What Is a PPR-AL-PERT Composite Pipe and Why the Layer Structure Matters

We have been producing multilayer composite pipes for plumbing and heating for over 15 years. The PPR-AL-PERT structure is not a random combination of materials. Each layer serves a specific engineering function. We use this structure because no single material provides all the properties required for a long-life underfloor heating pipe.

The innermost layer is PERT (Polyethylene of Raised Temperature resistance). This is the layer that contacts the hot water flowing through the pipe. PERT is chosen because it handles continuous temperatures up to 70°C and intermittent peaks to 95°C without deforming. It also maintains flexibility at low temperatures, which matters during cold climate installation when pipes may be bent in environments below 10°C. Unlike PEX, PERT does not require an expansion memory feature for underfloor heating because the pipe is installed in a fixed screed layer that constrains thermal expansion.

The middle layer is aluminum alloy (AL). We use a 0.2-0.4mm aluminum layer that provides two critical properties. First, it acts as an oxygen barrier. Second, it provides longitudinal shape memory. The aluminum layer means that when you cut the pipe and bend it, it holds the bend without returning to straight. This is a significant practical advantage during installation because the pipe stays where you route it.

The outer layer is PPR (Polypropylene Random Copolymer). This layer provides chemical resistance to the alkaline and acidic compounds found in screed and concrete. It also protects the aluminum layer from moisture intrusion at cut ends and fitting connections. We lap the PPR layer slightly beyond the aluminum edges to ensure complete coverage.

The Oxygen Barrier: Why EVOH Prevents System Corrosion

We explain the oxygen barrier to every customer because it is the difference between a heating system that lasts 30 years and one that fails within 5. When hot water circulates through a heating system, it absorbs oxygen from any dissolved air in the water. In an open system (vented radiators), this is managed by the expansion tank and periodic venting. In a closed underfloor heating system, the oxygen has nowhere to escape. Over time, dissolved oxygen concentrations build up and begin attacking iron and steel components in the system: the boiler heat exchanger, steel radiators, circulating pump impellers, and any mild steel fittings.

The EVOH (Ethylene-Vinyl Alcohol Copolymer) layer in our PPR-AL-PERT pipe blocks oxygen diffusion to below 0.1 g/m³/day at 40°C. This is the DIN 4726 standard for oxygen barrier pipes used in underfloor heating. Without this barrier, oxygen diffusion through the pipe wall would introduce approximately 0.5-1.0 g/m³/day into the system water, which is enough to cause visible corrosion in steel components within 2-3 heating seasons.

We see the consequences of missing oxygen barriers most clearly in retrofit projects. An installer used a non-barrier PERT pipe for an underfloor heating system in a residential building in northern China. Within 18 months, the cast iron boiler heat exchanger had corroded through and needed replacement. The system water was black with magnetite sludge. The root cause was oxygen permeation through the pipe wall. We were called in to supply barrier pipe for the replacement installation.

Cold Climate Thermal Performance and PERT Ratings

We design our PPR-AL-PERT pipes specifically for cold climate conditions. When we say cold climate, we mean environments where outdoor design temperatures drop below -15°C and where the heating system operates at near-full capacity for months at a time. This describes much of northern China, Scandinavia, Russia, Canada, and northern US states. In these conditions, the underfloor heating system runs continuously at high flow rates, and the pipe is subjected to thermal cycling from ambient cold during construction to operating temperature.

The PERT inner layer maintains its mechanical properties at temperatures down to -40°C. This matters during construction. An underfloor heating installation in Harbin or Urumqi happens in autumn before the building is heated. The pipe sits in cold screed overnight. With a properly specified PERT pipe, there is no risk of brittle fracture. We have tested our pipe at -40°C and verified that impact resistance remains above the minimum requirement of 500 J/m² at 0°C.

For cold climate installations, we recommend the following operating parameters. The supply water temperature should be set at 45-55°C for a properly designed underfloor heating system. This produces a floor surface temperature of 26-29°C, which meets thermal comfort standards while keeping the return water temperature low enough to allow the condensing boiler to operate in condensing mode. The result is 15-20% lower fuel consumption compared to a system designed for 60-70°C supply temperature.

Pipe Sizing and Heat Output Calculations for Underfloor Heating

We help installers select the correct pipe diameter and spacing for each installation. The heat output from an underfloor heating pipe depends on three variables: the temperature difference between the floor surface and the room air, the thermal resistance of the floor covering and screed, and the pipe spacing. Here are the key relationships we use for cold climate design:

For a tile floor finish (thermal resistance approximately 0.02 m²K/W) over 50mm screed (thermal resistance approximately 0.05 m²K/W) with 20mm insulation underneath, using 20x2.0mm PPR-AL-PERT pipe at 50°C supply temperature and 40°C return temperature, the heat output at 150mm spacing is approximately 65 W/m². At 100mm spacing, heat output increases to approximately 85 W/m². This is why perimeter zones near exterior walls require tighter spacing: the heat loss through the wall is higher, requiring more heat input per square meter.

For cold climates, we recommend designing for a minimum of 80 W/m² in perimeter zones and 60 W/m² in interior zones. This provides a safety margin for extreme outdoor temperatures and allows the system to maintain thermal comfort when the outdoor temperature drops below the design temperature for short periods. A system designed at the exact heat loss calculation will struggle during the coldest days of the year.

We supply PPR-AL-PERT pipe in the following sizes for underfloor heating: 16×2.0mm (for small zones and renovation projects), 20×2.0mm (standard for most residential installations), and 25×2.5mm (for commercial and large residential installations). The wall thickness is designed for 10 bar working pressure at 70°C, which provides a safety ratio of 4:1 against the typical 2.5 bar system pressure in residential buildings.

Installation: Manifold Configuration and Circuit Length Design

We always recommend a manifold-based distribution system for underfloor heating. The manifold splits the floor area into individual circuits, each served by a single length of pipe from the manifold outlet to the furthest point and back. This configuration provides balanced flow to each circuit and allows individual zone control. We see poorly performing systems where the installer tried to use a single long pipe run from a central point, resulting in temperature variation of 5-8°C between the start and end of the circuit.

The maximum circuit length for PPR-AL-PERT pipe depends on the pipe diameter and the available pump pressure. For 20×2.0mm pipe on a standard residential manifold pump (0.25 bar available pressure), we recommend a maximum circuit length of 100-120 meters. Beyond this length, the pressure loss in the pipe exceeds the available pump pressure, and the flow rate drops below the minimum required for even temperature distribution. We always tell installers: when in doubt, split the zone into two shorter circuits rather than pushing the maximum length.

For cold climate installations in buildings with poor insulation, we recommend designing shorter circuits (80-100 meters maximum) to ensure adequate flow rate even when the manifold balancing valve is partially closed for zone control. The pressure loss in 20×2.0mm PPR-AL-PERT pipe at 50°C is approximately 0.15 kPa/m at the design flow rate of 2.5 L/min. For a 100-meter circuit, total pressure loss is approximately 15 kPa plus fitting losses, which fits within the pump budget for most residential systems.

Screed Compatibility and Expansion Joint Planning

One of the most important details in underfloor heating installation is managing thermal expansion of the screed layer. Concrete and cement screed expand by approximately 0.012mm per meter per degree Celsius. For a 10-meter-wide room with a 30°C temperature rise from cold screed to operating temperature, the screed expands by approximately 3.6mm. If this expansion is not accommodated, the screed will crack and transmit cracks to the floor finish.

We supply PPR-AL-PERT pipe with standard installation practice of placing the pipe on top of the insulation layer with pipe staples or clips, then covering with a minimum 30mm screed layer above the pipe crown. We recommend installing perimeter insulation strips around the entire screed field to allow free movement of the screed during thermal expansion. The insulation strip is a simple foam strip that we supply pre-attached to the insulation board or as a separate roll. We have seen cracked floors and debonded tiles on installations where the perimeter expansion gap was omitted.

For large screed fields exceeding 40m², we recommend planning expansion joints through the screed. The expansion joint should align with any structural movement joints in the building slab. The PPR-AL-PERT pipe passes through the expansion joint in a protective sleeve (we use a 32mm conduit that is 300mm longer than the joint width on each side). This allows the pipe to move within the expansion joint without being compressed or kinked.

Water Quality and System Pre-Treatment

We address water quality with every customer because it directly affects the lifespan of the underfloor heating system. Closed heating systems accumulate dissolved solids, scale, and biological contamination over time. The quality of the fill water determines how quickly these problems develop. We recommend the following water quality standards for PPR-AL-PERT systems:

• pH: 7.5-9.0 (too acidic promotes corrosion, too alkaline promotes scale)
• Hardness: 75-150 mg/L CaCO₃ (softer water increases corrosion risk in steel components)
• Dissolved oxygen: Below 0.1 mg/L (critical for closed systems with iron components)
• Chloride: Below 50 mg/L (elevated chloride causes stress corrosion cracking in stainless components)
• Conductivity: 100-500 µS/cm (indicates dissolved solid content)

For cold climate installations where the system sits idle for 6-8 months of the year (summer shutdown), we recommend a corrosion inhibitor additive specifically formulated for closed heating systems. We supply suitable inhibitor products and provide dosing instructions based on system volume. We have seen underfloor heating systems fail within 5 years due to summer shutdown corrosion when no inhibitor was used.

Certification Standards: What to Verify Before Purchase

We produce PPR-AL-PERT composite pipes to meet the following certification standards. When you are evaluating a supplier, ask for copies of the relevant certificates and verify they are current (not expired). We have seen certificates for pipes that do not actually meet the standard because the test reports were from a different production batch or a different factory.

• DIN 4726 (Germany): Oxygen barrier requirement for underfloor heating pipes. Maximum oxygen permeability 0.1 g/m³/day at 40°C.
• ISO 22391 (International): Multilayer pipes for hot and cold water installations. Includes pressure ratings, temperature ratings, and long-term hydrostatic strength testing.
• GB/T 18742 (China): Polypropylene piping systems for hot and cold water.
• EN ISO 15875 (Europe): Cross-linked polyethylene (PE-X) and similar pipes for hot and cold water.
• WRAS (UK): Water Regulations Advisory Scheme approval for drinking water contact.

We provide certificates for all standards for each production batch. Our factory is ISO 9001 certified and we undergo annual third-party audits by SGS for quality system compliance. For project specifications that require specific standards, we confirm compliance before quoting.

Comparing PPR-AL-PERT to PEX-Al-PEX and PERT-Al-PERT: Why We Choose This Structure

We are often asked why we produce PPR-AL-PERT rather than the more common PEX-Al-PEX structure. Both are valid multilayer designs, but they serve different application requirements. We chose PPR-AL-PERT for underfloor heating because it offers specific advantages in the temperature range and installation environment of radiant floor heating systems.

PEX (cross-linked polyethylene) pipes are manufactured by cross-linking the polyethylene polymer chains with chemical or radiation treatment. This cross-linking improves temperature resistance and reduces creep (cold flow) under sustained load. However, Pex Pipes have a higher thermal expansion coefficient than PPR-AL-PERT. When installed in a screed floor, the PEX pipe expands more with each heating cycle, which increases stress on the pipe-to-fitting joints over time. We see more joint leaks in PEX-based underfloor heating systems after 10-15 years of thermal cycling compared to PPR-AL-PERT systems.

The aluminum layer in our composite pipe is critical. The thermal expansion coefficient of aluminum (23×10⁻⁶/°C) is approximately half that of PERT (200×10⁻⁶/°C) and significantly lower than PEX (130×10⁻⁶/°C). When the aluminum layer is placed at the neutral axis of the pipe wall (approximately one-third from the inner surface), it reduces the effective thermal expansion of the composite pipe to approximately 0.025mm/m/°C, which is close to the thermal expansion of steel. This means the pipe expands and contracts with the screed layer in a controlled way that does not stress the joints.

The longitudinal weld of the aluminum layer is another quality differentiator. We use laser welding for the aluminum seam, which produces a continuous hermetic seal around the entire pipe circumference. Some manufacturers use overlap welding, which leaves a small gap at the weld that can allow moisture to reach the aluminum layer over time. Our laser weld provides a true metallurgical bond that we test to 20 bar internal pressure without leakage at the weld line.

Frequently Asked Questions

How does the oxygen barrier on PPR-AL-PERT composite pipes prevent underfloor heating system corrosion?

The EVOH oxygen barrier layer in our PPR-AL-PERT pipes blocks oxygen diffusion to below 0.1 g/m³/day at 40°C, which meets DIN 4726 requirements. Without this barrier, dissolved oxygen enters the system water and causes oxidation in iron and steel heating components. We have documented cases where non-barrier pipe caused cast iron boiler heat exchangers to fail within 18 months. Our barrier pipe protects the entire closed heating system from oxygen-induced corrosion throughout its design lifespan of 30+ years.

What temperature and pressure ratings apply to PPR-AL-PERT pipes in cold climate underfloor heating?

Our PPR-AL-PERT pipes are rated for continuous operation at 70°C and intermittent peaks to 95°C, at a maximum working pressure of 10 bar. For cold climate installations where supply water temperatures may drop below 5°C during winter operation or storage, the PERT inner layer maintains flexibility and impact resistance at temperatures as low as -40°C, which prevents brittle fracture during construction-phase cold weather. The aluminum layer also provides UV resistance during storage outdoors before installation.

What installation spacing is required for PPR-AL-PERT pipes in radiant floor heating in cold climates?

In cold climate installations where floor surface temperature must reach 29°C at perimeter zones (within 1 meter of external walls) and 26°C at center zones, we recommend 10cm pipe spacing for perimeter zones and 15cm spacing for center zones. This configuration produces heat output of approximately 60-80 W/m² depending on screed thickness and insulation quality. The tighter perimeter spacing compensates for the higher heat loss through exterior walls and prevents cold spots near windows and corners. For high-performance buildings meeting Passivhaus standards, spacing can be increased to 20cm center spacing because the overall heat demand is lower.

Conclusion

We have supplied PPR-AL-PERT composite pipes for underfloor heating systems across northern China, Scandinavia, Russia, and North America. The installations that perform best are the ones where the pipe specification, the system design, and the installation practice are all aligned. The oxygen barrier is not optional if you want a 30-year system life. The correct pipe sizing and spacing are not optional if you want even floor temperatures. And the water quality management is not optional if you want to avoid early corrosion failures in the boiler and circulating pump.

If you are specifying PPR-AL-PERT pipe for a project, send us the heating load calculation, the floor construction details, and the climate zone. We will confirm the correct pipe size, wall thickness, and recommended spacing for your specific installation.