I'm putting in six 500ft horizontal ground loops of 3/4" tubing for geothermal.  No joints will be in ground; all loops will run to manifold inside.  I know hdpe (astm 3408) is almost universally recommended, but haven't seen any technical reason why.  Ideas?  Also, if I use pex, is an oxygen barrier needed?  I don't see why and don't think hdpe has oxygen barrier.

- Ted

Barrier tubing costs a bit more but it'll protect your system against corrosion. HDPE does not have a barrier. You can buy PEX tubing with or without an oxygen diffusion barrier. The thing that will determine weather or not to use barrier tubing is the use of ferrous components within your system. No iron allowed in systems without a barrier. I'd use PEX over HDPE because it's a smaller molecular structure and much more flexible.

HDPE is the standard. It comes with a 50 year warranty from the factory. I know of hundreds of geothermal systems using HDPE. I don't know of one using PEX.