Water pipe types: PPR, PEX, CPVC and PVC — temperature, pressure and correct use

The choice of pipe is a decision that lives with you for decades inside the walls. The most common mistake we see: a pipe meant for cold water used on a hot line, failing after a few years. This guide explains the differences using international standards.

1) First: what is an "application class"?

The international standard ISO 10508 sets a classification of service conditions for plastic pressure piping for hot and cold water, for a 50-year design life1. It defines three temperatures:

• TD — design temperature: the temperature the system is designed to run at.
• Tmax — maximum temperature: occurs for short periods only.
• Tmal — malfunction temperature: the highest temperature that may be reached when controls fail (e.g. a heater thermostat failure), allowed for up to 100 hours in 50 years1.

Source: Table 1 of ISO 10508:20061. The standard also states that any system meeting one of these classes must be suitable for conveying cold water for 50 years at 20°C and a design pressure of 10 bar, and that all materials in contact with drinking water up to 80°C must pose no health risk1.

2) PPR (polypropylene) pipes

• Standard: the ISO 15874 series for hot and cold water installations inside buildings, for domestic and heating systems within the application classes2.
• Joining: heat fusion (pipe and fitting are heated and fused), forming a homogeneous joint.
• PN rating: you'll see PN10, PN16 or PN20; the number is the nominal pressure in bar at 20°C. Hot water lines need a higher rating; refer to the manufacturer's tables relating temperature to allowable pressure.
• Note: plastic pipes expand more with heat than metal; exposed hot lines need supports allowing movement per the manufacturer.

3) PEX (cross-linked polyethylene) pipes

• Flexible and often run as one length from a manifold to each outlet, reducing hidden joints in walls.
• Joined with mechanical fittings (compression/rings) depending on the system.
• Its international standard is in the same ISO 10508 framework (the ISO 15875 series), and the application class is checked as for PPR.
• Multilayer versions (PEX-AL-PEX) with an aluminium layer reduce expansion and hold their shape when bent.

4) CPVC (chlorinated PVC) pipes

• ASTM D2846 covers CPVC hot and cold water distribution systems up to 82°C (180°F)3.
• Joined with a solvent cement made for CPVC; don't use ordinary PVC cement.
• Pressure capacity falls as temperature rises; follow the manufacturer's tables.

5) PVC-U (ordinary unplasticised PVC) pipes

• ISO 1452 for PVC-U pressure pipes covers water supply up to 25°C (cold water), usable up to 45°C with pressure derating factors4.
• So it is not used for hot water lines.
• Its most common use in homes: drain pipes (made to different drainage standards from pressure pipes), and main cold supply lines depending on the design.

6) Other types you'll hear about

• HDPE (high-density polyethylene): common for buried mains and network connections.
• Copper: metallic, withstands high temperatures, more expensive, needs specialised brazing/soldering.
• Galvanised steel: found in older homes; over time it corrodes internally, reducing pressure and discolouring water, so it's usually replaced in full renovations.

7) Quick comparison

8) In Saudi Arabia

Design and installation of plumbing in buildings is governed by the Saudi Plumbing Code SBC 701, which covers installation, alteration, repair, replacement and maintenance of plumbing systems and materials5. Make sure the pipes used comply with locally approved standards and that installation is supervised by a specialist.

9) The water path from source to point of use

A typical path in homes with tanks; order, sizes and pressure are set by the design and operating conditions, not a general rule.

Tips when renovating

1. Don't mix different systems without dedicated transition fittings.
2. Pressure-test the network before closing walls, and document pipe routes with photos and measurements.
3. Fit branch valves for each bathroom and kitchen to ease maintenance.
4. Insulate long hot water lines to reduce heat loss.