In an open (non-pressurized) system most will state that an oxygen barrier type tubing is not necessary because your unit is still vented to the atmosphere and absorbing oxygen into the water through that vent.  However, in a closed (pressurized) system an oxygen barrier type tubing is a must have.  The main goal of a closed pressurized system is to minimize the amount of oxygen entering the system creating oxygenated water.  Oxygenated water = corrosion.  In a semi-closed system such as the Heatmor units it is still firmly recommended to use an oxygen barrier type tubing because this type of system only "vents" when the system expands enough to build approx 2.5lbs of pressure.  Therefore, in general the Heatmor system is rarely in an open system state.  Keep in mind, that when in doubt about which type of tubing to use, if you pick an oxygen barrier type you can't go wrong.

non-barrier pex
-cheaper
-does not protect against oxygen entering the water through the tubing

oxygen barrier pex
-slightly more expensive than non-barrier pex
-the barrier is created using an EVOH coating either on the outside of the tubing or between layers of the pex tubing

pex-al-pex (every pex-al-pex is 100% oxygen barrier due to the aluminum layer)
-slightly more expensive than oxygen barrier pex
-you get a larger ID along with the tubing compared to straight pex tubing

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