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Not deep enough for earth tubes / GAHT ?

 
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So we started digging trench for earth tubes today with a mini excavator, and this is what we got:

After about 4 feet, you get water, and it freezes -- which on the one hand doesn't surprise me, because it's 20 degrees here, and on the other hand does surprise me, because isn't the ground that far down supposed to contain more heat?

The excavator can go deeper, but it seems like there isn't any point if we hit water at 4 feet.

Should I just abandon the earth tube idea, then, and consider other ways to heat the greenhouse?
IMG_20210109_201651.jpg
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If the water is exposed to cold air, and colder air sinks into the hole, the water will freeze.  Dig down as deep as you had planned, let it warm up in the sun during the day, cover with foam insulation, plywood, cardboard, blankets, and plastic and check it in the morning.  In theory it should not be frozen.  Run your pipes, cover with sand or pea gravel or whatever you plan to use around the pipe, put down a layer or two of 2" foam insulation and cover that with dirt.

Good luck.
 
Charles Rehoboth
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Michael Fundaro wrote:If the water is exposed to cold air, and colder air sinks into the hole, the water will freeze.  Dig down as deep as you had planned, let it warm up in the sun during the day, cover with foam insulation, plywood, cardboard, blankets, and plastic and check it in the morning.  In theory it should not be frozen.  Run your pipes, cover with sand or pea gravel or whatever you plan to use around the pipe, put down a layer or two of 2" foam insulation and cover that with dirt.

Good luck.



Thanks for the reply. I think you're right that the water won't freeze if I do that, but is the ground really warm enough at that depth to be worth digging the trench? All the geothermal stuff I've seen seems to say 6 to 10 feet. We did have our water supply pipe (at 4-ish feet) freeze several years ago, now that I think of it. Not here at the farm, but same area, and that has to mean that the soil at that depth doesn't stay at 50 degrees as I was hoping to see.

Argh.
 
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Ground/air heat transfer through groundwater could be problematic.
If you want to go deeper,  for the heat, switching to a PEX pipe liquid based system seems in order.
Hell, the hard part of any geothermal tends to be the digging.
 
Charles Rehoboth
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William Bronson wrote:Ground/air heat transfer through groundwater could be problematic.
If you want to go deeper,  for the heat, switching to a PEX pipe liquid based system seems in order.
Hell, the hard part of any geothermal tends to be the digging.



A PEX liquid system, that's brilliant. Do you then just rig up a radiator and fan, plus some sort of pump to circulate it?

And you don't think the groundwater will chill it too badly in winter?

I don't object to doing the digging, as long as I'm not just wasting effort -- the thought of doing all this, just to get whatever temperature air I was already standing in, was what made me pause.
 
William Bronson
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If the ground water is indeed liquid water, then its probably worth harvesting the heat from it.
DIY rigs like this often use an automotive heater core for the radiator part of such systems.
How deep you have to go to get stable temperatures is unclear.
8 feet might be enough,  30 almost certainly.
The internet is not super clear on this point, builditsolar.com has some decent but dense information.
 
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As William said, builditsolar.com has a lot of info on this.  This page Earth temperatures gives the earth temperatures for the US.  It also explains seasonal temperature changes for depths less than 30 feet, where the temperature remains nearly constant.
 
Charles Rehoboth
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Update: I kept digging, and ended up getting a good long trench at 5+ feet with no water. Close to the intended greenhouse site I hit bedrock and could only dig to 2-3 feet, so made basically a ramp down to the good part of the trench.

We then put 500 feet of 1" PE-RT pipe, insulated the part on the ramp, and backfilled all but the last 2 feet of depth (because I still need to run electrical service to it) before giving the excavcator back. Thanks everyone!
 
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