"Run 16 mm pipe for the underfloor" is a phrase where buyer and factory often picture different things: different wall thickness, different SDR and therefore different working pressure. Let's turn the markings into calculable quantities — outer diameter, wall thickness, SDR, PN and how they relate to temperature. This is the minimum you need to write a clean RFQ and audit a supplier.
Reading the size: OD × wall
Polymer pipes are marked by outer diameter × wall thickness in millimetres, e.g. 16 × 2.0 or 20 × 2.0. The outer diameter is the "fitting size"; the wall is what carries the pressure. Two pipes of the same OD but different wall are different pressure classes.
SDR: the key to everything
SDR (Standard Dimension Ratio) = outer diameter ÷ wall thickness:
The lower the SDR, the thicker the wall and the higher the pressure. PEX/PE-RT typically use SDR 7–9 (thick risers) and SDR 10 (underfloor); PP-R is thicker still, SDR 6–8. The same Ø20 mm at 2.0 and 3.0 mm wall gives completely different PN.
PN and the link to temperature
PN (Nominal Pressure) is the maximum working pressure at 20 °C for a 50-year design life. On hot water the margin shrinks as the polymer softens with temperature. It depends on the material's MRS (minimum required strength: PEX ~8.0 MPa, PP-R ~10.0 MPa) and a service (design) coefficient C:
| Class (indicative) | 20 °C | 40 °C | 60 °C | 80 °C |
|---|---|---|---|---|
| PE-RT / PEX PN10 | 10 bar | ~8 bar | ~6 bar | ~4 bar |
| PE-RT / PEX PN16 | 16 bar | ~12 bar | ~9 bar | ~6 bar |
| PP-R PN20 | 20 bar | ~14 bar | ~9 bar | not for continuous 80 °C |
These derating figures are indicative, for sizing only — take the real P/T curve from the batch data sheet and a live quote, as they depend on resin grade and C.
Common sizes and coil lengths
For underfloor and in-building distribution in Central Asia the workhorses are 16, 17 and 20 mm; aluminium composite comes in 16/20/26 mm, coiled or in straight lengths. A single heating loop is usually kept to no more than ~80–100 m, otherwise the pressure drop grows and the pump cannot drive the circuit.
| Size OD×e | SDR≈ | Typical use | Standard length |
|---|---|---|---|
| 16 × 2.0 PE-RT (EVOH) | 8 | underfloor heating loop | coil 120 / 240 / 300 / 500 m; ≤ 100 m per circuit |
| 17 × 2.0 PE-RT / PE-X | 8.5 | underfloor heating loop | coil 120 / 240 / 300 m |
| 20 × 2.0 PEX-AL-PEX | 10 | branches, tails, short risers | coil 100 / 200 m |
| 26 × 3.0 PEX-AL-PEX | 8.7 | heating mains | coil 100 / 200 m or 5 m straight |
| 20 × 2.8 PP-R | 7.1 | cold/hot water risers | 4 m straight |
How to use this when ordering
- Name the pipe fully: OD × wall × material × barrier class (e.g. "16×2.0 PE-RT, EVOH, DIN 4726").
- Give each circuit its length and diameter so the pressure drop stays within the pump budget (typically ≤ 0.2–0.25 bar per loop).
- Confirm the quoted PN has been derated to the working temperature, not just 20 °C.
Worked example: choosing the wall for a duty
Given: a heating main at 70 °C and 6 bar. Take PEX-AL-PEX or PEX at SDR 7–8 (about 2.2–2.7 mm wall on Ø20) so that, after derating at 70 °C, the pressure margin stays above 6 bar. For underfloor heating (60 °C, 4 bar), 16×2.0 (SDR 8) or 17×2.0 is enough. The logic is always the same: start from working temperature → coefficient C and allowable stress σ → pick the wall thickness from SDR, not the other way round.