Pex and the O2 barrier

[franck@geneseo.net]

I've read in a few treads that when installing a hydronic heat system that you should use PEX pipe that is rated as having an O2 barrier.
Could someone explain the reason why the O2 barrier is important and what bad things could happen if the incorrect material is used?
Please bear in mind, I'm not looking for a shortcut on an installation, I'm just trying to broaden my understanding and I'm curious as to why it's critical. Thanks!

[nhmaster]

The O2 barrier keeps oxygen from migrating through the pipe and into the water (or antifreeze ) Excess oxygen causes rust and other very nasty problems in the boiler and will void boiler warranties.

[hj]

Without the O2 barrier oxygen can permeate the wall of the tubing in enter the system. Heating systems are designed with many materials which can rust, but as long as the system's integrity is maintained, there is no oxygen to cause the rust. A leak which causes constant introduction of new, air containing, water or PEX without the O2 barrier destroys that integrity.

[franck@geneseo.net]

I did not realize that oxygen could actually "pass through" the walls of the PEX. Is this barrier physical or chemical in nature? Is there a chemical addative that may be added to a closed loop system to condition the water, one that might further reduce corrosion to all the metal components? Does the water in a hydronic system have to be changed from time to time?
(Please, no jokes about changing the air in my tires.)

[jadnashua]

The only way the systems stay intact is because the water in the hydronic system does NOT get replaced. Essentially all water from your tap has some air and other chemicals in it. Hydronic systems are designed to extract that air, leaving the water. Even if that water contains some other minerals or is not pH of 7 (neutral), typically, after awhile, they get neutralized as they react with stuff inside the system and are essentially extracted from the circulating water. If there are leaks or water allowing or forcing new to be added, and through the walls fo the pex would still be considered a leak, then you are constantly adding new reactive material to the circulating stream. This will play hell on the system depending on how much and how fast that happens.

For a potable water stream, the tubing itself doesn't really add anything, since the water is constantly being replaced and there's already a bunch of oxygen in there. And, except in older or poorly designed systems, there's probably little iron in there to rust that isn't protected in some manner (like the lining and anode rod in the WH). Other metals rust as well, but copper and brass are very stable in comparison to iron. Native elemental aluminum is almost unheard of - it rusts so fast, but chemically coats the surface with a molecule that is the same size. Iron oxide is bigger, thus bows and flakes off exposing more iron underneath.

[Redwood]

Normally in a system due to the heating process the water has a low oxygen level. Rust is AKA Ferric Oxide. Without Oxygen it can't happen. Constant replenishment in a leaking system or, the use of PEX without the oxygen barrier allow oxygen to enter the system and cause corrosion.