Barrier and non-barrier Pex

Unknown

about pex in general .

When non oxygen barrier pex is used on a ferrous boiler , how exactly does the oxygen get into the system to corrode the components ?

Can oxygen barrier pex be used on potable water ? If not , is it because the barrier can contaminate the water ? How is the barrier applied to the pipe ? Do any manufacturers rate any brand of pex for the dual use ?

Thanks in advance .

Alan(CaliforniaRadiant)Forbes

Diffusion

When you study the mechanism of diffusion in chemistry, liquids and gases always want to move from an area of high concentration to an area of low concentration because nature is always trying to balance inequalities; so the oxygen that is in the air will move through non-barrier PEX and into the water which has a very low concentration of oxygen.

As far as I know, barrier PEX can be used for potable, but each manufacturer will have their own listing so you must check with them as well as your local jurisdiction.

There are different application methods for the barrier; maybe somone else can elaborate.

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Unknown

magic!

Just kidding. the oxygen in the system is gradually consumed in the process of oxidation, or rusting.. for all the ferrous components in the system.

If you have an o2 barrier, once the o2 in the system is used up (i.e., it has rusted all it can with the initial amount of o2 in the system, which isn't too much), then it's done and oxidation stops.

If you don't, o2 can continually seep into the system through the walls of the pipe, allowing oxidation to continue until your components are destroyed.

technically most o2 barrier pex is the same as that company's non barrier pex, with a barrier added (typically to the outside or sandwiched in the middle of the pipe), so there isn't a "real" reason not to use it. However, it's more expensive typically, so why would you want to, and most companies don't bother to get their o2 barrier pipe rated for potable use for this reason; they expect people will just be happy buying the cheaper pipe that is actually rated for potable use. And in most places, it's not code to use anything not rated for potable use, whether it's "really ok" or not, so while you aren't hurting yourself you might be looking at a code violation.
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Mike T., Swampeast MO

Hope I'm remembering this right...

Diffusion is a two-way street. Just like radiation, for it to occur, it MUST be happening BOTH ways. However, if the concentration of whatever is diffusing is different on both sides of the enclosure, more diffuses from the space with a higher concentration than from the space with the lower concentration.

In a hydronic system, if there are ferrous components that can consume the diffused oxygen, they will do so and the water remains at an oxygen deficit relative to the atmosphere until all of the iron is converted to rust.

If I remember properly, PEX is very orderly--even crystaline--in molecular structure but there are imperfections. These imperfections are able to either "grab" free oxygen atoms and/or occasionally "pull" an oxygen atom from an oxygen molecule. When the imperfections "line up" throughout the structure, the oxygen atom makes its way through.

Think of a vertical wall that you're trying to climb. As long as there are some imperfections (and you're wearing rubber climbing shoes) you can slowly make your way to the top. Even if you fall numerous times before making it all the way to the top you occasionly achieve success.

An oxygen barrier has a molecular structure that refuses to allow oxygen to penetrate (or at least penetrate only with almost imfinite rarity). It only has to stop the movement of oxygen in one direction to stop it in both, so its position in the system is irrelevant.

With an oxygen barrier you are now essentially trying to climb a vertical glass wall. Rubber shoes might help you "stick" for an instant, but you can't proceed past the first step!

Mike T., Swampeast MO

If you notice, that's a bit different than the explanation you ordinarily see that makes you believe that diffusion is occurring only because there is more oxygen outside the system than inside. This would almost imply an "intelligence" in the oxygen with it somehow "knowing" that there is less inside the system than outside.

If there's nothing in the system to consume the oxygen, the levels inside and outside eventually match. When this happens, diffusion is still occurring--it's just that it's at the same rate in as out. It only appears to stop because there is no net change.

Unknown

Thanks guys for explaining the fundamentals

It takes a while for the info to sink in - that O2 from outside the tubing can be drawn into the system right through non barrier pex . Working with copper for 18 years really sets your mind to one way of thinking .

One other question - what type of material is used in the 02 barrier ?

Mike T., Swampeast MO

Most common seem to be EVOH (Ethylene Vinyl Alcohol) in PEX-C and aluminum in PEX-AL-PEX.

EVOH is essentially Saran Wrap. Remember the commercial with, "A lion won't eat what it can't smell?" Or was it a tiger?

hr

Some manufactures

are adding anothe protective layer over the EVOH to protect it during installation. Maybe help the squeak factor also.

I still prefer the aluminim layer inside the PAP as the best of the bunch O2 barrier. It's safe in there.

Check around for 20 foot lengths of pex. Some suppliers carry this in barrier, and it allows much nicer boiler room work.

hot rod

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Mike T., Swampeast MO

As best as I can understand oxygen diffusion through PEX, the oxygen isn't being "drawn through". Kind of like the answer to, "Why does a dog lick his $*((s?", it's happening because it can.

Also, the oxygen isn't passing through "holes" like through Swiss cheese--it's chemically latching on but the bond is weak and it finds new "footholds" in adjacent imperfections. Higher temps encourage the process.

There seems to be some debate over whether it is oxygen molecules (O2) or atoms of oxygen but it really doesn't matter.

Dale

It may be time

Ron, it may be time for you to get your boss to send you to the Wirsbo school in Minn. lots of fun and you get tons of great info. Kind of a learning vacation which is the best kind.

Dale

PE in general

Most soft plastic is really pretty leaky to gasses. If you look at new underground gas pipe it's all PE kind of a really cheap pex in that there is no cross link. And strangly the molecules in the pipe leak gas through them, about a cubic foot per mile per day at 60 psi. You may have read about the hydrogen economy? Won't work with normal PE pipe since the hydrogen would leak right through the wall. What will work is PAP with hydrogen. If you've been to Dan's water class he talks about helium balloons of regular rubber that leak out but the black mylar coated one's you get at 50 holding for quite awhile, also the rubber diaphranms in expansion tanks leaking air through to the water. Funny if you think about things the older we get the more we know that we often really don't know.

jerry scharf_2

dissolving gases

Most plastics one way or the other start with simple molecules (often oil based) and then through chemical processes create much longer, complex ones. The process is not infintely precise, so there are sure to be local variations and incomplete chemical actions in very small areas. The process engineers work really hard and they also make things relatively thick, so the effects of these local defects are minimized and they can work well with water under moderate pressure.

Gases are a whole differnet game, and are much more able to work their way through plastics. There are many amazing things that gases can do, including dissolving into fluids, plastics and metals. It turns out that most gases don't go into metals very well, but most polymers, including pex, are not designed to prevent gas dissolving into it. Once it can dissolve in, it can come out the other side.

The oxygen barrier does not prevent oxygen movement, only slows it down. The Europeans came up with a target based on how much oxygen can come through and how much could be tolerated in a heating system. This was based on a cast iron boiler and ferrous pumps. With the advent of stainless steel HXs, suddenly you've decreased the amount of iron in the system and the tolerable amount of oxygen must go down to save the pumps. So IMO even oxygen barrier pex should have water trated with an oxygen scavenger if you're using ferrous pumps and an SS HX. Thanks to Hot Rod for first waking me up to this!

jerry

Arthur

Oxygen Barrier.

? I would be surprised if O2 can leak thought PAP. Although I would tend to agree with you on pipe which is just coated.
I've been in the heating business for over 40 yrs and have yet to see a heating system rotting out from the inside out but have seen dozens & dozens rotting out from the outside in. most under ground pipework.

hr

UV

will also break down the external O 2 barriers. More than 30 days in direct sunlight, they tell me, will ruin the O2 barrier also.

Dragging pex over concrete walls, rebar, etc will scrape off the film also. The EVOH temds to curl and flake off after a deep scratch starts the process. I have examples of this, somewhere

I have heard UV from fluorcent lighting in mechanical rooms will also ruin the O2 barrier over time. Don't know how long, really!

With all this "potential" why would you chose pex with the barrier in such a fragile location?

There have been some pex manufactures that have the O2 layer between the walls of the tube. I think Weil offered this tube for awhile. An Irish extruded tube, I believe. A Weil awhile, actually

Another of the big reasons I use PAP now for all my radiant. Aluminum is a "for sure" O2 barrier. Thankfully the composite rubber manufactures acknowledged this, also

There are other pitfalls to consider with aluminum barriers, however. If for any reason the aluminum barrier gets exposed to the fluid it (alumunim) is very ph sensitive. Unless you crawl through all your PAP do you really know the layer is protected by the inner pex channel??

I'd still highly recommend hydronic fluid conditioners with PAP, or any system for that matter. We all know how nervous the industry is about the recent sighting of aluminum based HXers.

Proper fluid testing and care will be CRITICAL to the longevity of these castings, IMO Time will tell, it always does!

hot rod

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