Subterranean Heating/Cooling System - Continued

annalog_gwApril 1, 2014

This is to continue the discussion from the previous Subterranean Heating/Cooling System thread.

This post was edited by Annalog on Tue, Apr 1, 14 at 18:41

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Hi Anna,
I will be interested to see what the folks say about your lay out plan. Hope you are on spot!

    Bookmark   April 2, 2014 at 1:47PM
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Thanks, Waterstar. I think this plan is very close. My biggest concern is now the tube crossings. I could eliminate the ones from a single greenhouse by adding one more exit plenum on the south side of the greenhouses, nearly matching the north but with only the south lines while the north exit plenum would have the center and north lines. However, then all three lines would need to leave the greenhouse space through a 16 inch space in order to make room for the additional plenum. But since that would be at nearly the same distance for all the tubes, maybe that wouldn't matter as much.

My plan to have 1/2 inch hardware cloth below and above the heat sinks was confirmed Tuesday when I turned the soil in the raised bed near the chicken coop -- two underground nests of field mice! I don't know if they moved in during the winter when the bed wasn't being watered or earlier when I stopped cultivating as much after planting a perennial on each end of the bed. It may be that the mice killed the perennials and not the early cold snap.

While the lower level of hardware cloth will be permanent, the upper level will be removable if needed.

    Bookmark   April 3, 2014 at 8:23AM
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Hi Anna,

I kind of like your idea of "...leave the greenhouse space through a 16 inch space in order to make room for the additional plenum. ...".

Sounds like you need a greenhouse kitty!!! ( :

    Bookmark   April 3, 2014 at 11:01AM
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Hi Waterstar,
OK, a drawing of the idea with the extra exit and no crossing tubes from the same greenhouse.Tubes from the two greenhouses do overlap but offset by 6 to 8 inches vertically.


    Bookmark   April 3, 2014 at 8:33PM
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Hi Anna
You`ll be ok with the 6"-8" spacing. If the tube lengths and amount of bends differ a lot you`ll need to devise a plan to balance the air in such a way that the longer, more bendy tubes get a bigger share. If they`re all balanced equally, the longer bendy tubes will have a longer dwell time than the shorter less bendy tubes.

    Bookmark   April 4, 2014 at 5:32AM
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Thanks Hex2006,
All the tubes in each system are planned to be the same length (inside system tubes are longer than outside system tubes). The bendy bits are exaggerated but are to allow the two tubes with the shorter paths be the same length as the one that goes around the other tube.

There will be separate fans for the inside and outside systems so at they can be operated separately or at the same time. Since the outside of the outside heat sink won't be insulated, I plan on only using that one for cooling. I figured that it would be OK for those tubes to be a different length than the tubes in the inside systems.

I am planning on movable baffles or adjustable openings to balance the airflow if necessary.

I am eager to get started digging when the weather is less windy.

    Bookmark   April 4, 2014 at 11:22AM
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Looks a lot better to me. I'll be interested to see what Curlygirl says about the ones just for cooling...

    Bookmark   April 5, 2014 at 2:13PM
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The easy way to increase cooling capacity during the season is to make provision to connect the inlet and exhaust plenums to the outside air. That gives the option of running the system at night to lower the soil mass temperature to increase the cooling capacity for the following day. You may never need to use it but its worth incorporating into the overall design just in case you do.

This post was edited by hex2006 on Sat, Apr 5, 14 at 18:56

    Bookmark   April 5, 2014 at 6:55PM
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Please accept my apology. I meant to say I was waiting to see what YOU had to say about the tubes just for cooling. ( : By the way, thank you for all of your posts in the past years, they helped me learn a lot.

Wow! Your comment is going to change my design. In my hot humid summers here cooling will be an issue. I will talk to my hubby today so he can figure our how to "to make provision to connect the inlet and exhaust plenums to the outside air." THANKS.

By the way, in some of your old postings you mention that your greenhouse is 154sqft. I'd love to see a picture of it.

    Bookmark   April 6, 2014 at 6:07AM
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Hi Waterstar,
I dont know if Curlygirl has her system up and running yet but it would be nice to hear how other operational shcs systems are performing in different climates.
You can see some pics of the greenhouse and the shcs install on johns old forum. The shape of the floor and roof didnt lend itself to the standard 2 plenum tubing layout so i had to come up with something different.;postorder=asc&start=0

    Bookmark   April 6, 2014 at 9:53AM
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Hex2006, the tip to provide a means "to connect the inlet and exhaust plenums to the outside air" is a great one. I now remember reading that somewhere (probably on Sunny John's site) but it had not stuck in my brain. ;-)

I can see how to do that with the inlet plenums since they are in the north corners. The exhaust plenums for the outside systems are very easy as they are already outside and the exhaust could go through the wall or straight up. The inside exhaust plenums are a little trickier. I currently see three options. There could be additional holes in the foundation walls but I really don't want holes that near a corner. I could have the exhaust exit through the base of the greenhouse glazing. This is definitely the easiest. I could figure out how to use the foundation wall holes for the outside systems for the inside systems but I expect that will be too complicated and limit future flexibility options.

When I add a clear cover in the winter over the exterior frame over the outside beds and greenhouses (essentially putting the greenhouses inside hoophouses), this modification changes from cooling the inside greenhouses to heating the outside hoophouses. In the winter, the outside systems might be set to hoophouse air day and night while the inside systems could be set to greenhouse air during the day to capture heat and to hoophouse air at night to heat the hoophouses. So many ways to use the system. I love it!

Thank you very much!

P.S. Now I think I may want to insulate the ground around the outside beds. I am thinking of leaving the above ground uninsulated and adding nearly horizontal insulation underground below the walkways around the structure. Fortunately this can easily be added later. :-)

This post was edited by Annalog on Sun, Apr 6, 14 at 11:29

    Bookmark   April 6, 2014 at 11:03AM
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Hex2006, That link was great, I loved reading your info. Must admit gets over my head. I read it more than once and it still doesn't all sink in. Thankfully I will be able to get this SHCS in without totally understanding all of the physics, thanks to help from folks you, curlygirl, Steve, Anna and others. The info from Sunnyjohn, Roper and others is just so valuable, but I go brain dead off and on when I am reading all of it. Oh my. ( :

So, now I am confused about how to implement the summer idea of connecting to the cooler nighttime outside air to lower the temps during the day. If I put the intake outside at night to reduce the underground temps, then during the day it will be taking hotter air from outside, right? Wouldn't I want to intake the cooler inside GH air during the day? Are you talking about using 2 systems as vs. my one?

My GH will have the temperature controlled vent openers. The opening/closing temperature range is 59 degrees to 77 degrees, so I will be using them in warmer times of the year. Our temps here in both the summer and winter are usually about 20 degrees cooler at night than during the day.

Can I use the vents to control the intake and outflow of air? I was planning on having them open at night and closed during the day during the summer and then closed during the winter.

For further temp control in the summer I am putting up deciduous or annual fast growing vines outside the greenhouse on the west wall, but letting the sun hit the South and east walls so that my espaliered citrus trees can get light.

I just get so confused sometimes!!! DUH.

Anna, I'm sure glad mine is not as complicated to figure out as yours. ( :

    Bookmark   April 8, 2014 at 7:05AM
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Running the cooler night air through the shcs tubing removes some of the stored heat. The following day the shcs runs as normal, taking air from the top of the greenhouse and exhausting back into the greenhouse. A lot of the information out there is valid apart from the 5 airchanges/hr or 4ft/sec tube air velocity. Roper`s website advocates the 5 airchanges hr and

    Bookmark   April 8, 2014 at 11:17AM
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Hi Hex2006,

How do I take the air from the outside as vs the top? Just by opening closing vents? I hope so. We will return the auto temp opener/closers because they won't help up. Hopefully we can find some openers/closers that work on a timer (or are at least electric so hubby can modify them or hook them up to a remote control). I know I sound so ignorant, but that's because I am on this subject. ( : Sigh.

From your post on Posted by hex2006 (My Page) on Fri, Nov 6, 09 at 22:16 I figured my1,044 cu ft. GH would need about 1000 cfm after static resistance. But now I can't remember if that was based on 20 times...I guess I'll need to go back and refigure that. In the meantime though, I have used that 1000 cfm figure to estimate my fan requirements.

I chatted with a very good tech at Grainger and he says my static resistance is about .8-1 in. so that means I would need to get a fan that is 11-1200 cfm for baseline with target flow of about 1000 cfm at .8 to 1in static pressure. He sells one (14" though....bigger than I would like) for $775. (

I also chatted with a tech at Canarm and found I would have to go to thier HVAC devision to get on strong enough. He says the blades on his that Roper mentions are flat plastic at a set pitch and he is not convinced they will function well with a .8 static pressure. I have not priced this one yet...but will call Northern Tools and/or Farmtek? for a quote and will also research it more.

What kind of fan did you settle on? I know it is variable speed from what you said in some of your posts. I'm afraid to hear the price...but why go through all of this work and expense if it will not work right when I am done? I sure don't want 140F.

Thanks again for your kindness to help me learn.

    Bookmark   April 8, 2014 at 11:51AM
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You may not need to vent at night, a lot depends on the shcs system and the climate. If you live somewhere that has scorching hot days everyday, cooling the shcs at night may not be enough to offset the daytime gains without using conventional ventilation.
With a standard 2 plenum shcs all you need to do is attach flexible ducts to the plenums to pull air in from outside and exhaust it outside (bypassing the greenhouse completely). If the plenums are located on outside walls you can use a box on each with two sliding shutters to reroute the air, open the shutters to the outside air and close the shutters to the greenhouse interior.
The static pressure shouldnt be above about 0.2", a few long tubes with high velocity airflow will need more fan power than double the number of shorter tubes, both having the same total combined length.
If you blow down a 50ft hosepipe as hard as you can, you might go a bit red in the face ;) if you cut the hosepipe into five 10ft lengths and blow down all five at once it`ll be very easy.
I upgraded from 10" to a 12" 870cfm duct fan which was around ã80 ($134). Unless your greenhouse is enormous you wont need a 14" fan, especially with a pricetag of $775.

This post was edited by hex2006 on Tue, Apr 8, 14 at 14:33

    Bookmark   April 8, 2014 at 2:31PM
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Thanks so very much for the fan advise. What a huge relief. My hubby really thanks you, I scared him pretty good with the $$$$.

Also, now I understand about the venting to cool it at night. It makes a lot of sense and my hubby now can design what we need. My goodness you know a lot about this SHCS.

How hot are your temps in the summer and how hot/cool does your GH get? Is it dry or humid? Do you need to outside vent? I don't know your zone. We get very humid all and night, so that should work in my favor I think. It usually gets in the 90's in the day, but the humidity makes it "feel" warmer.

I liked your idea of having a place where we could track results with the SHCS in different climate zones. After I get mine up and running I'd be happy to add my info and I bet Anna will too. Curlygirl and Steve333 might join in and lots of folks here. I think it would be fun and helpful to create such a "database".

Blessings to you for helping this newbie. ( :

    Bookmark   April 8, 2014 at 10:13PM
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I`m in the uk so the climate is fairly moderate compared to other places, typical summer highs dont get much above 32C and winter lows maybe -8C. We dont use the zone system but its perhaps 7 ish. The shcs can keep the greenhouse temps within bounds in summer and frost free in wnter. I`m south facing with no tall trees or buildings to block low angle winter sun which helps.
I have passive vents all around the base of the greenhouse along with a single large vent at the top for venting stale air and bringing in fresh which is needed to replenish the CO2 levels. Your humidity is ideal for shcs, moist air reaches dewpoint much quicker than dry air.

    Bookmark   April 9, 2014 at 1:45PM
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Hi hex2006

You have a climate with which I am familiar. I lived in the NW for a long time. Less sun (by a long shot) than SC. The temps here are warmer, and with the humidity it feels much hotter. I have to be careful not to be in the sun too long in the summer or I feel like I am going to pass out.

I was hoping my climate would be ideal with all the humidity and sun. Thanks for telling me it is! I am, as they say here in the USA, in hog heaven.

Now, Hex, you have brought up something else with which I am unfamiliar. CO2 levels. I will have 4 top vents. Do I just open them? Or should I just have hubby cut holes in the walls with removable doors, that could alo be used later if I need to pull air in and out while bypassing the greenhouse to keep it cool enough? How often do the C02 levels need help? Daily, weekly???

I am amazed how many details there are to this and I must say I am impressed by how much you know about it. I am grateful to you for your help.

    Bookmark   April 18, 2014 at 11:30AM
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Greenhouses delayed again by chickens. ;-) Hubby and I have put up an additional chicken pen (10' x 10') with interior coop and may need to put up a second before work on the SHCS foundation. This fall is looking less likely.

    Bookmark   July 23, 2014 at 10:35PM
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Hi Curly Girl,

Just wondered how your project is going. I'd love to see some pictures. Mine is slow, but some progress. ( :


    Bookmark   September 2, 2014 at 9:24PM
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Hi Curly Girl,
I am also interested in your progress.

We are currently working on that second 10 x 10 chicken pen. (One of the ordered pullet chicks has turned out to be an aggressive cockerel. DH calls him Bitey and I tend to call him Dinner or Fricassee.) The only progress that I can report on the greenhouse is that the drainage in the existing dug area is sufficient to have no standing water the morning after over 2 inches of rain in one day (3 inches over two days and 4 inches over five days) with fairly saturated ground. The rain was such that it had filled the 10' x 10' hole that had been dug 8" deep in order to bury hardware cloth under the second chicken pen due to runoff from uphill. While we had not checked the greenhouse area, I suspect that hole was filled with over a foot of water as well.

My current goal is to finish at least the entire foundation, the west greenhouse and central shed by the end of spring. We will see if I meet that goal. ;-)

    Bookmark   September 24, 2014 at 10:34AM
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Finally started digging the foundation. 1 foot by 12 feet by 6 inches took 45 minutes (west edge). I don't want to know what that calculates as for all of it. Actually, the first six inches is the worst as that is where most of the rocks are (especially as there was a pile of rocks and asphalt in this location when the road was worked on before we bought the acre.. ;-) I will do a calculation after I get to the planned depth. I expect it to be much better than the current 6 cubic feet per hour. :-D

Edit: Geek that I am, I could not resist doing the calculations. 288 square foot area, 6 cubic feet per hour for the first foot and at least double that for the rest of the depth works out to 48 hours for me to dig the first foot and another 48 hours to dig the rest, not including either soil transport time or DH helping. Figuring a 6 hour work day is 16 days or 8 weekends of good weather. It is possible that we could be working on laying the foundation walls in March if DH can dig an hour or so on good weather days. :-)

Second edit: Dug 3'x12'x6" in the next 45 minute session and then 2'x12'x6" in a 35 minute session. My estimate above seems more realistic.

This post was edited by Annalog on Wed, Dec 31, 14 at 15:00

    Bookmark   December 29, 2014 at 6:36PM
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Still on track. DH and I are making good progress on the 12'x24'x3' hole; it is over 1/3 dug. Hoping to finish the hole by the end of February and start the foundation wall and subterranean heating/cooling system at the beginning of March. I need to redraw the plans for the size change of the central shed as it affects the tubing layout.
Update: DH has been a digging demon. It looks as if we will be starting on the foundation walls by mid February.

This post was edited by Annalog on Tue, Jan 27, 15 at 8:05

    Bookmark   January 22, 2015 at 10:59PM
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The hole for the greenhouse project is now over 80% dug. The hole will be 12'x27'x3' for the foundation containing the underground part of the subterranean heating/cooling system. Both DH and I are getting excited about putting in the foundation and filling the hole back up. :D

Some photos:

The yucca that needs to be removed.

West end of hole. The pipe and pole are the result of electrical work before the design was changed.

View from the house looking east.

    Bookmark   February 10, 2015 at 9:04AM
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Earlier in the thread there is mention of what type of controller is used to manage the fan for these underground systems. Could I get some more info on that and the "differential thermostat' that maintains a set difference between air/soil temp. Can't find anything like that in stores. Tested out mine for the first time using a reversed shop vacuum, probably 500 cfm. Inlet was 84f/74rh, outlet was 53f/84rh. So it seems to be working. But I don't know the best way to control it.

    Bookmark   last Friday at 8:54PM
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hairmetal4ever(Z7 MD)

Few questions about this system for my climate – low elevation (700’), Mid Atlantic, zone 7, MD. In my climate I’m probably more interested in the COOLING aspect.

My thought is that a subterranean system might be BETTER for cooling on sunny days than ventilation from outside air, once the outside air is over a certain temperature.

If the subsoil temperature is 60F (guessing) and it’s 95 in the GH, and the outside temp is 84, if I could utilize that cool subsoil temperature, could I cool more effectively than using a ventilation fan??

Also – would this system do anything about CO2 levels? Would CO2 levels change in any way from this system or would it be “neutral”?

    Bookmark   Yesterday at 8:09AM
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Glad to hear to updates on this site. My hubby got suddenly ill, so our progress has been halted for now....but hoping to start it back up late spring or early summer. He has the concrete poured for the corners and the frame up, so the next step would be me adding some dirt to the bottom and laying the tubbing in. Even if he is still ill, he may be well enough to show me how to cut the holes for connecting the tubes. Blessings to all and I will be following your progress!

    Bookmark   14 hours ago
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Waterstar, best wishes for your hubby's complete recovery. Glad to see the progress to date. :-)

Hairmetal4ever, my system is not in place yet but I am more concerned about cooling than heating. My understanding is that the most significant part of the heat exchange is due to the water phase change that takes place (water vapor condensing from the air when cooling the hot and humid air from the greenhouse while underground or evaporating water from the warm and moist soil when warming the cool and dry air from the greenhouse during the winter. This should be our first summer to see this work. :-)

Nbet, I am looking forward to answers to your questions. DH and I have been focusing on digging and I had put off looking for the hardware and controllers. It is probably time for me to be thinking about those. Like the photos and the idea of an early test. :-)

    Bookmark   55 minutes ago
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