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Wednesday, June 29, 2011

Subterranean Heating and Cooling

Installing a Subterranean Heating and Cooling System

Last night CTD commented on my post "If I could design a system from scratch...," saying:

You may have already thought about a subterranean heating and cooling system, but if not, here's a couple of links. I'm considering one of these for its year-round efficiency (off grid if I can get the math to work out) and possibly a small rocket mass as a supplement for those really cold stretches...

Such cool stuff! The subterranean heating/cooling system is primarily three layers of Underground Air Circulation Tubing (UACT), 4 inch thin-walled perforated drainage tubes (less than $6 per 10 foot length at Home Depot). The tubes are spaced about every 2 feet horizontally in each layer, and the UACT layers are each a foot deep. The tubes are connected to a plenum (e.g., a 55-gallon drum) at each end, and a fan blows the entire volume of the greenhouse through the underground tubing every 10 minutes.

During days when it gets toasty in the greenhouse, air enters the system warm and moist and comes out cool and dry, leaving the moisture and heat energy in the soil. When the greenhouse air gets colder than the soil (e.g., winter nights), the fan pumps cool air through the ground and it comes out warmer and moister.

John Cruickshank of says he's created greenhouses in Zone 4 (average minimum temperature of -25 degrees F) that maintain a Mediterranean environment all year round without any supplementary heat.

I still want to include a rocket mass heater. One, I like to burn things (wood). Two, I like to cook things (pizza...). Three, my greenhouse isn't a double-glazed marvel of heat retention, so it will probably need supplemental heat, even though I'm in Zone 7 (average minimum temperature of 5 degrees F).

Here are some sketches of how I think I could have both a Subterranean Heating and Cooling System (SHCS) and a Rocket Mass Heater (RMH).


  1. I thought you couldn't dig down. How will you install this system?

    1. I actually toyed with the idea of installing geothermal heat for my home, with the option of tapping off the geothermal system to heat/cool my garden. The up front expense was a bit high, but we'd save enough on heating/cooling to pay for the investment after 12 years. Which would give us another 13 years of savings before we'd have to replace the unit.

      Alas, times are a bit tight, plus in isn't clear that we can dig a 300 foot geothermal well in this community (covenants, bad surface conditions, etc.).

      For random folks who happen upon this page, I ended up getting tapped to write The Complete Idiot's Guide to Aquaponic Gardening for Penguin Book Group. The book is now available for purchase at It includes lots of DIY plans as well as everything I wished I could have found in a book back when I was starting out (which wasn't very long ago...). So far the reviews are good!

  2. It was one thing to dig down just so my grow beds could be a few inches lower. I wasn't willing to dig just for that (count me lazy). But the idea of having easy cooling in the summer and robust warmth in the winter, plus making my grow beds lower, makes a difference.

    Granted, it would be easier if I had an open field where I could use a back hoe or something.

  3. HI, me again.
    Not to throw more ideas at you, but for an application your size it might be possible to set up a solar panel and a battery (during the night) to power an SHCS fan. I'd like to try that myself, as well as use solar to power the pumps for my tank. I recently bought some how-to material on building panels. It doesn't look too difficult, I'll let you know what I learn.

    On another note, I'm actually concerned that in our region we might have more difficulty with summer cooling than I first thought. I've read material on the heat advantages of SHCS, have you come across any numbers on the cooling power? If so I'd be interested, you can reach me at

    Finally, I would be very interested in participating in the type of study you proposed (after all I'm interested in the exact same type of system). One drawback for me, I have not yet even begun to develop my project(s), they are all in the planning stages.

    Thanks for putting up the images of your design. I'm still struggling with incorporation of an RMH, but you can see my work in progress here:

  4. John Cruickshank over at SunnyJohn comments on the cooling issue here. Basically, he suggests adding underground air circulation tubing outside the greenhouse perimeter and venting and misting and shadecloth.

    If I were in a zone with more light and had the head room to have my roof significantly higher than my growbeds, I would remove my 'glazing' or plastic entirely and replace it with shade cloth. As it is, I simply removed my glazing once it started getting nasty hot. That would be at the point where the underground thermal battery is so thoroughly "charged" that it can't effectively cool the greenhouse, and I don't know how early in the season that would occur for folks.

  5. How would this work in our humid environment? (I'm in the DC-area too.) Fortunately, my yard is big enough to get a back-hoe in, so I need to look at this before I plop down a HF 10x12 greenhouse.

    As far as solar goes, Harbor Freight always has the 45 Watt PV kit on sale for $150 or so. You can often find coupons for additional % off. There are folks doing solar air heaters and using PV to power DC fans with snap-switches. I see no reason why both couldn't be adapted to this setup. The solar air heater could be very large, and pump heat into the buried thermal mass. Lots of discussion on the simplysolar yahoo group.

  6. We're humid here, but not that humid.

    I'm actually loving on the idea of a temporary greenhouse, like the one I've got in my back yard. I can put plastic on the frame when it gets cool, and keep it on until it starts getting nasty hot in the summer. Adding a rocket mass heater ought to mean I can keep the "thermal battery" under the garden heated throughout the winter.

    I'm not sure what the water table is under my property, but I think it is low enough that the underground tubing wouldn't get flooded (and stay flooded).

    I've got one of the Harbor Freight PV kits. One of many projects that is on my to do list.

    I think I should be good to go, to install an underground heating/cooling system. Just want to double check with someone at the county and re-read my community covenants one more time...