A plastic pipe never rusts itself — but it breathes. Oxygen from the air migrates through the pipe wall into the water and, carried by circulation, reaches every steel component in the system. Within a few heating seasons a perfectly good closed loop turns into a producer of black sludge and leaks. Here is how the oxygen barrier actually works, and why in heating it is a requirement, not marketing.
The mechanism: how oxygen gets in and what it destroys
In a closed underfloor-heating loop the water touches a very large pipe surface. O₂ molecules dissolve in the polyolefin (PEX, PE-RT) and diffuse inward, then travel with the flow into:
- the steel boiler heat exchanger — pitting and through-corrosion;
- steel or cast-iron radiators and coils — rust and blocked channels;
- brass valves and the manifold — dezincification and creep;
- the circulation pump and temperature controls.
The reaction product is magnetite (Fe₃O₄) — a black, magnetic sludge. It settles at low points, erodes pump impellers, cuts heat output and eventually causes valve leaks and cracked exchangers.
DIN 4726 and barrier classes
The German standard DIN 4726 limits the specific oxygen permeation of heating pipe to roughly 0.1 g/(m³·day). By construction:
| Construction | O₂ permeation (indicative) | DIN 4726 | Barrier reliability |
|---|---|---|---|
| Plain PEX / PE-RT (no layer) | high (tens–hundreds × the limit) | fails | no barrier |
| PE-RT / PE-X with EVOH | low, mg/(L·day) range | passes | low-diffusion, not absolute |
| PEX-AL-PEX (aluminium layer) | practically 0 | passes with margin | 100 %, permanent |
EVOH vs aluminium — the real difference
EVOH (ethylene vinyl alcohol) is a co-extruded layer on the outer or inner side of the wall. It cuts diffusion by roughly 100× and more, but it does not zero it out: EVOH is hygroscopic and sensitive to damage. If the layer is scraped, sanded or exposed at a fitting, the barrier is compromised at that point; UV and improper pipe prep degrade it over time.
The aluminium layer is metal — oxygen physically cannot pass it. In PEX-AL-PEX it is sandwiched between two shells and adhesive layers, so it is protected from abrasion and UV. Diffusion is near zero and does not age: the barrier stays 100 % for the whole service life of the pipe.
How to verify the barrier on delivery
- Read the print. The wall should state the heating duty, a DIN 4726 reference, "O₂ barrier" / "sauerstoffdicht", date and batch standard.
- Cut a sample. On composite pipe you see the bright aluminium layer on the end face; on EVOH pipe you see a thin, colour-contrasted layer you can peel.
- Ask for the mill test report. Request an oxygen-permeation test per DIN 4726 / ISO for the specific batch, not a catalogue average.
- Check EVOH thickness and side. Outer EVOH abrades on site; for high-temperature duty prefer a protected inner layer or aluminium.
- Specify it in the design. For closed loops with a boiler, barrier pipe goes the full length, including the tails to the manifold.
Where else oxygen comes in — and what to do
Barrier pipe does not solve everything: oxygen also enters via fresh make-up water, old steel sections and leaky fittings. A practical set: keep the closed loop topped up with deaerated or demineralised water, use anti-diffusion (barrier) pipe on every hot section, and dose a corrosion inhibitor when dissimilar metals meet. But the first and biggest step is not to skimp on the pipe's own barrier.
When the barrier is a "must" — and when its weight fades
The professional rule of the German-speaking market is direct: a closed heating circuit must not be fitted with plastic pipe that has no EVOH or aluminium barrier, and the certification-body position is that only pipe meeting the oxygen-tightness requirement can do without additional corrosion protection. In procurement terms the barrier becomes a hard requirement when the circuit contains oxygen-sensitive steel — a steel boiler, plate heat exchanger or steel radiators; when the loop is closed with pressurised top-up; when the design documents follow an EN 1264 calculation with a long maintenance cycle; or when a renovation connects to an existing network that already carries corrosion products (then oxygen control and water treatment must be read together). In an all-plastic, steel-free or non-closed system the weight of oxygen permeation drops — that is decided by the circuit, not by how premium the words "oxygen barrier" sound.
How to read a barrier figure: the unit is the trap
Oxygen-permeation requirements for plastic and composite pipe in heating and radiator connections are usually cited from DIN 4726. A widely quoted value is 0.32 mg/(l·d) at 40 °C, but a per-unit-area-per-day basis also circulates — the test basis and unit differ, so the numbers cannot be compared side by side. Edition matters too: DIN 4726 is now 2024-12, replacing 2017-10; the new edition adds barrier thermal stability and layer thickness for heating polymers, compatibility with heating-water additives and bonding with the system, and assigns the oxygen-tightness test method by pipe outside diameter. The buying-side rule is to demand all four elements together — standard number, edition year, test temperature and unit; missing any one makes the value a reference, not a judgement.
Permeation figures are indicative and vary with wall thickness, EVOH and batch — confirm against a test report and a live quote.