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 amvicsystem.com). 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