Make sure to use oxygen barrier PEX in any closed-loop hydronic heating system that includes PEX tubing. New water is not routinely introduced to closed-loop systems. The same water cycles through them, under pressure, continuously.
Oxygen corrodes ferrous metals. Any system components made from iron or steel will rust over time if the system contains fresh oxygen. Since water contains oxygen, the purpose of the barrier is not immediately obvious. 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 and will gradually develop a black or brownish color.
PEX without an oxygen barrier actually allows oxygen from the air to permeate it. This happens even if the PEX is embedded in concrete. Cast iron circulator pumps will rust when oxygen is present in the water. Bronze or stainless steel circulators are necessary when this is the case. Many hydronic heating sources cannot handle fresh oxygen and would need to be isolated from an open-loop system or any oxygen exposure with a heat exchanger.
The oxygen barrier on standard PEX is normally a layer of ethylene vinyl alcohol (EVOH) applied to the outside of the tubing. The aluminum layer of PEX-AL-PEX serves as its oxygen barrier.