PE-RT Pipe Extrusion: Floor Heating Tube Without Crosslinking

What PE-RT is

PE-RT is polyethylene of raised temperature resistance. It is a copolymer of ethylene with a small amount of a higher alpha-olefin, arranged so that the comonomer sits on the long chain rather than in short branches. That arrangement raises the melting point and, more importantly, changes how the material behaves under long-term stress at temperature.

The result is a polyethylene that holds a hydrostatic rating at 70 °C for 50 years without crosslinking. PEX achieves the same by joining the chains chemically; PE-RT achieves it by controlling the chain architecture. Both work, and the difference is in everything else.

Because PE-RT is not crosslinked, it can be recycled, it can be fusion welded like any polyethylene, and it is easier to extrude because there is no crosslinking step. It is also less resistant to very high temperature and to some chemical environments than PEX.

Table 1 — PE-RT grades. Type II is a higher-performance polymer and is specified where the same diameter must carry more pressure.

The rating, and how it compares with PEX

Both PE-RT and PEX are rated at 20 °C and both derate with temperature. The curves are close up to about 70 °C and diverge above it. At 70 °C a PN16 PE-RT pipe is good for about 8.5 bar against 9.9 for PEX-a; at 80 °C the gap widens to 7.2 against 8.6.

For floor heating, which runs at 35 to 50 °C, the difference is irrelevant. For radiator systems at 70 to 80 °C it is worth checking. For anything above 80 °C, PEX or a different material is the right choice.

The advantage PE-RT has is not in the rating but in the installation. Because it is a thermoplastic rather than a crosslinked one, it can be joined by fusion welding, it can be recycled at end of life, and a damaged section can be cut out and re-jointed on site. PEX needs mechanical fittings and cannot be fusion welded.

Table 2 — Allowable pressure against temperature. Below 60 °C the two are within 15 percent of each other.

Extrusion: easier than PEX, stricter than HDPE

PE-RT extrudes on standard PE equipment at 190 to 220 °C. There is no crosslinking step, no silane dosing, no electron beam — the pipe comes off the line finished, which is the main process advantage over PEX-b and PEX-c.

The temperature profile should be slightly cooler than HDPE at the same wall, because PE-RT has a lower melting point and a narrower band between melting and degradation. 200 °C at the die is typical; 220 °C is the upper limit.

Cooling is critical for a different reason than in HDPE. Floor heating tube is supplied in coils of 100 to 300 metres, and a coil must be round enough to uncoil without kinking. A pipe that is cooled too fast has residual stress that makes it coil unevenly, and the coil will have a memory that fights the installer.

Sizing is by vacuum sleeve, and the shrinkage allowance is 1.5 to 2.5 percent, similar to HDPE. Because the wall is thin — a 20 mm SDR 6 pipe is 3.3 mm — the line runs fast and the tank is short.

Dimensions: the SDR system again

PE-RT uses the same SDR system as PPR and HDPE. A 20 mm SDR 6 pipe has a 3.3 mm wall and is rated PN20 at 20 °C. Floor heating tube is almost always 16 or 20 mm outside diameter, in SDR 6 or SDR 7.4.

The dimension that matters most in practice is ovality, because the tube has to push through a clip, a manifold fitting or a screed without jamming. An oval tube also does not seal properly in a compression fitting, and a leaking manifold connection in a floor is a very expensive repair.

Table 3 — Common PE-RT sizes. Floor heating is dominated by 16 and 20 mm.

Floor heating: what the tube has to do

A floor heating loop is a tube buried in screed, carrying water at 35 to 50 °C, that must last the life of the building without a joint under the floor. There is no maintenance access, so the tube is specified for 50 years and the joint is designed out.

That means the coil must be joint-free, the tube must be oxygen-barrier if the system has metal components, and the bend radius must be respected during installation. A PE-RT tube with an EVOH oxygen barrier layer is standard for systems with steel radiators or boilers, because oxygen diffusing through the tube corrodes the metal.

The barrier layer is coextruded, usually 0.1 to 0.2 mm of EVOH on the outside. It is thin, it is not structural, and it is easily damaged by a sharp clip or a careless pull through a stud. A barrier tube with a scratch through the EVOH has no barrier at that point.

Testing and the checks that matter

PE-RT pipe is tested by hydrostatic pressure at 20, 60 and 95 °C, and the results are extrapolated to 50 years. The 95 °C test is the one that separates a genuine PE-RT from a standard PE that has been relabelled.

On the line, the checks are dimensional: outside diameter, wall thickness, ovality and length. A laser gauge at the haul-off gives a continuous record and catches drift before it becomes a coil of scrap.

For floor heating, the installation test is a pressure test before the screed is poured and a second one after. A tube that passes before and fails after has been damaged by the screed work, and the repair is a joint under the floor that the specification was designed to avoid.