EVOH for Underfloor Heating Pipe (PE-RT/PEX)

Oxygen Ingress Thresholds in Residential PE-RT Floor Heating Circuits

In residential radiant floor heating, oxygen diffusion through the pipe wall is the controlling corrosion parameter for ferrous pump housings, manifold bodies, and heat exchanger internals. DIN 4726:2017 requires warm-water surface heating pipe to demonstrate oxygen permeation not exceeding 0.1 g/(m³·d) at 40°C. A five-layer PE-RT/tie/EVOH/tie/PE-RT pipe achieves this threshold when the EVOH core thickness is held at 0.08–0.10 mm in a 16×2.0 mm pipe, equivalent to 4.0–5.0 vol% of the total wall volume. In a typical layer split, the inner PE-RT layer is 0.88 mm, the two tie layers are 0.06 mm per side, the EVOH core is 0.10 mm, and the outer PE-RT layer is 0.90 mm. The EVOH grade is normally selected from ethylene contents of 32–38 mol% and a melt flow rate of 1.6–4.0 g/10 min at 210°C/2.16 kg, because lower ethylene content raises barrier performance but increases moisture uptake and processing sensitivity. The oxygen barrier test is conducted according to ISO 17455, and the acceptance limit is expressed per cubic metre of internal pipe volume per day. Because water vapour still diffuses through the inner PE-RT layer into the EVOH core, the specified EVOH thickness includes a long-term margin for moisture plasticization of the copolymer.

On the extrusion line, the barrier core is coextruded between two maleic anhydride grafted polyethylene tie layers; direct PE-RT-to-EVOH contact without tie resin produces low peel strength and delamination during coil winding and installation bending. EVOH pellets are pre-dried in dehumidified-air hoppers at 80–90°C to a moisture content below 0.3 wt-%, and the melt temperature is maintained at 210–225°C to prevent gel formation at the die lip. The die gap and melt temperature are set to control interfacial wave instability between the tie resin and EVOH; a melt viscosity ratio outside 0.5–2.0 at typical pipe extrusion shear rates produces visible layer nonuniformity and local thin spots. Vacuum calibration with cooling water at 15–20°C and closed-loop diameter control are used for finished pipe. Terminal product configurations include 12×2.0 mm, 16×2.0 mm, and 20×2.0 mm coils in lengths from 120 m to 600 m for residential manifolds and floor-loop layouts.

In high-temperature radiator distribution networks where supply water is controlled at 80–90°C and return water is allowed to fall to 40–60°C, PEX/EVOH/PEX multilayer pipe is specified to prevent oxygen ingress into closed hydronic circuits. ISO 15875-2 and ASTM F876/F877 govern the PEX layers, while the oxygen barrier function is verified against DIN 4726:2017 with the same 0.1 g/(m³·d) limit at 40°C. For a 20×2.0 mm pipe, the EVOH core thickness is typically 0.10–0.12 mm, corresponding to 5.0–6.0 vol% of the total wall, and for continuous high-temperature service the upper end of 0.15 mm is used. EVOH grades with ethylene content of 38–44 mol% are preferred in this segment because the higher ethylene content improves flexural fatigue resistance when the pipe is routed through joist spaces and around radiator tails. Since the oxygen diffusion coefficients of polyolefin and EVOH increase with temperature, qualification at 40°C per DIN 4726:2017 is not automatically sufficient for 80–90°C service, and additional ISO 17455 testing at elevated water temperature is performed by pipe manufacturers. Tie resin layers of 0.08–0.10 mm per side isolate the EVOH from the PEX layers and maintain adhesion after repeated thermal cycles.

Production of PEX/EVOH/PEX pipe involves coextrusion of the PEX outer layers, tie layers, and EVOH core through a multi-manifold or feedblock die, followed by vacuum calibration and, for silane-crosslinked PEX-b systems, moisture crosslinking in a separate chamber. The coextrusion feedblock must maintain laminar flow between the PEX, tie, and EVOH streams, and melt piping is chrome-plated to reduce polymer degradation. The EVOH melt stream is held at 205–220°C, and the line speed is adjusted so that EVOH residence time does not exceed 15 minutes; excessive residence time produces cross-linked gel particles that appear as visible specks in the barrier layer. Finished radiator connection products are supplied as 16×2.2 mm, 20×2.0 mm, and 25×2.3 mm coils or straight lengths, used with press or push-fit fittings at service pressures up to 10 bar.

How Does Oxygen Barrier Demand Shift in Low-Temperature District Heating?

Low-temperature district heating and heat pump distribution systems operate at supply temperatures of 55–70°C with design lifetimes of 50 years under ISO 22391-1/2 for PE-RT type II; this longevity shifts the barrier requirement from short-term oxygen exclusion to long-term stabilized oxygen permeation after water saturation of the pipe wall. The EVOH layer does not contribute to calculated hoop stress under ISO 22391 because the hydrostatic design curves are based on the pressure-bearing PE-RT layers alone. ISO 17455 is used to measure oxygen permeation after high-temperature water conditioning, and DIN 4726:2017 is applied as the oxygen ingress threshold even when the installed circuit is not a floor-heating loop. For dimension ranges from 25×2.3 mm to 63×5.8 mm, the EVOH layer is specified at 0.10–0.18 mm, corresponding to 4.0–5.5 vol% of the wall; a 32 mol% ethylene content grade is commonly selected to minimize oxygen transmission while maintaining enough melt strength for large-diameter annular flow. Tie layer thickness is 0.08–0.12 mm per side. At sustained water temperatures above 70°C, moisture plasticization of the EVOH core can increase oxygen transmission, so barrier performance must be re-verified at the upper service temperature rather than assumed from 40°C data alone.

The pipe is extruded on a five-layer line with gravimetric dosing of PE-RT, tie resin, and EVOH; EVOH melt temperature is held at 205–220°C. Ultrasonic or infrared wall-thickness scanning measures EVOH layer eccentricity, with a concentricity tolerance of ±0.02 mm across the circumference. Post-extrusion cooling is performed in segmented vacuum calibration sleeves to prevent inner-wall sag in diameters above 40 mm. Extrusion line speed is often governed by EVOH screw recovery capacity; when barrier layer thickness exceeds 0.15 mm, barrier extruder output may need to be increased by raising screw speed within thermal degradation limits. Finished products include PE-RT type II/EVOH multilayer pipes in 25 mm, 32 mm, 40 mm, 50 mm, and 63 mm OD, supplied as straight lengths or coils for buried distribution between heat substations and buildings; service pressure ratings are typically SDR 11 with water-side temperatures up to 70°C.

Snow-melt pipe embedded in exterior concrete is subjected to a wider thermal envelope than indoor floor heating, with stored-water temperatures as low as −20°C during winter shutdown and surface cycles from −20°C to 60°C under solar gain. The EVOH oxygen barrier layer in PE-RT/EVOH/PE-RT snow-melt pipe is sized at 0.10–0.15 mm for 20×2.0 mm and 25×2.3 mm products, equal to 5.0–6.0 vol% of the wall. EVOH grades with 38 mol% ethylene are specified for low-temperature impact resistance and reduced notch sensitivity in the barrier core. Tie layers of 0.10 mm per side are mandatory because glycol-water mixtures and concrete cure water impose different moisture uptake profiles on the inner and outer PE-RT layers, which can otherwise create asymmetric swelling stress at the EVOH interface. The outer PE-RT layer isolates the EVOH from wet concrete with high alkalinity during curing; any damage to the outer layer before concrete placement can expose EVOH to cement water and should be rejected by site inspection.

During production, the long-coil formats required for snow-melt layouts—300 m, 400 m, and 600 m—demand stable EVOH melt delivery through gear pumps and a low-shear barrier screw with L/D ratios from 24:1 to 30:1. EVOH pellets exposed to warehouse humidity above 60% RH are pre-dried to 0.3 wt-% moisture before feeding, and the hopper inlet is blanketed with dry air to prevent moisture regain. Line speed and coil winding tension are coordinated so that the EVOH layer is not subjected to compressive wrinkling at the inner radius of the coil; buckling of the core layer appears as intermittent white bands in finished pipe. The finished products are installed in hydronic snow and ice melting systems for ramps, pavements, and loading docks, with heat transfer fluid containing up to 40% propylene glycol. Oxygen permeation is verified under ISO 17455 after conditioning in water at 40°C, and DIN 4726:2017 remains the oxygen ingress ceiling.

When a Combi Boiler Merges Potable Hot Water and Hydronic Oxygen Barrier Duty

In combi-boiler installations, the same appliance supplies domestic hot water and space heating, and the heating-side hydronic loop must prevent oxygen ingress that would accelerate corrosion of the stainless-steel or aluminium heat exchanger. Multilayer PE-RT/EVOH/PE-RT or PEX/EVOH/PEX pipe is used on the heating side with the EVOH core isolated from potable-water contact by the full inner polymer layer. The relevant system standard for such multilayer piping is the ISO 21003 series, with the oxygen barrier function independently assessed by ISO 17455 and limited to 0.1 g/(m³·d) at 40°C under DIN 4726:2017. For 16×2.0 mm through 32×2.9 mm combi-system pipes, the EVOH layer is 0.08–0.12 mm, representing 4.0–6.0 vol% of the total wall; tie resin layers are 0.08–0.12 mm per side. Because the potable-water-contact layer is on the inner side, the EVOH core is separated from chlorinated hot water by the full inner PE-RT or PEX thickness during normal service.

The downstream process for potable-compatible heating pipe requires dedicated regrind control because EVOH-containing edge trim cannot be reintroduced into the inner potable-water-contact layer. Production lines use melt pumps and screen packs for the EVOH layer, and the EVOH melt temperature is restricted to 205–220°C. Finished pipe is flushed and pressure-tested before close-wound coiling; any exposed EVOH at the cut edge is sealed during fitting installation by joint design or manufacturer instruction. The terminal products include 16×2.0 mm, 20×2.0 mm, 25×2.3 mm, and 32×2.9 mm coils and straight lengths for combination boiler heating loops, low-loss headers, and hot-water recirculation return lines. Potable-water approvals are evaluated on the complete pipe wall by national schemes; the EVOH core is not in direct water contact in an intact pipe, but exposed layer edges at fittings must be sealed to prevent water wicking into the barrier core.

For commercial hydronic risers in hospitals, schools, and multi-storey residential buildings, the shift to 32–63 mm pipe diameters changes the relationship between EVOH layer thickness and oxygen ingress because the internal water volume increases with the square of diameter while pipe circumference increases only linearly. The EVOH layer is specified at 0.12–0.20 mm for 32×2.9 mm through 63×5.8 mm pipes, but the EVOH volume fraction falls to 2.6–5.0 vol% as the total wall thickness increases; compliance with the 0.1 g/(m³·d) at 40°C limit of DIN 4726:2017 must therefore be verified by ISO 17455 testing rather than assumed from volume fraction. Tie layers are 0.12–0.15 mm per side, and EVOH grades with 32–38 mol% ethylene are used to balance barrier performance with large-diameter annular melt strength.

Large-diameter multilayer pipe extrusion requires the barrier extruder to operate at the upper end of its output range, often making the EVOH layer the rate-limiting step in line speed. The barrier extruder may operate near maximum screw speed, and screw cooling is applied where necessary to keep melt temperature within 205–220°C. The pipe is cooled in long vacuum calibration sleeves with internal air pressure control to prevent inner-wall collapse, and wall-thickness and layer distribution are monitored by ultrasonic sensors at four points around the circumference. Finished products are supplied predominantly as 5 m straight lengths for riser installation, with manifold assemblies using press or compression fittings; service conditions are governed by ISO 21003 for multilayer piping and ISO 22391-2 for PE-RT material performance.