PEX tubing with an oxygen barrier should be installed in any closed-loop system. New water is not routinely introduced to closed systems. The same water cycles through them continuously.
Since water contains oxygen, people often ask us why the barrier is needed. The answer starts with oxygen itself. Oxygen corrodes ferrous metals. Any system components made of iron or steel (such as a cast iron circulator pump) will rust over time if oxygen is in the system. The oxygen initially present in the water chemically reacts with ferrous equipment to form iron oxide. Once this takes place, all corrosion stops. This type of mixture, which no longer contains oxygen, is ideal for closed systems.
PEX without an oxygen barrier actually allows oxygen from the air to permeate it. This happens even if the tubing is buried in concrete. Standard pumps will rust when oxygen is present in the water. Bronze or stainless steel pumps are necessary when this is the case.
