Geothermal closed loop sizing

gwbubMay 10, 2007

I have an open loop geothermal system now, using well water that is then sent to the sprinklers or back into the ground on the other side of the house. The system works very well. But at 10 GPM I'm burning a lot of electricity running the 1.5 HP pump on the well. I want to convert to a closed loop system.

My home is at the beach with dense medium sand. Water table is about 4 feet below surface. I want to dig some trenches down to the water table, and bury PEX pipe below the water table for a closed-loop system.

I am trying to size the amount of pipe I need. All the tables I've found are based on dry soil, not wet soil with the pipe below the water table. I need 30KBTUHR of heat shedding. Anyone have experiences or references for sizing the ground loop in these circumstances (below water table in wet sand)?

Also, since my house is elevated 10' above grade on driven pilings (no grade beams) 12' o/c, is there any reason that I can't trench under the house between the pilings for the ground loop?

From an engineer's perspective, using 0.25 Btu/hr-ft-degreeF for PEX thermal conductivity, I calculate I need about 600 feet 1/2" pipe (ideal). A 0.7% efficiency factor makes it 850 feet. Another 0.7% safety margin brings me to 1200 feet. I'm planning four 50' trenches, using one 300' spool of 1/2" PEX for each. The 10 gpm flow will then be split 4 ways in the manifold, so it is only 2.5 gpm per loop giving about a 4 psi pressure drop. Added to the 8 psi drop on the coil in the unit, plus fittings, I only need to pump 10 gpm at 15psi.

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I know practically nothing about this, but a couple thoughts anyway :)

It seems like the loop length for systems in dry ground would set the upper limit for loop length, and the loop length for the type of system where the loops are sunk in water (e.g. in a lake) would set the lower limit for loop length. It seems like both of these loop lengths should be available from the suppliers of these systems. I suppose you should be somewhere between these two lengths, but I am guessing thats a pretty larger range?

I suppose the key question is how fast does the water migrate away from the coil and bring in new water as the coil heats or cools the sand/water near it. Seems like this is going to depend a lot on how tight the sand is, and (maybe) how much water is in the sand?
I guess that you could do a small test trench and see how well it performs. I think you would need to run it for a long enough period to bring the thermally induced water movement around the pipe to equilibrium -- maybe days?


    Bookmark   May 11, 2007 at 10:29AM
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