed rock and hold more water.  It is usually the cheapest fill too.  However, it is important that this type of fill not have heavy traffic on it.  It does compact to concrete densities if you are not careful.

Ferrocement is my recommendation for constructing planter walls. Ahhhh, ferrocement a mystery for you... no worry, it is an easily shattered mystery. It's quick, simple, dirt cheap, massive, bombproof and very compact. And it's a breeding ground for a skill set that simply does away with a lot of construction problems that the usual farming environment simply would be much better with. Wooden construction is universally a solution only to carpenters. Just because you're a hammer doesn't mean everything has to be a nail... In case you were wondering, I've had a lot of bad experiences with wooden walls in greenhouses. So far, all have been disappointing ultimately.

There is a photo tour of the Distro Greenhouse Construction process of ferrocement walls on the site (Distro Greenhouse with Ferrocement Walls.) Check it out. Then do a Google on ferrocement. Ferrocement is the answer for permanent walls, benches and such in greenhouses.

Yes, it is possible to talk to me! And I am thankful for any serious interest in the SHCS for solar heated greenhouses.

I work in the realm of the SHCS design in my spare time, intent on spreading the good news of our success so that more experience from others would help the understanding of these systems mature in the public domain.

First use the Contacts page for email.  I can be reached via the Forums too.

My spare time is not that abundant, but if you do need to talk on your nickel, be my guest. When I am available, I can be reached directly at (970) 215-4710.  But sorry, I can really only receive and return calls to established clients of Going Concerns Unlimited.

I don't normally charge for my time if you call as a existing client. So you will have to get used to me simply helping you out on the phone for free. It does happen, even in these daze...

Call me on your nickel, my schedule. If I am not available, you can leave a message when you will call again, and I will try to meet your needs. If there are any basic questions you'd like answered from here, then fire away. I am always available via email...

Go here for an outline of what I provide. 

Currently I am fully engaged at < href="http://www.strawjet.com"Strawjet Inc so can only consider production work for public domain projects. In the past, the basic plans package included:

  1. Custom Floor Plan Layout with the locations for the trenches and stacked layers of UACT, plenum, return air bunkers and any noted fixtures included (please provide dimensioned sketches to get me started.) This drawing includes the total amount of UACT and the number of tubes.
  2. Electrical Schematic with the blower(s) and two thermostats laid out for night and day operation (with two speed fan circuits if that is what is being spec'd.) I will specify the exact fan and thermostats as part numbers from Grainger. I include the performance specifications so that you can source the hardware from your own resources.
  3. Blower Plenum elevation and description so that you can build you own.
  4. Greenhouse Side View Elevation above and below ground to show the typical spacing of the UACT matrix in your soil.
  5. Spreadsheet Schedule of your project's tubing and fan size calculations. You can see the design goal for the air speed in the tubes and time spent in each one. You can use this data to compare with your actual performance after you've built the system.

As little as $100 up front and I can put the necessary time aside to do this work for you. I need some time discussing your specific details and some dimensioned sketches or drawings from you of your project. If your project is complex and involves more time, I will let you know that before I begin.

My fee Schedule escalates as commitment of time and energy to a project grows:

  • $25/hr for plain ordinary time discussing details of job we agree to work on together.
  • $45/hr for time spent producing client materials for a job - drawings, schedules, schematics, making and assembly of parts and products...
  • $75/hr plus expenses for on-site directing of work - doing any of the above plus whatever else that needs doing; supervision of contractors, managing laborers, scheduling etc.

For simple tubing layouts I do, it takes me about 2 hours of drawing time plus an hour of basic interchange and considerations, fact gathering etc. That comes out to $115. If the scheme is brain dead simple and straight forward (single plenum, one house, etc) I am usually satisfied with $100. I have to get into the project before I know just how much real productive time I can provide that will truly accelerate things for you (or how much time I will be spending sorting out my own understanding of it. There's no use charging someone else for my education if that is clearly all that is happening...). Currently I am fully engaged at < href="http://www.strawjet.com"Strawjet Inc so can only consider production work for public domain projects.

Scan and email 'em if you can. If your scanning software can create black and white gifs or compressed tiffs, then the resolution would be the best for sketches and drawings. JPG's are ok too, but not the best for file size/resolution issues. If you can keep it all below 500k my tired old 49.33k dial up shouldn't have a problem. You could also take a digital camera picture of your sketches and send them along as digital files.

Otherwise, you can mail them to me at:

John Cruickshank
5765 Colver Road
Talent OR, 97540
The perimeter insulation...? Yes, the SHCS does need a thermally isolated mass of soil under the greenhouse. If you are in Ag Zone 5 or 4 you can dig a perimeter trench and put 24" of 2" thick foam board vertically, or you can place it backfilled horizontally with a 1/4" per foot slope around the outside of the building. Either way is ok, and your call. Obviously, the horizontal install is a lot less digging. Keep in mind that trenching near your hoop house posts WILL loosen them! If you are doing a horizontal install, take a look at the Frost Protected Shallow Foundation specifications manual for details on how to do it. Just make sure you don't use white bead board! It is not the same as "blue board" (closed cell Extruded Polystyrene Foam.) Bead board is not designed to work underground. If you are going to pour a concrete stem wall, you can trench the foundation location and use the foam boards for the outside of your forming.
Were you thinking of square galvanized tubing? It is the framing material of choice if you can locate it. Check out Allied Tube and Conduit. They make virtually all galvanized tubing for greenhouses. Greenhouse manufacturer's usually stock it, metal dealers seldom have it in retail quantities, but they might in your area. Talk to your metal fencing dealers, they buy their round galvanized tubing from the same manufacturers who make the square tubing.  They should be able to look after you in their next factory order.  I use 1 1/2" square 16 gauge and 2" square 14 gauge to great advantage here. That builds a very solid investment and is a piece of cake to assemble in quite complicated forms with nothing more than a 4" portable metal cutting bandsaw and a good 1/2" drill and self drilling #10, #12 and #14 self drilling hex head fasteners. Touch up the cut zones with galvanized primer and you are good to go for 100 years or more... and if you have a welder handy, you can go a long way to building a custom whole greenhouse frame. Take a look at some details on my last project in Colorado. And videos of my last project in Alpine, Arizona, at 8500 feet, high in the snow country...
Lots of steel in that one!
For power, at least one 20 amp GFCI circuit for a SHCS with a single fan (may be less than 5 amps at 110 VAC. Inflation blowers for double skin films draw approximately 1/2 amp at 110 VAC plus what you need for the fan to move all the gh air underground at least 5 times an hour) That could just be an underground branch circuit off of an existing panel as long as you have a disconnect at the greenhouse. But you can install a complete 220 VAC split phase system if you like, just be sure to run a Neutral conductor out to power up your other 120 volt devices. If you are considering an independent photo voltaic solar system, you will need at least 240 watts in panels and charge controller, at least a 300 watt 110 VAC inverter and a 50 AH battery.

Yes, there is at least two things you can do that I know will improve performance post construction.

  1. Use a vapor barrier under your pathway surfacing. By limiting how much evaporation of moisture occurs in the pathway at night, you limit the uncontrolled night time migration of heat from the soil to the air. Uncontrolled losses this way will dissipate heat unnecessarily. Typical pathway sizes could mean you'd be preserving as much as 20% of the heat loss from the entire exposed soil surfaces. 6 mil poly sheeting under your paving will stop the vapor migration from deep below and keep them dryer too. Make sure you perforate the plastic to allow for water to move on through. You can drill holes in the roll before installing. You shouldn't need more than one or two holes per square foot.
  2. Tune up your UACT. If your tubing lengths are all different, they will have different amounts of air moving in them. The shorter tubes will have the least air resistance and will take more air than the rest. Each tube should take the same amount of air so that your tubes are all doing the same amount of heat exchange. You can use a wind speed manometer to measure each one. By pinching the ends shut on the faster ones, you can balance the whole system so that each tube is doing all the work it can do.

And the best way to be sure is before you build!!  Use the free SHCS Design and Payback Calculator  and design your system with underground culverts as manifolds so that each tube is EXACTLY the same length. Here's an example.

Yes, the air is circulated, with the return air near the exhaust bunkers close to the ground, scrubbing the plant zone with dry, cool air. In really larger houses though, this effect is minimal. The air moves back to the plenum rising quite high and only slightly turbulent. You will find that large houses can use circulators to advantage if the ceilings are high and benefit with extensions to the pan inputs so they draw into the SHCS from near the top of the house.
In small houses, the shading effect of a large vertical duct in the middle is too much of a shadow. We are growing plants aren't we? And we've found from smoke tests that in small houses (less than 1000 sf with low ceilings) there is enough of a tumbling and turbulent effect to mix the hot air effectively so that the heat is circulated without raised intakes or ceiling fans. Bigger houses will benefit with raised intakes. Shading and suction losses at the blower have to be taken into account though.

In higher, larger houses, high intakes are recommended. Costs sometimes need adjustment in very high and large greenhouses (over 3000 square feet with 8' side walls.)  If your system is this large, the cost of the SHCS gets a little scary, at least the cost per square foot starts creeping up too high.  In such cases, you can trim down the size of the various package elements - but be sure you use higher temperature inputs.  For large volume systems I recommend putting in a large diameter inlet duct that is accessing the hottest possible free air source. And paint it flat black if it's in the sun. And we don't do the 'octopus' tubing installs to a single buried barrel plenum! Stick to buried culverts as manifolds, and put the barrel plenums at opposite corners. Here's an example.

No, using heavily perforated tubing, and 4" size, the soil microbes influence the tubing zone so intimately that 'bad' mold doesn't have a chance to get established, there is too much diversity for any one variety to take over. Circulating organically laden air might be different though - so air from composting and animal housing might be a problem - it could provide the nutrient to get something going that is unusual in normal soil strata. I've never seen problematic mold in normal scenarios.
Composting in greenhouses is far too tricky to manage for the inexperienced. Stay away unless YOU understand the process - ammonia and methane in a greenhouse will certainly attact the plants, building, equipment and environment not to mention the soil if a SHCS is in operation. Great in theory, but one of those "buyer beware" situations... at least vent the composting gases outside.
You could use the top layer of tubing as a conduit to run some drip lines in. But flood irrigation through the entire system wouldn't work - too many perforations. And you could be slowly migrating the surrounding soil into the tubes.
You can do the numbers yourself. If designed accurately, you can expect to raise the soil temperature to 65-75F by the end of the summer. That is the soil temperature of all the soil in the greenhouse to a depth of 3 feet!! You can expect to keep it cycling in that range for 3 months after the summer season before it begins to discharge seriously. That represents an enormous influence on the zone above it. For a 30' by 20' greenhouse I believe that is in the neighbourhood of 75 tons of soil and represents several million BTU's of heat. How this all pans out for season extension is a guess, but I assure you, it will extend your season!! For Zone 4, we can grow Eliot Coleman style in simple hoop houses for 12 months of the year using just a SHCS. Adding insulated north walls, double skins, propane CO2 generators and tighter building envelope we can grow tropically well for 10 months.
I normally recommend thermostatically controlled CO2 generators for supplemental heat. They are not vented, adding humidity and extra heat as well as the CO2 the plants will need when growing in a closed winter space. And do install a Carbon Monoxide meter and Oxygen sensor with a digital ppm readout and alarm to be safe! Some applications in some jurisdictions
    require
forced air heaters that can melt all snow fall to prevent catastrophic collapse.

Discussion Forums

Two forums are currently available.  

  • Go here for General Permaculture discussion relating to all matters connected habitat design and optimization including SHCS systems and solar heated greenhouses.  
  • Go here for specific SHCS discussion involving some of my clients

Costs

  • Go here for a fee schedule for my services
  • Systems costs are outlined with the free online SHCS System Calculator
  • Basic 2006 ballpark for the hardware costs of a SHCS - $200 per 1500 per square foot of greenhouse space PLUS fifty cents per square foot.

 

 


Going Concerns Unlimited specializes in designing
  Subterranean Heating and Cooling Systems 
for solar greenhouses. 

John Cruickshank at Going Concerns Unlimited

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