What pipe diameter and PN/SDR mean
Numbers on a pipe card are not decoration. They decide whether the line survives pressure and temperature. Basics: guide.
How to read 20×2.8 PN20
Rozma 20×2.8 PN20: 20 — outer diameter mm; 2.8 — wall thickness mm; PN20 — pressure class. Larger diameters: 25×3.5, 32×4.4. Glass-fibre / reinforced variants: {{product:44250|}} / {{product:44251|}} — read the exact card.
SDR and hot water
SDR links wall thickness to diameter. Hot water often needs a thicker wall / another class than cold. Use the manufacturer table, not “forum PN”.
Why you do not downsize “by eye”
Smaller diameter = higher losses, noise, weaker flow for several points. A mixer’s 1/2″ thread does not mean the whole house may be 16 mm “because it fits”. Follow the project or the existing line.
Typical mistakes
- Confusing inner and outer diameter.
- Ignoring PN on boiler/hot lines — water heater.
- Buying a fitting for another diameter “because it will tighten”.
FAQ
Is PN20 always enough?
Often yes for domestic water, but temperature + the manufacturer table beat a single number in an ad.
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