In a DX system they use copper tubing, backfilled with sand or grouted in. In a Closed Loop system they use plastic tubing grouted in. Because the copper is more efficient in thermal transfer, the DX system's wells are less than 100' deep as compared to 100' to 400' deep for the Closed Loop. If the above is correct, why don't they use copper in the Closed Loop system?

Paul

The DX copper loop is already a closed loop system.

I'm assuming you mean a 'water loop' when you say 'closed loop'.

If they were to use copper in the water loop system, they may have problems with electrolysis inside the tubing. That's my guess on why. Other than that, I don't know. Maybe because of the cost of the copper.

As someone else pointed out in another topic thread, although copper has better thermal transfer, a 1 inch PVC pipe has something like 10 times the surface area for the thermal transfer, so even thou the PVC pipe isn't a good, the larger size of the pipe (thus larger surface area) used more than makes up for the difference.   

I don't see why copper couldn't be used for a closed loop water system, but it's proably more cost effective to use PVC piping.