Anyone know if 3/4" PEX is okay for geothermal ground loop? Should it have oxygen barrier?
As far as I know, itÂs not an approved material for a ground loop anywhere in North America; I personally wouldnÂt even consider it!
Irrespective of whether PEX is allowed or not, a 1.25" ID pipe has 2.78 times the capacity of a 3/4" pipe. So for a given flow rate, the resistance of the PEX pipe will be way higher than a standard 1.25" HDPE pipe. This resistance would require a more powerful pump to overcome which would lower your COP since you'd be wasting energy in the pump itself. A fusion-joined HDPE pipe set should last indefinitely.
Paul in Montreal
If you use multiple circuits of 3/4" pex the resistance to flow can be made to equate or even exceed that of a 1.25" system. A suitable design is to connect each of your pex circuits to larger inlet and outlet manifolds above ground. Make sure you use one continuous run of pipe for each circuit so that there are no connnections being made undergound.
Take a look at geothermaldiy.com I think they are using 3/4" PEX for their systems.
Also, here are some specs on the thermal conductivity of PEX and PE pipe http://www.engineeringtoolbox.com/physical-properties-thermoplastics-d_808.html