how deep soil needed for geo thermal?

suz1023June 30, 2012

a house we're interested in needs a big addition and i wonder if the site will allow for geothermal for it. it's pretty ledgie, probably with large sandy pockets throughout.

can geo thermal be used in shallow sites?

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Annie Deighnaugh

Actually geothermal wells don't need soil. They just need the earth's temperature. When we had our wells drilled (two 250' wells) they came in with a giant rig (I didn't see it buy DH said it looked like it could drill oil wells) and they drilled both wells in one day. It really didn't seem to matter what they hit. Once they drill the hole, they sink a closed loop down the well and then fill it with bentonite. The fluid (nontoxic) from the geothermal system is then circulated through the loop where the earth either warms or cools the fluid, depending on if you are heating or cooling your house.

How deep the well starts is based on your has to be below the frost line in your area. Our wells start 4' below ground and go down from there. They are in the front of the house, but no one sees them as they are below ground. The only other thing has to do with spacing of the wells...they need to be far enough away from each other that they won't freeze the earth between them in the winter time.

Econar geothermal unit. Note hot water tank next to unit is not a heater, but a storage unit. We have a desuperheater which pre heats the hot water when the unit is operating so we burn less gas for our tankless hot water heaters. When the unit is in air conditioning mode, we get hot water for free.

Pipes going out to the wells

    Bookmark   July 2, 2012 at 11:15AM
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oh thankyou---what an excellent tutorial!
i was having trouble understanding how it worked, so really, thanks so much for your answer.
this place is mostly granite ledge. they will drill through that?

btw, we have a 300 gallon storage tank in our basement so it's room temp before feeding the hot water heater. we have another one behind the barn i plan on bringing with us to our next home.

    Bookmark   July 2, 2012 at 3:46PM
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Annie Deighnaugh

From wikipedia:

The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 30%.[9] Since a heat pump moves 3 to 5 times more heat energy than the electric energy it consumes, the total energy output is much greater than the input. This results in net thermal efficiencies greater than 300% as compared to radiant electric heat being 100% efficient. Traditional combustion furnaces and electric heaters can never exceed 100% efficiency.

    Bookmark   August 1, 2012 at 2:06PM
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So where does 600% come from? I've never seen that in a residential installation...


    Bookmark   August 2, 2012 at 10:41AM
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Annie Deighnaugh

Rounding error?

There seems to be a seasonal component to the % efficiency, but don't ask me...I'm no expert.

Here is a link that might be useful: US Dept of Energy

    Bookmark   August 3, 2012 at 8:50AM
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Re: US Dept of Energy

Interesting that the info on the link provided was on the 'Your Home' (residential) page. "Fairly high efficiencies (300-600%)" is in error in my opinion.

The latest, 'greatest', most efficient GSHP with variable speed everything (compressor, blower fan motor & flow center) boasts a COP of up to 6 (600%) with their 6-ton HP in partial load heating mode in an open loop configuration (see page 38 in link below). They don't mention from what depth they're pumping this water. One would have to assume that unlike a closed loop which is a balanced system (water coming up equals water going down) that the water table would have to be very near the surface to avoid high wattage pumping from a deep well and that according to their published specs requires an Entering Water Temperature (EWT) of 80 degrees F.


If you're living somewhere that the ground temperature is 80 degrees F in the winter - there's a good chance you won't be needing much heat!

Regarding COPs above 6 (600%), I have been involved in projects where the proposed system efficiency was 700%. One such example was an integrated single geothermal system operating between 2 closely spaced municipal facilities. One was an indoor skating arena, the other a municipal garage. The way it works is that the heat extracted from the water to make the ice for the skating rink would be pumped into the in-floor radiant slab of the municipal garage floor - before the use of the loop field in either case. So rather than having 2-separate GSHP systems each with a COP of 3.5 (350%), one integrated system instead of 2 is said to have a COP of 7 (700%) or the addition of the combined output of 2-separate systems.


Here is a link that might be useful: Waterfurnace 7 Series

    Bookmark   August 3, 2012 at 11:25AM
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