Technical Notes
Radiant Panel Assocation, PO Box 717, Loveland, CO 80539 (970) 613-0100
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.
- Use a tubing with an oxygen barrier as mentioned above. Almost all
synthetic tubing is now available with an oxygen barrier option.
- Use non-barrier pipe with the knowledge that there is a potential for
accelerated corrosion of ferrous metal parts which may or may not significantly
reduce the life of those parts affected, and plan accordingly.
- Corrosion inhibitors (water treatment) designed for hot water heating
systems can be added to the water to control corrosion. This method requires
a routine inspection and maintenance program.
- Because only ferrous metals are at risk, a heat exchanger can be used
to separate the heat transfer fluid in non-barrier tubing from that in
ferrous components such as cast iron boilers.
- The system can be designed and installed without any corrodable ferrous
metal components.
