A specifier in Riyadh, Lagos or Seville reads PN20 on a PP-R pipe and PN16 on a PE100 coil, and the site runs at 45 °C ambient for four months a year. Every one of those ratings is silently a 20 °C number. Polymer pipe UV and temperature derating comes down to one question: how much of the stamped rating survives at the wall temperature the pipe will actually see.

Under ISO 15874-2 a PN20 PP-R pipe in 60 °C hot-water duty is an 8 bar pipe. PE100 keeps 0.73 of its rating at 40 °C under ISO 13761. PVC-U keeps 0.70 at 40 °C on the ISO basis and 0.62 at 38 °C on the ASTM basis, two reference conditions. PEX is rated 100 psi at 82 °C and 80 psi at 93 °C.

This page puts the four materials on one axis, works the PN20 example through the standard’s own tables, and ends with the yard rule for storage in sun.

Two minutes on the sun half of this page: Hubbell Power Systems, a utility-hardware maker rather than a pipe supplier, shows what UV does to an unprotected polymer surface and why a black, well-dispersed compound resists it.

Every Pressure Rating Has a Hidden Temperature, and It Is Not the Same One

Start with the condition printed nowhere on the pipe. ISO 4427-1 defines the strength behind a PE rating as the 97,5 % lower confidence limit of predicted hydrostatic strength at 20 °C for 50 years, and names 20 °C as the reference temperature. ISO 15874-2 builds PP-R the same way. ISO 1452-2 for PVC-U is different: its base case is water up to 25 °C, it allows service to 45 °C with a reduction curve between, and it stops there.

The ASTM world moves the base again. ASTM D2837-25 obtains a hydrostatic design basis by regressing stress-rupture data over at least 10 000 hours and reading the 100 000-hour intercept at 73 °F (23 °C). That is 11.4 years, not 50, and an intercept rather than a lower confidence limit. ASTM D1785-26 and D2241-25a PVC pipe is rated on that basis, and so is PEX to ASTM F876-26, which is rated for water at three temperatures.

Three reference temperatures, two design philosophies. A factor table is only valid against the rating it was derived for. Multiply a PN16 ISO pipe by an ASTM-basis 0.62 and you have under-rated it; multiply a Schedule 80 PVC rating by an ISO 0.70 and you have over-rated it. The rest of this page keeps each material on its own basis and says so in every row.

The Four Materials on One Axis: Published Factors From 20 to 60 °C

The first table is the ISO-basis view. The PP-R column is not a table anyone publishes. It is the ISO 15874-2:2013 reference line for PP-R, clause 4.2 equations (3) and (7), solved for 50 years at each temperature and divided by the 20 °C value. The 20 °C result, 9.71 MPa, matches the standard’s Table A.2 (6.93 MPa design stress times the 1.4 cold-water coefficient) within 0.01 MPa, which is the check on the arithmetic.

The PE100 column is ISO 13761:2017 as republished by the PE100+ Association. The two PVC-U columns are the Australian PIPA TN003 table, selected from ISO 4422-2 on a 20 °C base, and the ISO 1452 factors on a 25 °C base as printed on a certified manufacturer’s datasheet.

Source: PP-R calculated from ISO 15874-2:2013 clause 4.2 equations (3) and (7) at 50 years; PE100 from ISO 13761:2017 via PE100+; PVC-U from PIPA TN003 (2005) and a UNI EN ISO 1452 datasheet (2019); read 12 September 2026.

Read across the 40 °C row and the three ISO-lineage materials land within 0.03 of each other: 0.71, 0.73, 0.70. The differences that matter are at the edges. PE100 has a published 0.63 at 50 °C, which EN 12201-1:2024 trims to 0.62. PVC-U on the ISO 1452 basis stops at 45 °C unless producer and user agree a case above it. PP-R keeps going to 60 °C because hot water is its design duty, and at 60 °C it still holds half its 20 °C strength.

The second table is the 73 °F view, where PEX lives and where a Schedule 40 or 80 PVC rating comes from. The PVC column is the manufacturer-published table on the ASTM basis; JM Eagle’s Technical Bulletin TB09 and GF Piping Systems’ Schedule 80 specification agree at every step except 110 °F and 130 °F, where they differ by 0.01. The PE column is Table A.2 of the PPI Handbook of PE Pipe.

The PEX column is not a curve. PPI TN-52 states the rating PEX to ASTM F876 carries: continuous operation up to 100 psi at 180 °F, and 80 psi at 200 °F where the tubing is listed for it. A 630 psi hydrostatic design stress gives an SDR 9 tube 157.5 psi at 73 °F, so those ratings are 0.63 and 0.51 of the base. Nothing is published for 40 °C, and this page does not invent a number.

Source: JM Eagle TB09 (January 2009); GF Piping Systems PVC Schedule 80 specification; PPI Handbook of PE Pipe, 2nd edition (2008), Tables A.2 and 1-1; PPI TN-52 (November 2024). PEX 73 °F value computed from HDS 630 psi and SDR 9.

Now compare the two PVC columns at nearly the same temperature. At 100 °F (38 °C) the ASTM-basis factor is 0.62; at 40 °C the ISO-lineage factor is 0.70 to 0.72. The bases differ, a 100 000-hour intercept at 73 °F against a 50-year lower confidence limit at 20 or 25 °C, and the curves were fitted by different bodies. Neither is wrong. Applying one to a pipe rated on the other is.

Worked Example: What a PN20 PP-R Pipe Is Allowed to Carry at 60 °C

PN20 is a trade label, and ISO 15874 does not use it. In the standard the same pipe is series S 3,2 (SDR 7,4): dn 20 with a 2,8 mm wall, dn 25 with 3,5 mm, dn 32 with 4,4 mm, dn 63 with 8,6 mm, all from ISO 15874-2 Table 5. Check one: (20 minus 2,8) divided by (2 times 2,8) is 3,07, inside the S 3,2 series. The next series down, S 2,5, is the PN25 wall. Once you know the S number, the standard gives the allowable pressure for any class in two lines of arithmetic.

Line one is the design stress. ISO 15874-2 Table A.2 states it for PP-R by class: 6,93 MPa for 20 °C cold water over 50 years, 3,02 MPa for Class 1, 2,12 MPa for Class 2, 3,29 MPa for Class 4. The classes are temperature-time profiles evaluated with Miner’s rule for a 50-year life. As tabulated by PPI, Class 1 is 60 °C for 49 years, then 80 °C for one year, then 95 °C for 100 hours. Class 2 swaps 70 °C for 60 °C. Class 4 is underfloor heating: 20 °C for 2,5 years, 40 °C for 20, 60 °C for 25 and 70 °C for 2,5.

Line two is the pressure: design stress divided by S, and the MPa figure times ten is bar.

Source: ISO 15874-2:2013 Tables 3, 5, A.1, A.2 and clause 4.2; arithmetic by this page, 12 September 2026; class profiles per PPI TN-52 (2024).

The row that answers the search is Class 1. A PN20 pipe in 60 °C hot-water service is good for 9,4 bar by the design stress. Table 3 turns that into a qualification: S 3,2 sits below the 3,8 ceiling for an 8 bar Class 1 system and above the 3,0 ceiling for a 10 bar one.

The honest spec line is “PN20 PP-R, Class 1, 8 bar”. A 10 bar hot-water design needs S 2,5. Class 2 at 70 °C is harsher; the same pipe qualifies for 6 bar. The 80 °C year and the 95 °C 100 hours inside each profile pull these numbers below the continuous-60 °C figure of 10,1 bar; the excursions cost about 0,7 bar on this pipe.

Two cautions. These design stresses are the ISO minimum reference values a conforming compound must meet, not the measured strength of any maker’s pipe; a compound tested above the reference line may justify more, and only its own ISO 9080 data can show it. And the arithmetic assumes the pipe wall sits at the class temperature, which a later section explains how to establish. A buyer sourcing PP-R pipe and fittings for a hot-water riser should write the class and design pressure into the order, not the PN.

Back to top

Heat Also Costs Years: What the Same Tables Say About Service Life

A derating factor keeps the 50-year life and gives up pressure. The same test data can be read the other way: keep the pressure and give up years. PIPA POP013, the Australian rerating guideline for PE pipe, prints both in one row.

Source: PIPA POP013, Temperature Rerating of PE Pipes, Issue 3 (July 2019), Table 2; water service only.

Read the 50 °C row as a buyer. Even after dividing the rating by 1.4, the minimum life is 22 years, as far as the ISO 9080 extrapolation rules let the test data stretch at that temperature. At 60 °C it is 7 years. A PE100 pumping main in a 50 °C well-field is a 22-year asset on the guideline’s own terms, and the tender should say so. The PE detail, the MOP equation, the design coefficient and why fittings derate differently, is in this site’s HDPE pressure rating and SDR guide, which uses the same ISO 13761 figures.

PP-R has its own cliff. Solve the ISO 15874-2 reference equations at 50 years and the ductile branch governs to 60 °C, where it gives 4,87 MPa against 5,12 MPa on the brittle branch. At 70 °C the order flips: 4,07 MPa ductile, 3,21 MPa brittle, and the lower number wins. That knee is why the standard treats 70 °C service as a class profile with a 49-year block, and why a PP-R return line that runs at 70 °C around the clock deserves the maker’s own regression data rather than the reference line.

PEX draws its line at 180 °F. Continuous service is rated to 100 psi there; the 200 °F listing is for intermittent use, and TN-52 states that PEX should not be installed in a system designed to run above 200 °F for any significant time.

Which Temperature to Plug In: Pipe Wall Is Not Ambient

Every table above indexes on pipe-wall temperature, and 45 °C air is not automatically a 45 °C wall. PIPA POP013 gives three permitted ways to set it. Assume a constant wall temperature typical of continuous service, the cold-water case. Or, where variation is predictable, take a time-weighted average: each temperature multiplied by the fraction of life spent at it. Or, where large unpredictable swings occur, as on an exposed irrigation line, take the maximum service temperature less 10 °C, capped at 80 °C.

For a buried line with flowing water, PIPA TN003 gives the rule of thumb: mean wall temperature equals two thirds water temperature plus one third soil temperature. A 38 °C process stream through 26 °C soil is a 34 °C wall, which on the PVC-U table is a 0,80 factor rather than the 0,72 a 38 °C assumption gives. The same note adds the check most designs skip: when flow stops, water and soil equalise, so the no-flow case must be run too.

The US basis says the same about spikes. Footnote 1 to the PPI Handbook’s Table A.2 states that temporary, relatively minor increases beyond a sustained temperature have little effect on long-term strength and can be ignored; the multiplier presumes the listed temperature is continually sustained. A rooftop line that hits 55 °C for two hours on August afternoons and runs at 35 °C the rest of the year is a time-weighted pipe, and the weighted figure is what enters the table.

Back to top

Sun Is a Different Problem From Heat: What UV Does to Each Material

Heat lowers strength reversibly; a warm pipe recovers its rating when it cools. Sunlight does not work that way. PPI TR-18, the industry’s weathering report, describes the sequence: first a slight discolouration of the pigment, a chalky surface or whitening, then reduced tensile strength and ductility. Pipe under stress degrades faster: bent, highly stressed PE strips showed UV damage considerably sooner than relaxed control specimens. And a degraded surface must be removed before the pipe can be heat-fused or solvent-bonded, which makes it a jointing problem on the yard as well as a strength problem in service.

Each polymer is protected differently, and the protection is a compound decision the buyer cannot see. Black PE uses carbon black as a UV diffuser; ASTM D3350 requires a minimum of 2 %, and TR-18 states that this loading of a well-dispersed fine grade such as N-550 protects PE for more than 50 years of continuous outdoor service. Non-black PE, such as yellow gas pipe, relies on hindered amine light stabilisers instead. Polypropylene follows PE: carbon black at two to three percent gives good weathering resistance, and pigmented PP without it has a limited outdoor life.

Standard PVC compounds to ASTM D1784, the PVC 1120 cell class 12454 most pressure pipe is made from, carry enough titanium dioxide to screen UV in storage; long-term outdoor PVC needs a formulation built for it. PEX is the special case. ASTM F876-26 includes a UV-resistance requirement, yet ASTM F2657-25, the outdoor-weathering test method, states in its own scope that PEX tubing is not designed for outdoor use.

PP-R needs its own note. ISO 15874-2 has no weathering or UV requirement; its only light clause, 5.2, limits a pipe declared opaque to 0,2 % visible-light transmission, which is about light through the wall, not resistance to it. Any UV claim on a PP-R datasheet is a manufacturer claim backed by whatever weathering test the maker ran, and colour is no proxy. The single-material pages on this site, what temperatures damage cold-water PP-R and whether HDPE resists UV degradation, should be read with that in mind.

The Yard Rule: How Long Each Pipe Can Sit in the Sun

The published answers are shorter than most yard managers assume; the table collects them from PPI TR-18 (2019) and PPI TN-32 (May 2025), with the protective mechanism named.

Source: PPI TR-18/2019 sections 4.0 to 6.0; PPI TN-32 (May 2025); PPI presentation UV Resistance of PEX Tubing (2021) quoting ASTM F876 Table 1.

Three months for non-black polypropylene is the number that surprises people, and it applies to the green and white PP-R most of the world buys. A container that lands in June and sits uncovered on a 45 °C Gulf yard until October has used its published allowance before the first joint is welded. PEX is stricter in spirit: TN-32‘s recommended label reads “Do not store unprotected PEX outdoors” and “UV damage is not visible to the naked eye”.

The second digit of the PEX designation code (PEX 1106 against PEX 5306) tells you which exposure class the tubing was tested to, and potable-water PEX must carry at least a 1. ASTM F2657 accumulates a set UV energy near Phoenix, Arizona, facing south; PPI notes that a month there may equal two to three months elsewhere, so the label’s day count is conservative for a European yard and about right for Riyadh.

The yard practice is short. Keep PEX and PP-R in original packaging under cover from day one. Rotate black PE and PVC so nothing sits past a season, and cut back any chalked surface before fusion or solvent welding, because TR-18 is explicit that a weathered surface will not bond. Date the bundle tag; the limit runs from the day the pipe left the factory’s shade.

Back to top

What to Ask a Supplier For, Including Us

Turn the tables into three lines on the request for quotation. First, the basis: which standard the rating is declared to, at what reference temperature, and the factor table from that same basis for the wall temperature you calculated. A supplier who sends an ASTM-basis chart for an ISO 4427 coil has not read the question.

Second, the evidence at temperature. For PP-R, ISO 15874-2 conformance is shown by hydrostatic tests at 20 °C, 60 to 70 °C and 95 °C with at least 97,5 % of results on or above the reference line. Ask for that plot or the compound supplier’s ISO 9080 classification, and for hot-water duty ask for the class and design pressure on the offer.

For PE, ask for the carbon-black percentage and dispersion grade, or the HALS package if the pipe is not black. For PEX, ask for the four-digit designation code; the second digit is the UV class. For PVC, ask for the D1784 cell class and whether the compound is a storage grade or an outdoor-service grade.

Third, apply the same test to this site. IFANPRO’s live catalogue pages for HDPE, green and white PP-R, PEX and PVC were checked on 12 September 2026, and none states a UV-stabiliser package, a carbon-black loading, or a hydrostatic result at 60 °C or 95 °C. Ask for it in writing before a hot-climate order, and treat any rating in the offer as a 20 °C number until the class, design pressure and basis are named.

For a buried main, start from the HDPE pipe and fittings range with a line that reads “PE100, SDR11, PN16 at 20 °C, ISO 13761 factor for 40 °C wall, 50-year design”; that is a specification a technical desk can answer with numbers.

Specify the class and the wall temperature, not the PN

For project procurement teams and importers ordering HDPE, PP-R, PEX or PVC by the container for 40 °C-plus sites, who want the basis, the design pressure and the compound data written into the offer. IFAN runs all four lines, plus PPH and brass, from one plant.

See the HDPE pipe range

Conclusion

The risk in a hot-climate pipe order is a rating read off the pipe wall and never corrected, or corrected with a factor from the wrong basis. A PN20 PP-R riser specified for 10 bar at 60 °C is under-walled by the standard that rates it; a PE100 main accepted at PN16 for a 50 °C well-field carries a 22-year life on the guideline that governs it. Neither shows at commissioning. Both show in year eight.

Before you sign, fix the pipe-wall temperature by one of the permitted methods, apply the factor from the same basis as the rating, and write the class or the corrected pressure into the order. Then check the yard plan, because a PP-R bundle left uncovered through one summer has spent its published exposure allowance before installation starts.

Frequently Asked Questions