Installing multilayer pipe step by step: tools, fittings and compression joints

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Installing multilayer pipe step by step: tools, fittings and compression joints

Multilayer pipe (also professionally referred to as multilayer or PE-AL-PE, resp. PEX-AL-PEX) is used today in most new buildings and renovations of heating systems. It is a material that combines the shape memory of an aluminum liner with the chemical resistance of polyethylene – and this combination is exactly what makes it so popular among plumbers and skilled DIY enthusiasts alike. Even though the material itself forgives mistakes more readily than copper, installation without the correct procedure can result in leaks or mechanical joint failure. In this article, I'll walk you through the entire process – from tool selection through bending and cutting to sealing compression fittings – based on what I've seen and done on dozens of jobs.

If you're still wondering whether multilayer pipe is the right choice for you, first read the topics How to choose the right heating pipe: copper, multilayer or plastic? and Multilayer vs. copper pipe: comparing properties, price and lifespan. Here we assume the decision has already been made and we're moving on to installation.

Structure of multilayer pipe – what's hidden inside

Before an installer picks up a pipe cutter, they need to know exactly what they're working with. Multilayer pipe generally has a five-layer structure:

Inner layer (PE-RT or PEX) – smooth, chemically resistant, prevents limescale buildup.
Inner adhesive – bonds the inner layer to the aluminum foil.
Aluminum foil (AL) – welded with pentagonal or butt welds, forms a diffusion barrier and gives the pipe its shape memory.
Outer adhesive – same function as the inner adhesive.
Outer sheath (PE or PE-RT) – protection against mechanical damage, UV radiation and corrosion.

For heating, the most commonly used sizes are 16×2 mm, 18×2 mm and 20×2 mm (outer diameter × wall thickness). For example, the Ivar Turatec 16×2 multilayer pipe is certified for PN10 at T=+70°C as well as PN10 at T=+95°C, which places it in a class suitable for both low-temperature underfloor heating and classic radiator systems. For larger distribution lines and higher flow rates, choose the Ivar Turatec 20×2 multilayer pipe, which can handle even more demanding two-pipe circuits in family homes. Sizing and selection are covered in more detail in the topic What pipe diameter do I need for my heating system?

Required tools – complete list before starting work

One of the most common mistakes I encounter is an installer starting work with an incomplete set of tools and then improvising. With multilayer pipe, improvising usually means damaging the aluminum layer or making a poor-quality cut. Here is a complete list of everything you need to have on hand before you start:

Special multilayer pipe cutter – not a regular plastic pipe cutter. The cutter must cleanly cut through the aluminum layer without deforming the cross-section. Use a type with hardened blades (rotary blade cutters). A regular plastic cutter will worsen the cut and may prevent fittings from seating fully.
Calibrator / reamer – after cutting, the inner diameter is slightly deformed. The calibrator restores proper roundness and simultaneously chamfers the inner edge. Without this step, the O-ring in the compression fitting will not seat correctly.
Expander pliers (for press fittings) – required if you use press fittings (crimping). Not needed for compression fittings.
Wrench (spanner) size 24–32 mm – depends on the size of the compression fitting. For DN 16 this is usually a 24 mm wrench, for DN 20 a 27–30 mm wrench.
Pipe bender (manual or spring type) – the aluminum liner holds its shape, but too tight a bend radius will kink the pipe. The recommended minimum bend radius for 16×2 pipe is 5× the outer diameter, i.e. at least 80 mm. For 20×2 it is at least 100 mm.
Tape measure / steel ruler – accurate measurement is essential, especially for embedded piping.
Fixing clips – for heating systems, we recommend metal or reinforced plastic clips with heat resistance up to at least 95°C.
Sealing tape or liquid sealant (for threaded parts) – PTFE tape or liquid thread sealant, e.g. Loctite 577. Applies to threaded connections on fittings, not to the compression joints with an O-ring themselves.
Welding torch / heat gun – not for the pipe itself, but possibly for heating the compression sleeve when disassembling for repairs.

On a job involving the renovation of underfloor heating in a 140 m² house, we also used a set of labeling tags – simple plastic tags with circuit numbers. This may seem unnecessary until you have to deal with a problematic circuit a year after completion. At that point, identifying the piping embedded in the screed is priceless.

Step 1: Measuring and planning the pipe route

Before any cutting, it pays to sketch the pipe route on paper or directly on the wall. Note every bend, every connection and every clip. For each room, determine where the circuit inlet and outlet will be, where the pipes will pass through walls and ceilings, and where expansion zones will be located.

Multilayer pipe expands with temperature – at a medium temperature of 70°C and an ambient temperature of 20°C, the linear expansion is approximately 0.025 mm/(m·K), which for a 50 K difference and 10 m of pipe means 12.5 mm. This is not negligible at rigid corners. For long horizontal runs, plan for expansion loops or compensators. In underfloor heating, this issue is eliminated – the pipe is embedded in the screed and expansion is absorbed by the system.

How to measure pipe length as accurately as possible? When the pipe is coiled on a reel, always measure from the start of the reel and keep track of how much you've measured off. Don't forget the allowance for insertion into fittings – each fitting "consumes" 15–25 mm of pipe from each side, depending on its size. For a more detailed guide on calculating length, see the topic How many meters of pipe do I need for a heating circuit: calculation and planning of piping.

Step 2: Cutting the pipe – how to achieve a clean cut

Set the rotary cutter on the pipe. Do not start cutting without the tool being properly calibrated – too much pressure on the first turn can deform the aluminum layer. Correct procedure:

Mark the cutting point with a pencil or electrical tape (the tape also holds the pipe steady during cutting and prevents deformation).
Insert the pipe into the cutter perpendicularly – the cut must be exactly 90° to the pipe axis. An angled cut will prevent the fitting from seating fully and cause the O-ring to seal unevenly.
Only lightly tighten on the first pass. Slightly tighten the screw with each subsequent pass – about ¼ turn. Do not force it all the way at once.
After cutting, check that the cut face is straight and that the aluminum layer does not protrude above the plastic.

Common problems: A dull cutter blade (should be replaced after every 50–80 cuts on AL pipe), tightening too quickly (deforms the cross-section), and cutting on an unsuitable support surface (the pipe bends). In practice, I've seen a case where an installer cut 16×2 pipe with a regular plastic blade cutter – the cut was angled, the aluminum layer twisted inward, and the O-ring was immediately pushed out during the hydraulic pressure test. Result: a failure in a freshly finished ceiling.

Step 3: Calibration and reaming – the most commonly skipped step

The calibrator serves two purposes at once: it restores the roundness of the cross-section (after cutting, the inside is slightly elliptical) and chamfers the inner edge (reaming). Reaming is important because a sharp edge would damage the O-ring when the fitting is inserted, leading to a leak over the long term.

Calibration procedure:

Select the correct calibrator size – it must match the outer diameter of the pipe (16, 18 or 20 mm).
Insert the calibrator into the pipe with a rotating motion while applying pressure. You will feel resistance as it overcomes the aluminum deformation.
Once the calibrator passes through smoothly without resistance, the cross-section has been restored.
Flip the calibrator around – most combination tools have a reaming blade for the outer edge on the other end. Use a few rotating motions to remove the sharp outer edge.

On a heating renovation job in an apartment building, we timed the calibration: an average of 45 seconds per cut including reaming. For 60 joints, that was an extra 45 minutes – but the pressure test passed without a single issue. When we skip calibration under time pressure on a job, at least 2–3 out of 20 joints start leaking during the pressure test.

Step 4: Bending the pipe – manual and machine methods

Multilayer pipe has one of its biggest advantages over copper precisely because it can be bent by hand. The aluminum liner holds its shape, and the pipe does not spring back to its original position. Nevertheless, certain rules apply:

Minimum bend radius: For 16×2, at least 80 mm; for 18×2, at least 90 mm; for 20×2, at least 100 mm. Some manufacturers specify 5× the outer diameter, which is equivalent.
Manual bending: Suitable for 30–90° corners. Bend slowly, in small increments, not in one motion. Bending quickly at a single point causes the aluminum layer to wrinkle – creating an internal fold that reduces the flow cross-section and creates a turbulent zone where deposits can build up.
Spring bender: Insert into the pipe before bending (internal type) or slide over the outside (external type). Prevents flattening. Suitable for precise corners.
Mechanical (lever) bender: Gives the best result, essential when making a large number of identical corners (e.g. a series of 90° bends on a radiator distribution line).

Real-world example: When installing underfloor heating in a kitchen (16×2 circuit, 65 m long), we made all the loop turns manually with the help of an internal spring. The room temperature was 6°C – the pipe was stiffer. We heated the bend point with a heat gun to about 30–35°C, which significantly improved flexibility. Higher heating is neither necessary nor advisable.

Step 5: Installing a compression fitting – from preparation to sealing

The compression fitting is the most practical solution for multilayer pipe: it requires no special crimping tools and can be disassembled and re-tightened. It consists of the fitting body (with an internal spigot, O-ring and seat), a compression sleeve (ferrule), and a nut. For PEX-AL-PEX pipe there are two basic types:

Classic compression fitting with a ferrule – the ferrule deforms as the nut is tightened, ensuring both tightness and mechanical grip. Suitable for PB (polybutylene) and some PE pipes.
Compression fitting with an internal spigot and O-rings – the fitting has an internal spigot that is inserted into the pipe, and an outer nut with a compression sleeve. Sealing is provided by O-rings between the spigot and the pipe wall. This is the standard for multilayer pipes.

For copper pipe or when transitioning between materials, for example the compression fitting for copper pipe – 15×1-EK is suitable, designed directly for copper with an outer diameter of 15 mm and a 1/2" thread. When transitioning between different systems (e.g. copper → multilayer pipe), special adapters exist – the 15×1 EK compression fitting for copper or PB pipe is exactly such a solution for a 15 mm diameter, when you need to connect an existing system or fitting with a different threaded end.

Detailed step-by-step procedure for installing a compression joint:

Preparation: Cutting, calibration, reaming – as described above. Do not proceed without this preparation.
Sliding on the nut and sleeve: Before inserting the fitting, don't forget to slide the nut onto the pipe first, followed by the compression sleeve (in the correct order, otherwise you'll have to take everything apart and start over – a classic mistake).
Inserting the fitting spigot: Insert the internal spigot into the pipe with a rotating motion while applying pressure. The spigot must go all the way in – the end of the spigot should be flush with the pipe face or protrude slightly. Check by sight or by feel.
Sliding the sleeve onto the pipe: The compression sleeve must fit tightly against the pipe and touch the edge of the fitting body.
Tightening the nut: Tighten the nut by hand until snug, then use a wrench for 1.5–2 turns. This is the value specified by manufacturers for most compression fittings on multilayer pipe. Do not tighten with force all the way – this could deform the O-rings or the compression sleeve.
Checking: After tightening, check that the pipe does not move within the fitting. A light attempt to pull it out should fail. The fitting must not be tilted – proper alignment is important for long-term tightness.

Step 6: Pressure test – do not backfill or plaster over without it

A pressure test is a mandatory step before covering any piping. For heating systems, a hydraulic pressure test with water is used at 1.5 times the operating pressure, but at least 6 bar, for a minimum of 30 minutes (for piping embedded in screed, standards recommend 24 hours). The pressure must not drop by more than 0.1–0.2 bar during stationary measurement.

Procedure:

Fill the system with water and bleed all air vents and radiators.
Use a manual pump or an electric pressure pump to raise the pressure to the test value. For typical family homes, the operating pressure is 1.5–2.5 bar, so the test pressure is 3–4 bar.
Close the water supply and monitor the pressure gauge for 30 minutes.
Visually inspect all joints – if there is a leak, you'll see drops or a wet spot.

If the pressure drops: First check all compression joints by tightening ¼ turn and repeat the test. If the problem persists, locate the leak using a dry cloth or soap solution. For more on the causes of leaks, see the topic Common heating pipe faults and leaks: causes and repairs.

Step 7: Fixing, insulation and covering the piping

Fix multilayer pipe running above ground with clips every 500–800 mm on straight sections, and every 200–300 mm at corners. A spacing of 500 mm is a safe standard for 16×2, while for 20×2 you can go up to 700 mm. Clips must be loose enough to allow longitudinal expansion – that is, not tightly clamped, only guiding – except at fixed points near compression fittings.

For piping in exterior areas or unheated spaces (e.g. garage, basement), thermal insulation with a thickness of at least 9 mm for interior piping and at least 19–25 mm for exterior or partially exterior runs is required. Without insulation, condensation forms on cold pipes carrying hot water, which damages the building structure.

Piping in the floor (underfloor heating) is embedded in a screed mixture with additives to ensure good thermal conductivity. The minimum thickness of the covering layer above the pipe is 45 mm for anhydrite screed and 65 mm for cement screed. Fix the pipe in the floor to the insulation board with clips every 300–500 mm.

Specific real-world situations: transition joints, tees and elbow fittings

In most heating installations, you'll encounter situations where a straight pipe alone isn't enough – you need to branch off, change direction by 90°, or connect to a fitting with a different thread. Here's an overview of solutions:

Tee (three-way fitting): For branching off the main line to a radiator. Important: the tee must be correctly oriented – flow in the main line must go straight through, with the branch off to the side. Reversed installation causes hydraulic imbalance.
Transition fitting (reducer): For example, from 20×2 to 16×2 when transitioning from a larger line to a radiator connection. Always reduce in the direction of flow, not against it.
Elbow (90° bend): Used instead of bending on short sections near fittings. Bending the pipe too close to a fitting (less than 10× the outer diameter) is problematic – the calibrator can't reach inside.
Transition to male thread (MT) or female thread (FT): For connecting to a radiator valve, manifold, or circulation pump. Always apply PTFE tape (3–4 wraps) or liquid sealant to the threaded part. Tighten the thread with a wrench, not by hand.

When installing Ivar Turatec 18×2 multilayer pipe in a two-pipe radiator system, an 18 mm diameter is sufficient for horizontal distribution lines in a family home with an output up to about 12–14 kW per circuit. For larger homes or long circuits, we recommend 20×2.

Most common installer mistakes and how to avoid them

Over years of practice, I've compiled a list of recurring mistakes. These aren't beginner errors – they're made by anyone who rushes or underestimates preparation:

Skipped calibrator – leak at the O-ring during the pressure test. Solution: always calibrate, always ream.
Wrong order – nut and sleeve slid on after inserting the fitting – you'll have to disassemble everything. Always slide these on before inserting the spigot.
Over-tightening the nut – cracked sleeve or damaged O-ring. Result: the fitting holds, but leaks. Tighten exactly according to the manufacturer's instructions (usually 1.5–2 turns past hand-tight).
Too tight a bend radius – deformation of the aluminum layer, narrowing of the flow cross-section. Result: reduced flow, noise in the pipe.
Angled cut – the fitting doesn't seat fully, and the O-ring is loaded unevenly. Always cut perpendicular.
Skipping the pressure test before covering – a classic mistake. A cracked joint under a screed layer or behind drywall is a very costly repair.

For more on common faults and their solutions, see the topic Common heating pipe faults and leaks: causes and repairs, and for regular maintenance, see Maintenance and inspection of heating pipe systems: what to watch for each season?

Material and temperature parameters – what PN10, T=70°C and T=95°C mean

Multilayer heating pipe is certified for certain combinations of pressure and temperature. The designation PN10 means a nominal pressure of 10 bar at a reference temperature of 20°C. At higher temperatures, the load capacity decreases – which is why manufacturers specify combined values:

PN10, T=+70°C – suitable for underfloor heating and low-temperature systems (condensing boilers, heat pumps).
PN10, T=+95°C – suitable for classic radiator systems with higher output, where the primary water temperature reaches 70–90°C.

In practice, the operating pressure of a household heating system will not exceed 2.5–3 bar, so PN10 offers a large safety margin. The critical factor is long-term resistance under temperature cycling – and this is exactly where multilayer pipe with an aluminum liner is more reliable than pure plastic, because the aluminum limits shape fatigue. Read more about this in the topic Pipe temperature and pressure: what do PN10, T=70°C and T=95°C mean in practice?

Frequently Asked Questions (FAQ)

Can I use a regular plastic cutter for multilayer pipe?

This is not recommended. A regular plastic cutter cannot cleanly cut through the aluminum layer – it will either crush it inward (making calibration harder) or produce an angled cut (which compromises joint tightness). Always use a cutter designed for AL-PE or multilayer pipes, ideally with hardened rotary blades.

How many turns should the compression fitting nut be tightened?

The general recommendation from most manufacturers of compression fittings for multilayer pipe is: tighten the nut by hand until snug, then add 1.5–2 full turns with a wrench. The exact value depends on the manufacturer – always check the installation manual for the specific fitting. Over-tightening is just as harmful as under-tightening.

Is it necessary to calibrate every cut, even if it looks fine?

Yes. Deformation of the aluminum layer during cutting is sometimes visually barely noticeable, but it's enough to prevent the O-ring from seating evenly. Calibration takes less than a minute and is cheaper than repairing a leak in a finished floor or behind drywall. Never skip it.

Can a compression fitting be disassembled and reassembled repeatedly?

Yes, a joint with a standard compression fitting using a ferrule or O-ring can be disassembled. However, when reassembling, you must check the condition of the O-rings and sleeve – if they are deformed or cracked, replace them. Some manufacturers recommend always replacing the O-rings and seals on reassembly as a precaution. Document repeated assembly so you know which joints have been opened.

What is the difference between a press fitting and a compression fitting for multilayer pipe?

A press fitting (crimping) requires special hydraulic pliers and direct control of the crimping profile – once made, the joint cannot be disassembled. Its advantage is high reliability when crimped correctly and speed when making a large number of joints. A compression fitting requires no special pliers, can be disassembled, and is suitable for smaller jobs and repairs. For large installations (e.g. an entire apartment building), press fittings are more efficient, while for family homes and renovations, compression fittings are predominant.

How long can multilayer pipe be left unused in an uncoiled state?

Multilayer pipe left uncoiled on a construction site is not a problem in terms of material properties (unlike some films and plastics). However, protect it from direct UV radiation (leave the packaging on it as long as possible), severe frost, and mechanical damage. Keep uncoiled pipe coiled at a diameter no smaller than the minimum bend radius for the given dimension.

Conclusion: installation without shortcuts pays off

Multilayer pipe is a material that forgives a lot – but only if handled correctly. Tools, procedure and inspection are the three pillars on which every reliable installation stands. Don't skip calibration, pay attention to the correct bend radius, tighten fittings exactly according to the instructions, and always perform a pressure test before covering. What may seem like a waste of time during installation is actually the assurance that the system will run trouble-free for 30 years or more.

If you're planning a larger project and aren't sure which dimensions to choose, check out related topics in our Knowledge Center: What pipe diameter do I need for my heating system? or Frequently asked questions about heating pipe: diameters, fittings, temperatures and installation. Products available in the heating pipe category cover both standard family homes and more demanding projects – from 16×2 for underfloor heating to 20×2 for main distribution lines.

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your home? Write to us - we'll be happy to help.