Problems resulting from use of non-oxygen barrier tubing surface to the homeowner in a number of ways – through heating system failure, continuing repair bills, repeated expansion tank change-outs, or poor heat delivery. With uninformed people creating confusion and panic by telling the homeowner 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 tanks, 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 And 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.
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.
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 the 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 utilizing 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.
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 manufacturers. The Europeans went through the same learning curve we’re going through now. Once the Europeans recognized the problem, they reacted by introducing barrier pipes and rescued the radiant floor industry.
There was a lot at stake. The hydronic market in Europe is about 12 times that of the U.S. and Canada. We also have a lot at stake. Our hydronic 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 pipes 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 to 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 dominant 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 the system into a primary boiler and a 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 the 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 a 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 their 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 the 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