My geothermal system is being installed as we speak.  They dug the supply-return header trench yesterday and it was about 3 ft deep.  I was under the impression that it should be at least 4 ft deep.  I can't find any information on the web and they're wanting to get it rigged and backfilled today.  Help!  With what we have invested in this I would hate to have it compromised by something easily fixed now.  We live in the Nashville, Tn area and the frost line is about 18".  Any info or suggestions would be greatly appreciated!  Thanks

In Nashville, 3 feet deep should be sufficient for freeze/thaw purposes.
However, if there are other considerations, such as a driveway over, then 3' may not be deep enough for that.