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Oxygen-diffusion corrosion in radiant heating systems

Too much oxygen is bad for any hydronic system because it corrodes the iron and steel. You know this is true if you’ve ever lived with a boiler relief valve that keeps popping off. The oxygen in the cold fill water comes out of solution when the boiler fires and goes to work on the metals that are lowest on the metallurgical totem pole. Usually, the iron gets the worst of it. In plain English, we call this rust. You know rust, right?

Anyway, oxygen makes metals that are ferrous (that means made of iron or steel) rust. The more oxygen there is, the worse the rusting will be. Now here comes the spooky part. The wrong sort of hydronic radiant tubing can allow oxygen to pass right through it -

even if it’s buried under concrete floor, a driveway, or a sidewalk. This phenomenon has nothing to do with the pressures in the system. You could have 100-psi water pressure inside the tubing and the oxygen will still get in – if it’s the wrong sort of tubing.

And how come those Mylar balloons take a lot longer to go limp? You know the sort I’m talking about? The shiny silver ones that have things like, "Over the Hill," and "Happy Birthday You Old Fart" printed on them? How come they last longer? It seems that some materials do a better job of holding gases than others, eh?

And therein lies the great answer to how the hydronic radiant tubing sucks up the oxygen. It has nothing to do with the pressure of the liquid, it has to do with the concentration of the gas we call oxygen. Mother Nature likes to balance things, and if she finds more air molecules inside an ordinary balloon than she finds outside, she will move the air molecules right through that old balloon. She can do this because an ordinary balloon is a semi-permeable membrane. That’s a fancy way of saying there are tiny holes in the balloon. These holes are so tiny that liquids can’t pass through them, but gases can. If you filled that balloon with water instead of air, that balloon would stay filled until you tossed it.

Now, follow this. When you fill a heating system with cold water, the oxygen that leaves the heated water will react chemically with the iron and steel in the system. The reaction eats up the oxygen and forms a compound called iron oxide. Iron oxide, in plain English, is rust. It’s the black stuff you see suspended in the water. The longer you can leave water in an all-metal heating system (one without plastic or rubber hydronic radiant tubing) the better it gets. That’s because once the oxygen reacts with the ferrous metals and forms iron oxide, the corrosion stops. That nasty, stinking, black water is like fine wine to a hydronic system. It’s the good stuff. And as long as you don’t keep adding more fresh water, you will have no more corrosion.

So here’s where the tubing comes in. As soon as the oxygen burns itself up, chemically speaking, the water inside the system finds itself in oxygen deficit. In other words, there’s more oxygen in the air that’s outside the tubing than there is in the water that’s inside the tubing. Since the tubing is a semi-permeable membrane, Mother Nature begins to shove oxygen molecules through the wall of the tubing and into the water. And that causes more corrosion. If this goes on long enough, you wind up with this sludge (rust) that flows around with the water, messing up anything that has a close tolerance to it, such as a control valve or the business end of a circulator. The oxygen also loves to go to work on the thin steel in a diaphragm-type compression tank. Oxygen won’t usually damage a cast iron or steel boiler because most of these boilers have relatively large passages. The flow of water, however, will carry away the sludge and dump it in places you never even knew your system had, and that’s why oxygen-diffusion corrosion is something you need to avoid.

Now, you should know that temperature plays a part in this. Generally, the hotter the water is the worse things get. If you keep the temperature below 140 F, the corrosion doesn’t show itself in such a dramatic way. And since most slab installations will have the temperature of the water down around 110 degrees F, or so, oxygen-diffusion corrosion is less of a big deal. The jobs that will get you, though, are those staple-up jobs because the water will be hotter here, as it will be on those snowmelt jobs.

But let’s get back to those balloons for a minute. How come the Mylar balloon doesn’t lose its air so readily? And have you ever noticed when you’re eating a bag of potato chips that the bag you’re holding looks a lot like a deflated Mylar balloon?

What you have in your hand, my friend, is an example of an EVOH, which stands for a very long word I will not get into right here. An EVOH is a material that does not readily let gases through. Air can’t easily get in or out. That’s why the expiration date on the potato chips is something like 2055. It’s also why your Mylar balloon hangs around your ceiling so long. This material is not a semi-permeable membrane.

Hydronic radiant tubing manufacturers use an EVOH that’s similar to Mylar on their tubing to keep the oxygen from working its way into the water. Lately, some manufacturers have switched to a thin layer of aluminum, which they sandwich between layers of either PEX or rubber. Aluminum is a really good oxygen-diffusion barrier because it is a metal. Gases will not pass through metal. A balloon made from aluminum may not be practical, but it will never deflate.

If you use tubing with an oxygen-diffusion barrier such as aluminum or an EVOH coating, oxygen will not enter the tubing and you will not have any abnormal corrosion. The controversy that took place during the early Nineties revolved around the accusation by some manufacturers that other manufacturers made tubing that did not have a suitable oxygen-diffusion barrier. They based their accusations on something called the DIN Standard. DIN, loosely translated, stands for Deutsche Industry Norm, the "Deutsche" being the Germans. Some of the folks who were selling hydronic radiant tubing in America back then adopted DIN Standard 4726. This standard demands that hydronic systems not allow in any more than one-tenth of a milligram of oxygen, per liter of water, per day, when the water is 40 degrees Celsius (104 F). In case you’re wondering, this is an extraordinarily small amount of oxygen. Nowadays, every tubing manufacturer has a product that will meet DIN4726, but that wasn’t the case in the early Nineties, thus the controversy.

But having a tubing that meets DIN4726 isn’t the only way you can protect a hydronic radiant or snowmelt system from oxygen-diffusion corrosion, and tubing manufacturers who weren’t meeting the strict German standard were very vocal about this. You see DIN4726 doesn’t demand that you do it a certain way; they just say DO IT! And you can do it in a couple of other ways.

First, you can separate the ferrous (iron and steel) parts of the system from the nonferrous parts of the system by using a stainless steel heat exchanger. The most popular are the braised-plate exchangers. Those are those tiny ones that look like silver bricks. They keep the water in the plastic or rubber tubing isolated from the steel and cast iron parts of the system, and they’re perfectly acceptable.

You can also install a system that has no iron or steel in it, of course. Brass valves and circulators, copper boilers -

these will do nicely because these materials won’t rust. Copper boilers are also very popular with snowmelt systems because they allow you to use nonbarrier tubing and thus lower the cost of the installation.

Another way to prevent corrosion and meet the standard is to add corrosion-inhibiting chemicals to the system water. The trouble with chemicals, though, is that you have to monitor them from year to year because they lose their potency over time and can actually cause corrosion. In Europe, where everyone is very concerned about the environment and pollution, contractors avoid chemicals because there are few places to dump them when it’s time to get rid of them. And that’s why you find so much oxygen diffusion-barrier tubing in Europe. When those European tubing manufacturers began to market their goods in North America, they didn’t want contractors considering chemicals or heat exchangers as a viable option to what they were selling. I’m sure you can understand why.

Nowadays, literally every tubing manufacturer marketing in North America sells a tubing that meets DIN4726. What’s fun to watch, though, is how some of them are now encouraging American contractors to use heat exchangers or chemicals and nonbarrier tubing, which they offer at a lower price. Why would they do this? Because this tubing is less expensive! It gives them a better shot at the job.

So there you have it in a nutshell. Is oxygen-diffusion corrosion a real concern? Yes, but mainly on systems where the operating temperature is above 140 F (staple-up and snowmelt systems). Should you protect every system you install by using either DIN-Standard tubing, a heat exchanger, or corrosion-inhibiting chemicals? Absolutely! It’s all basic science and it’s as close to you as that next bag of potato chips or that big red party balloon.

Like your engineering in plain-English?

I’m not an engineer so I have to work hard to figure this stuff out so I can explain it to others in a way that makes sense (hopefully!). What you just read is an excerpt from my book, Hydronic Radiant Heating - A Practical Guide for the Nonengineer Installer. If you liked what you just read you might want to hear the rest of the story. You’ll find it in our Online Store. Thanks.

This article was written by Dan Holohan,
 who is a well respected leader in our industry
 

This article and may others can be located at:
www.heatinghelp.com

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Non-Oxygen Barrier Piping


The concerns of home owners in Canada who have non-oxygen barrier tubing in their hydronic heating systems are expressed in a variety of ways. The most common reaction we see is disbelief and shock at the cost of remedies that are offered by any number of people who claim to have the answers to their problem.

 

Problems resulting from use of non-oxygen barrier tubing surface to the home owner in a number of ways – through heating system failure, continuing repair bills, repeated expansion tank change outs or poor delivery of heat. With uniformed people creating confusion and panic by telling the home owner that plastic piping used in their heating system will make it very hard to sell their home, homeowners want their systems repaired.

 

There are three basic remedies to ensure the longevity of a hydronic heating system. The first option would be to put a heat exchanger to separate the boiler water from the heating water. What this does is isolate the boiler from the oxygen barring water and thereby preventing rusting the system out. The second option would be to install a new non-ferrous boiler system with new pumps, expansion tank and related products. This will slow and prevent the rusting and corrosion that can and will develop in a hot water heating system. The third solution is to treat the water on a regular basis with chemicals to help stabilize the water and reduce oxidation. These options are discussed in more detail below.

 

Oxygen & Corrosion

In most heating systems, certain parts of the system are made of metal components – both ferrous and non-ferrous. Most metals, including non-ferrous ones, when put in contact with water will corrode readily. The extent and type of corrosion will vary with each material. In a hydronic heating system, the surface of non-ferrous metal components will passivate quickly. However, due to the inherent corrosion resistance of non-ferrous materials used in these systems, it is unlikely that these components will corrode sufficiently to fail by this process alone. However, ferrous iron components without some form of barrier- galvanizing or other plating-may, in time, corrode so severely as to fail. In most hydronic heating systems there will be at least one primary component made from ferrous materials-the boiler.

 

One of the necessary ingredients for the corrosion of ferrous metals is oxygen. The oxygen content in the circulating water may have an influence on the corrosion rate. Oxygen can penetrate the wall of standard, non-oxygen barrier plastic tubing and ingress into a closed hydronic system. The process by which oxygen passes through the tubing wall is complicated and relatively slow. However a radiant panel heating system will utilize several thousand feet of plastic tubing providing the potential for significant oxygen ingress into a closed loop system. In response to these failures, considerable research has been compiled on the subject of oxygen ingress into hydronic heating systems through the wall of the plastic tubing.

 

Process of Oxygen Ingress

The terms most commonly used to describe the leakage of oxygen through plastic are oxygen permeation and oxygen diffusion. While plastics look solid, they do in fact have tiny microscopic holes that traverse the many polymers. The size and quantity of these paths are different with the different types of plastics available. In the majority of plastic used, the holes are large enough for air to pass but not large enough for the passage of water molecules. As oxygen is depleted in the hydronic system through the corrosion process, or through another avenue, the water has a negative saturation level. This will create a differential across the tubing wall which in turn creates the driving force to effect oxygen permeation through the tubing wall.

 

The most common way to slow down or eliminate the passage of oxygen across the tubing wall is to apply a thin layer of oxygen resistant polymer to the tubing. This has been accomplished in several ways including sandwiching the barrier in the middle of the middle of the tubing or applying it directly to the outside.

 

Solutions

The first option would be to install a heat exchanger to enable the separation of the heating water from the boiler water. There are several different heat exchangers available to do this. This option enables use of the existing boiler. With the installation of the heat exchanger, you would also need to install a new pump and expansion tank along with an independent fill valve and necessary fittings to ensure proper installation. In essence, what you get is two systems running without the mixing of water media. The benefit of this method is the saving of the cost of a new boiler. If the existing boiler is still in reasonable shape this could be a viable option

 

The second method of repair can be utilized a non-ferrous boiler with all new non-ferrous pumps, expansion tanks and accessories. This would be the most expensive way to go, but if the boiler is on its last legs it may be necessary. The benefit of this option is that additional pumps and expansion tanks are not required.

 

The third option is to treat the water chemically with the intent of stabilizing the water. The treating of water in the boiler is something that should be done on a regular basis regardless of the condition of the boiler. This can help to ensure the longevity of any heating system and also allow a qualified professional to check the system to ensure it is working to its optimal performance. The long term benefit to a heating system that has non-ferrous tubing might not be huge, but it can be of benefit for short term relief.


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How to stop rubber and plastic system corrosion

Tens of Millions of feet of rubber and plastic tubing have been installed in the U.S. and Canada for the past 10 years or so to connect all types of radiation and convection units back to the boiler. Most of it however has been used in both residential and commercial radiant floor systems.


Recently members of the hydronic industry have recognized that oxygen diffusion in rubber and plastic tubing can cause corrosion in heating systems (January 1994. P.1) This, unfortunately, was dismissed by leading U.S. pipe manufacturers for years as nothing more than unsuitable water conditions or mechanical system air leaks.

 

This is not to fault American or Canadian manufactures. The Europeans went through the same learning curve we’re going through now. Once the Europeans recognized the problem, they reacted by introducing barrier pipe and rescued the radiant floor industry.

 

There was a lot at stake. The hydronics market in Europe is about 12 times that of the U.S. and Canada. We also have a lot at stake. Our hydronics market is growing, we’re looking to increase our market share and radiant heating is a way to do it. But we must do it well!

 

Oxygen diffusion barriers on non-metallic hydronic heating pipe are now finally standard in our industry. The DIN 4726 German oxygen diffusion standard was officially adopted by the U.S. Hydronics Institute as the guide to prevent oxygen corrosion in plastic systems. But remember, this is an endorsement, not a law. In Europe, it’s a law!

 

The unfortunate fact is that now thousands of building owners and contractors have to live and deal with these problem systems for years to come.

 

My answer is stay away from chemicals. Use the mechanical method. Even though both methods are listed in DIN 4726 as two of the three solutions to combat oxygen corrosion caused by diffusion, the mechanical method is the dominate method for old existing non-barrier piping systems in Europe.

 

Grade 316 stainless steel heat exchangers are used to separate the mechanical equipment (the boiler side) from the rubber or plastic heat distribution system, thus dividing system into a primary boiler and secondary radiation loop exposed to the non-ferrous tubing.

 

The system components in the secondary loop exposed to corrosive water conditions are the secondary circulator, the distribution manifold, the hard piping from the heat exchanger to the manifold and the secondary loop expansion tank. All of these components are available in the 316 stainless steel except manifolds for which brass or copper is acceptable.  High grade 316 stainless has proven to be the best long term solution. A 316 stainless tank might be difficult to find, though in Europe they are readily available. For the heat exchanger, it is best to use the plate and frame type which is most economical, compact and adds little to the pressure drop of the system.

 

After the rubber or plastic system has been hydraulically isolated from the rest of the system with non-ferrous components, the second most important consideration is to convert the system over from intermittent to constant circulation by means of an outdoor reset control on the primary side of the flat plate. This can be accomplished via mixing valve or boiler reset.

 

The reason for this is the lower system water operating temperature and the associated dramatic reduction of the oxygen diffusion rate through the pipe walls. Constant circulation however may not be suitable for all tubing. Check with the piping manufacturer to see if there tubing is suitable for constant circulation. The potential for premature internal tubing erosion could be an issue.

 

Before piping up the new system, flush the rubber or plastic system with water to remove any corrosion sludge in the tubing system and to restore the btuh output of the tubing. Chemical solvents can be used for this only upon consultation and approval of pipe manufacturer.

 

The initial cost of mechanical system separation is a relatively high one-time expense but definitely the preferred long term solution to dramatically extend the useable service life of the system.


By Joe Fiedrich
Hydronic Heating Authority


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About Entran II Hose

The Goodyear Tire & Rubber Company designed, manufactured, and sold approximately 25 million feet of Entran II hose from 1989 until 1993. That hose was then distributed throughout the United States and Canada.

There are various types of installation for the Entran II hose. Most forms of installation are as follows:

Installation Type

Description

In-slab

“In-slab” is Entran II installed in foundational concrete for purposes of hydronic radiant heat. “In-slab” does not include Entran II hose installed in a “Thin-set” application.

Staple-up

"Staple-up" is Entran II installed below flooring attached to sub-floor, floor joists, or other sub-floor structure with staples or similar fasteners for purposes of hydronic radiant heat. “Staple-up” does not include Entran II installed as “Exposed Staple-up."

Exposed Staple-up

"Exposed Staple-up" is Entran II hose installed using the “staple-up” method above on an unfinished ceiling, in a crawlspace or unfinished basement, or above an acoustic tile or “drop” ceiling.

Thin-set

"Thin-set” is Entran II hose installed in light-weight concrete, gypcrete, or similar materials for purposes of hydronic radiant heat.

Baseboard

"Baseboard” is Entran II installed as a conduit between baseboard heating elements and the boiler or other heat source.

Snowmelt

"Snowmelt” is Entran II installed for purposes of melting snow and/or ice.


Problems/Types of Damages

Entran II can cause various types of property damage including leaks at connections, corrosion, damage to the boiler or other parts, elements or components of the heating system. It can also cause serious or catastrophic failures of the heating system including major water damage and/or in-line breaks causing property damage.

How to Identify Entran II Hose

Entran II is reddish-orange in color and is usually stamped clearly on the outside with the word “Heatway” or “Heatway Systems” and a tradename such as Twintran, Nytrace, Entran II Trace, Entran II Wire, Entran 2, Entran 2 Trace, or Entran 2 Wire. The inside diameter of Entran II hose is about 1/4 to 3/4 of an inch and the outside diameter is 3/4 to 1 inch. Entran II manufactured by Goodyear has a date code printed on the hose and lists “Entran” in capital letters and lower-case letters. The Goodyear date code has both date numbers and a letter “A,” “B,” “C,” “D,” or “Z.” (Some Twintran counterflow hose manufactured by Goodyear in the year 1989 has a date code without the letter designation.)

Entran products manufactured by other companies such as Dayco, and Entran III or Entran 3, or Entran EPDM products manufactured by Goodyear, are not a part of this Amended Agreement.

 

 

Examples of Entran II Hose with "Heatway Systems" stamp.

 

 

Example showing a furnace room with Entran II Hose connections to various manifolds.

 

Example of Entran II Snowmelting Installation











Miscellaneous examples of Entran II Hose


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Radiant Floor Heating
 
      
Here are some things you should be aware of when purchasing a new home with a radiant heating system or having work done to your current radiant heating system...
     
          Radiant Floor heating is not new. The ancient Romans built fires under masonry floors. Over the ages it has appeared and reappeared. Thankfully now ,do to modern Materials you don't have to be a Roman Emperor to have warm feet. Of course the best feature of Radiant heat is the level of comfort, but I personally am very excited about the energy savings. By using radiant heating systems homes can be heated with very low water temperatures.
              

I have witnessed, over the years, different technologies become popular, some fade away and some stick. In the mid nineteen-seventies wood boilers & furnaces became the rage. It seemed like everyone was building & selling them. I was fortunate because at my school was the "Energy Testing Laboratory of Maine" (This was the national listing agency, the U.L. of solid fuel ). I saw what happens when a type of product becomes in vogue, and the market becomes flooded by manufacturers out to cash in. "A Lot of Garbage hits the Streets".

              
In The late Seventies and early Eighties, Solar Hot Water Heating was the ticket. Once again it happened hundreds of solar manufacturers were out there peddling their stuff and once again, "A Lot of Garbage hits the Streets".
                
Now we are in the age of "Radiant Floor Heating" and it seems that everyone and his brother is making it & selling it. You guessed it, "A Lot of Garbage hits the Streets". Whether it be their method of installation, their tubing, the fittings, their insulation, or any other of the multitude of accessories, consumers should be cautious.
                       
Two major tubing failures have already occurred resulting in multi million dollar lawsuits, & settlements. Those are Polybutylene, & Heatway Entran (rubber tubing). It seems certain now we have shaken out the bad tubing. Maybe.
                 
PEX is the right stuff. But now the challenge is ,everyone & his brother that can extrude polyethylene is making PEX. (By the way PE stands for polyethylene & X stands for cross linking of the PE at a molecular level, giving the tubing great strength, to withstand pressure & temperature) As with all products there is good, and not so good.
                
I visited a plant on a hillside in the middle of nowhere in Ireland. Among my group was an American boiler company representative, he was checking out the products to consider importing their tubing & re-labeling it with their name. While it was truly fascinating watching the production, I felt I was visiting an illegal moonshine operation. It was an eye opening experience. Seeing, what type of  operation is probably, producing a lot of the obscure PEX products flooding the market.
                    
The Internet has become a source for a lot of shaky science & products. Beware! Internet is wonderful in many ways, but people to often use it to find bargains. Being a frugal yanked , I love a good deal but I try to recognize "if it sounds to good to be true, it probably is". I strongly suggest sticking with the big name products - "Wirsbo", "Stadler", and "Rehau", just to name a few.
                    
Buy tubing that is manufactured by the company whose name is on it. A product may say "ACME TUBING" on it but that does not mean "ACME" produced it. There are many companies that change their supplier based on  a penny-or-two price difference per foot, and you may end up buying a different product than what your neighbor bought last month.
               
Another important consideration is the "Oxygen Barrier". I was surprised and very skeptical when I first heard that oxygen will pass through the walls of plastic pipe (and concrete). There was a great deal of debate when plastic tubing was first being used for radiant heat. Why should it Matter..? Well, first of all fresh water is very corrosive (it will eat through steel (even low grade Stainless, or cast Iron). In our "closed" heating systems the water becomes "dead". The systems do not continually add water as some people believe, but it is in fact the same water reheated and re-circulated. The air (with oxygen) is expelled from the system and the water loses it corrosive characteristics.
                
It is hard to believe that this water we drink can ruin a boiler vessel, but it does.
                
Polybutylene tubing was used extensively for radiant heat, it had no barrier. We replace 2-3 failed boilers per year, because of non Barrier tubing. When we perform the replacement we install a heat exchanger to protect the new  boiler from the old tubing.
                 
In the rush to radiant floor heat systems, people are working like hell to find a way to do it cheaper.
               
Because the water Temperatures are relatively low many people have decided a simple water heater can do the job. There are some hybrid water heaters, (but they cost as much as a boiler) available for this type of application and they do work, but the regular household models are not intended for this usage. Because they can stand fresh water, some Internet companies propose there use and then sell non barrier tubing, to go with them.
           
There are several reasons not to try this. First of all it is against Maine State Plumbing Code. While you can do it yourself and it can be made to work, you might  get away with it for a while, but someday for some reason you may have to call a professional to work on it and you will probably find it hard to get a licensed pro to touch it. Also, it can affect the sale of your property. There is also potential health risk if you are circulating domestic water through your floor, and then shower in it later. Legionnaire's Disease is one of the problems that can occur as well as other bacterial  growth that can happen in stagnant water.
                 
I hope I have not scared anyone away from Radiant Heat, this was not my intention. I strongly believe it is the best heat system going.
                   
Methods of installation have been developed to install the heat tubes in ceilings & walls, as well the floor.

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There are several methods for installing the tubing in floors. In "concrete slab" is hands down the best method the mass is a great sink, and you can run very low temperatures.

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Many homes are conventional wood frame construction, so methods have been developed to incorporate tubing, into the structure. The most common & effective is a light weight "concrete over pour".

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Methods of attaching the tubing to the underside of the floor are very common in retrofit applications. the most effective of this type employ aluminum plates to transfer the heat by way of conduction directly to the sub-floor.

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The least effective is called "joist bay" heating.The idea being that the tubing heats the air  in a space in the joist bay. This is the fastest, easiest, and cheapest installation method, but it is also the leading method of systems that do not work effectively. Operating costs are also higher than any other system.

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Getting the tubing closer to the surface is most effective. It allows the use of  lower water temperatures, and delivers much quicker response times. Some very well developed products are available which make these systems easy to install. "Wirsbo Quick Trak" and and "RauPanels".

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(I think this happy couple is looking at some photos of our work.)
       
There are many factors that must be considered in the installation of Radiant floor systems, but are too numerous to fit into in this short article.
       
Whatever you do, make sure it is a decision you are willing to live with for years to come. Initial cost is only part of the selection process. Over time the cheaper system will cost more.
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