ASK THE EXPERTS: Earth Tube Follow-Up

Earth tubes should be installed as deeply as is practicable.

I read Jim Riggins’ “Mailbox” response on earth tubes in HP159, and would like some follow-up information. I live in Spanish Fork, Utah, and am building a geothermal greenhouse. I’m considering using earth tubes to bring in slightly earth-tempered air. What size and type of pipe did Jim use, and how deeply are they buried? My application won’t allow me to go deeper than 4 or 5 feet, so I’m wondering if it will provide any benefit at all.

Darren Brown • Spanish Fork, Utah

I used ECOAIR 8-inch PVC pipe with antimicrobial coating (see I installed it 10 feet deep. I am in Climate Zone 5, as is the Provo area, and roughly at the same latitude. In this climate zone, the soil reaches a year-round constant temperature at about 8 feet in depth. I added 2 feet to be conservative. The tube is 100 feet long.

I expect that these numbers are about the same for your area. You can contact a local university or do some research to confirm. At 5 feet of depth, you would certainly see some beneficial heating, but not as great as you would at 8 feet. The key would be to make sure you are below the frost line. One way to research this is to find out from your local building department the depth they require foundation stem walls to be built. If you cannot get below the frost line because of rocks or boulders, it is probably not worth installing the tube.

The antimicrobial-coated product is very expensive. Since you are heating a non-living space, you could consider uncoated, conventional PVC (white) or ABS (black) 8-inch pipe.

Jim Riggins • EnerSmart Energy Solutions

Comments (1)

puttermeister_52's picture

Interesting article and comments. This page brings up some questions I have about earth tube heating of a small storage space, rather than en entire house.

I’ve been interested in earth tubes for some time, although not soon enough to incorporate them into the new house I built a few years ago. However, I’m planning on building a garage/storage unit this fall and wondered thought of using an earth tube to provide tempered fresh air into the storage area. As this is a small (24 x 14 x 8’; 2700 cu.ft.) space which will be well insulated (including the slab floor) and has no windows it seems like a good space to experiment on. As I will soon have a small excavator I’ll be able to do the trenching and digging myself, so the outside costs will be minimized.

It would be a bonus if I could temper the garage as well but given the rather optomistic R-values quoted on even good insulated garage doors, I think this might be unrealistic.

I had originally thought of using 100’ of 4” corrugated plastic drain pipe (which we call “Big O” here in Canada) with a small AC fan at each end to push-pull the air but I have come to the conclusion that there would be too much air resistance and possibly not enough contact area between the pipe and the earth. I’m in coastal BC so although we don’t have really cold winters, we can get stretches of below-freezing temps., with the occas. dip into the teens. Cooling in the summer is not a consideration.

What I’m now considering is using a length or two of 4” or 6” PVC pipe from the air inlet to a 30” dia. concrete caisson pipe (such as is used forshallow wells and drainage) buried at least 3’ down, with gravel at the bottom to absorb any condensation, and another length of PVC in the cover, leading up to the storage area, again with a small fan at both inlet and outlet. The thought is to minimize the air resistance while the concrete caisson provides the earth contact area. The storage space would also have a small ceiling fan to exhaust the stale air.

Does this seem like a viable proposition or am I missing a lot of crucial details? Of course, I could just use s small space heater, with a ceiling fan and a small outlet vent, which would be a lot simpler…

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