Oxygen Permeation (Diffusion)
Definition: The ability of oxygen molecules to pass through a material due to the material's molecular structure and a difference in the partial pressure of oxygen on each side.
Relevance: Oxygen can be introduced in significant enough quantities that, when combined with other factors, might cause unacceptably high rates of corrosion on ferrous metal components.
Discussion: All hydronic heating and cooling systems are susceptible to oxygen entering the system through numerous sources such as threaded fittings, air vents and gas permeable materials. Excess amounts of oxygen in a system can lead to premature failure of ferrous metal components due to corrosion. While copper tube is, for all practical purposes, impervious to oxygen migration through its walls, all synthetic tubes display a degree of permeation unless specifically designed to prevent the intrusion of oxygen. Whether this characteristic can lead to problems for ferrous metals in radiant floor heating systems is dependent on a variety of factors. The amount or length of tubing in a system can play a significant role since it is the available wall surface that makes the tubing suspect. Water temperature must be given a strong consideration because as the temperature of the tube wall increases it becomes more permeable. The internal system pressure and the speed of flow are less a factor. Water quality can also greatly impact the corrosion process and cause acceleration beyond acceptable limits.
Although oxygen permeation is measurable in the laboratory, there has been much debate as to the degree of long term affects in actual installations. With radiant floor systems numbering in the hundreds of thousands over the last twenty years in the United States, there has not been strong evidence to indicate that oxygen permeation in synthetic tubing has contributed significantly to wide spread system failures. It should be noted, however, that there have been a number of incidences of premature expansion tank failure and excessive corrosion of cast iron pumps and cast iron boilers which can be linked directly to excess oxygen in the system.
Oxygen Barriers: Manufacturers offer synthetic tubing which incorporates an oxygen diffusion barrier which dramatically reduces the measurable amount of oxygen permeation to the point where it is no longer in question. The German DIN Standard calls for an oxygen permeation rate of less than 0.1 gram per cubic meter per day and is generally accepted as the measure of an oxygen tight radiant heating tube. Keep in mind that this is only a concern when ferrous metals are present in the system. Also note that oxygen permeating a non-barrier tube may or may not be in significant quantities to cause corrosion problems.
Action: Where oxygen permeation is in questions there are several ways to deal with the potential corrosion problem.