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How wide a hole to still benefit from geothermal radiation?

 
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I've seen some semi-buried greenhouse on youtube. Dig down 2-3 feet and you get a little bit of warmth radiated back from below grade soil.

At what width does this not work? I saw someone post a comment that wider than 15ft hold you wouldn't benefit much from below ground radiation. I hadn't thought about that before.

Any thoughts on this? I know some underground greenhouses (like ott kim) are wider than 15ft.
 
pollinator
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Can you give more details about the concept?
Are you talking about frozen soil, desert or temperate areas?
 
pollinator
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The quality of greenhouse is a huge variable. For the excavated area, the ratio of depth / surface area probably has a magic number that is just right for whatever latitude. I think a good starting point for figuring this out is looking up the maximum frost depth for your area.
 
gardener
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A government entity where I live was promoting half-buried greenhouses for a while. We tried it and abandoned it after the first winter. The south wall cast a long shadow in midwinter, but we get a lot of solar gain so that was a major loss. Also we had to peel away a corner off the top and climb down in to maintain, water o r harvest, which of course isn't good for retaining heat.

If your region has very little solar gain in midwinter it might be worth it, I don't know. At very north latitudes the shadow would be even longer though.
 
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For a horizontal loop you only need to dig between 6 - 8 feet deep. For a vertical loop you need to drill between 250 and 300 feet deep.
 
kevin nachman
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Thanks for the replies.
Latitude 34 degrees, north Georgia zone 7b, almost at top of hill heavy red clay soil. According to older maps frost line is 12 inches but I imagine that is rare or out of date, would guess a lot shallower now.

What sparked my interest was this post:
http://www.bananas.org/f2/my-semi-pit-banana-greenhouse-18518.html

He claims to have kept above freezing just going down a few feet.
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