Upper Pond – Pig Drinking Resevoir
I got the following letter in email this morning asking about our house’s earth air tubes. I’ve gotten quite a few other letters and comments asking about this. To the uninitiated, the basic idea is to draw air into the house via tubes in the ground to either warm or cool the house. A fan can be used to drive the air or it can be completely non-electrical using a solar chimney or simply the natural drafts of the house. I’ve mentioned this a few  times before in regards to our old farm house and cottage. I share my response to D.P. below so others will benefit from the questions and experiences.
Hi, my name is D. P. and I work for a tribally owned health consortium in Anchorage, Alaska in the Department of Environmental Health and Engineering. Soon I will be assisting villages with various air quality projects they see are needed (control of open burning, woodstove-replacement etc.). While researching ways to improve air quality both in and outside of the home, I often run across renewable/clean energy projects that not only cut down on cost and resources but would also have a significant positive impact on air quality, such as the earth tubes (improved ventilation without great heat loss). I am very interested in learning more and was hoping you could provide more information on how you installed them, did you have plans to build from?
I didn’t have any plans – this is the sort of thing that is a bit unique. Seven years ago I designed and built our earth air tubes to fit our situation based on experiments I had done and reading (see below). For the old farm house, basically we dug a 70′ long by about 2′ deep by 4′ wide trench (I’m doing these numbers from memory, the 70′ is right, the depth is approximate). The trench went from the basement of our house down hill and exited the earth at a point lower than the house penetration. This allows for drainage of cold air down and water down as well as rising warm air inflow. Realize I’m designing for a cold environment to warm incoming winter air. If I was designing for a southern climate where I wanted cooling I would change the configuration to drain cool air downward into the house.
In the trench we placed stone for drainage (2″ washed crushed stone). On the bed of stone we placed four parallel 70′ long (10′ interlocking smooth walled sections) 4″ diameter drain pipes giving a total of 280′ of pipe length. The pipes were spaced a few inches apart from each other and wrapped in filter fabric. Most of the pipes were solid walled, the last 10′ of each tube, furthest from the house, was perforated with the holes on the bottom for drainage. This was probably overkill in retrospect but I was worried about water in the pipes, especially condensation during the warm summer months when the system would act more like an air conditioner / dehumidifier.
Drainage is very important. Figure out how to make sure the pipes are not going to fill with water, have mold build up, etc. You don’t want to get Legionnaire’s disease, black mold, etc. The incoming winter cold air is wonderful at drying the pipes. In southern parts of the world the incoming high humidity could cause problems due to condensation. In wet areas water could pool in the pipe. Being on a slope helps a lot. Hard in some areas… We’re in Vermont and everything is sloped.
We covered the pipes with some dirt, perhaps 4″, and then two layers of 2″ closed cell insulation. The first layer was two sheets side by side and the next layer was one sheet that overlapped the seam of the previous two sheets of insulation. The reason for the insulation is the pipes are not very deep – we generally have shallow soils on our land and in much of Vermont for that matter. In Alaska you’ll want the insulation too I would expect due to the deep frost penetration. We then covered the insulation with dirt to bring the trenches back to level. Each layer should be well packed to prevent settling.
It is advisable to put hardware cloth 1/4″ screening over the ends of the pipes inside and outside to keep vermin out. Window screening inside of that helps keep insects out. A dust settling chamber can be setup if dust is an issue from local roads, pollen, etc. I would use a ~2’x2’x2′ chamber with a deep stack of furnace filters. Slow moving air through filters is very effective for dust removal. Clean the filters periodically. Put the filter system on the incoming end, that is to say away from the house, to keep the pipes cleaner.
Our winters get down to about -45°F and we have months below -20°F many years although this past winter was quite mild. We have high winds. It’s not to dissimilar from Alaska – I lived in Fairbanks, Anchorage and Kenai long ago. The earth air tubes have made a huge difference for our old farm house here in Vermont. I would think they would help folks in Alaska too for exactly the same reasons. The winter air in our house went from being stale to fresh. Our sinuses were much clearer afterwards and the air is no longer so dry during the winter – an effect I had not predicted. The outdoors can be -45°F and the air coming into the house at the pipe exit in the basement is 35°F. That gives us a 80°F heat gain from the earth on the fresh air. We heat with wood and now we have no back drafting so the wood furnace works much better.
A big issue in our area is radon which is a radioactive gas put out by the granite below us. I had a concern that the earth air tubes might collect radon but the opposite is the case. They are reducing the radon concentration in the house by diluting the air through fresh air exchanges. “The solution to pollution (radon) is dillution.”
I’m very pleased with the results and have come up with some improvements for our new system. We have been building a new house further up the hill and I’m incorporating improved designs of the earth air pipes there.
- Extending the insulation wider to create more of a thermal shadow.
- Longer pipes to get more effect.
- Out-flowing air pipes parallel and below in-flowing pipes to act as a heat exchanger.
- Pollen/dust filter – put it out at the input end of the system so the pipes stay clean.
- Solar stack to drive system or electric fan or chimney stack around wood stove – I went with the latter on the tiny cottage and that works quite well in addition to being a non-electric, passive system. For our old farm house I simply used the draft of our wood furnace to drive the system and that worked well. Also the farm house ceilings/attic leaks some so that drives the system some too.
- Clean-outs on pipes at ends and any bends.
- Build a little housing of stone to protect the ends of the pipes at the downhill side.
I would love to incorporate probes for humidity and temperature logging in the soil around the tubes and in the tubes themselves to gather more data but it costs too much. I’ve been doing it with simple digital min/max thermometers I stick in and monitor daily.
For details of our new Tiny Cottage. Our family built the new house all by ourselves for under $7,000 and it requires only about 3/4 cord of wood for heating. This is a design that would work very well in Alaska and can easily be scaled up. The cottage is on a floating slab which works well with permafrost from what I’ve read. Cost was kept down by some salvage such as the windows which are from an office building. In our case we anchored the slab to the ledge of the mountain but in Alaska one could float it on the p
ermafrost. In that case I would pour the slab a little wider to get even more floatation – engineer it. The cottage uses ferro-cement for the barrel vault roof which minimizes the cement volume while maximizing the strength, easily done by people with some simple form work. This makes it strong enough to take any snow load – no need to shovel the roof. We are living in it although like with most owner built things we still have some things to finish. We were able to build most of it in about two months – two adults, a 14 year old and a 10 year old plus the help of a three year old. If it were built on a slight mound on the permafrost then earth air pipes might be able to be put under and around it.
What resources did you find helpful in your research before installation?
Mostly I spend years thinking about it and doing experiments. I built progressively larger models and took measurements with temperature probes. Build one and run it for a year to see how things go. The articles I found on the net all talked about burying the pipes far deeper than is possible where we are (10′ minimum depth) and about using far larger pipes than possible (18″ minimum pipe diameter). It is rare to be able to dig down deeper than 18″ before hitting ledge. While those might be ideals the reality is one has to adapt to the situation and the 4″ pipes worked at a shallow depth with insulation.
Putting the insulation above the pipes was my way of achieving much greater depth. Soil has an R factor of about 0.1 per inch. Insulation is about R5 per inch. Thus with 4″ of insulation I effectively gained 5×4/0.1 = 200″ = 16′ of depth. Not really because the heat curves around the edges of the 4′ board but still it was a vast improvement over simply burying the pipes.
I just did some Googling and here is a search pattern that will bring up some links about earth air tubes for further reading. Another good source on this general topic comes from earth sheltered housing. John Haite has written some excellent stuff on this under the heading of PAHS which stands for Passive Annual Heat Storage which uses similar ideas. There are two good overviews on Wiki .
Do you think its possible to run a series of earth tubes to an entire village?
Sure. Although personally I like doing things on a per home basis. That way if there is a failure it only knocks out one set, not the entire village. Think if an animal gets into a pipe and dies… or a skunk sprays… or some practical joker throws a dead fish in… If the pipe serves the entire village it’s a nightmare. If the pipe serves just one house it’s an inconvenience. Small scale is beautiful.
I am finding mostly earth tubes are used for cooling,
Yes, that was what I saw too in the few articles I found. For us the tubes are for heating fresh air. Our climate is more akin to yours so warmth is more of an issue than cooling. Our summers rarely get above the 70’s here on the mountain. Interestingly, 1,000 to 1,500 feet below in the valley the climate is very different. Down there cooling is an issue. It always amazes me when I go down off the mountain just how hot it is a mere few miles away.
and most do not address the problem of permafrost.
Permafrost will have some interesting issues. A little bit like our ledge. It may be that you need to make a much wider insulating cover which will produce a vertical shaft of heat. Consider placing pipes directly under the homes so the thermal shadows of the homes themselves help the system. It is definitely an interesting variation problem…
I am not an engineer and I’m sure there are places in Alaska this would/wouldn’t work but just wanted get more information and advice from someone who’s seen/constructed these first hand.
Speaking as an engineer, don’t let them get their hands too tight on the project and make it too big. There is a serious distraction among engineers to over build projects, to make them bigger than necessary. It’s an ego thing: “Who has the biggest pipes? er, tubes…” You know what I mean. :)
Outdoors: 75°F/56°F Sunny
Farm House: 78°F/68°F Flipped compost pile
Tiny Cottage: 68°F/62°F