How to choose PEX/AL/PEX press fittings: type, size and compatibility

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How to choose PEX/AL/PEX press fittings: type, size and compatibility

Press fittings for PEX/AL/PEX pipes are today's standard for heating distribution systems, underfloor heating, hot and cold water piping, and sanitary installations. Although the pressing technology looks simple - push the fitting on, press it with pliers, done - choosing the right type, size and system compatibility is a matter where mistakes can easily happen without proper knowledge. And a mistake in pressing will show up either immediately (leakage during the pressure test), or after years, when the joint fatigues and starts dripping behind a ceiling or under a screed floor.

In this article, I'll break the whole topic down into its components: what PEX/AL/PEX press fittings are, what types exist and what they are used for, how to correctly determine the size, what system compatibility means, and what to watch out for when combining different manufacturers. If you're looking for a specific installation procedure, check out the separate article Installing PEX press fittings step by step: tools and procedure in the Knowledge Center.

What is a PEX/AL/PEX pipe and why it matters for choosing fittings

Before we get into fittings, we need to clarify what pipe we're talking about - because the market has several types that look similar on the outside but have fundamentally different properties and different fittings.

PEX/AL/PEX (also labelled PE-Xb/Al/PE-Xb or PE-Xa/Al/PE-Xa depending on the polyethylene cross-linking method) is a five-layer composite pipe: an inner layer of cross-linked polyethylene (PEX), an aluminium layer (AL), and an outer layer of cross-linked polyethylene. The aluminium layer serves three functions at once - a barrier against oxygen diffusion (important for heating), shape memory of the pipe (a bend holds its shape), and mechanical protection. Precisely because the pipe has a solid aluminium layer, the pressing technology differs from that used for plain PEX pipes without aluminium.

Press fittings for PEX/AL/PEX are designed so that, during pressing, they clamp the outer surface of the pipe via a sleeve (ferrule), while the fitting's inner mandrel holds the pipe from the inside and prevents deformation. Fittings for plain PEX (without aluminium) have different tolerances and also differ in the profile of the pressing groove - which is why you cannot interchange PEX/AL/PEX fittings with fittings for regular PEX, PERT or HDPE.

Overview of press fitting types: what belongs where

PEX/AL/PEX press fittings are manufactured in several basic shape categories. Each has its place in an installation, and they are not interchangeable - the shape of the joint determines its function in the piping system.

Straight couplings (sleeves)

A straight coupling connects two pipes of the same or different diameter in a single axis. You use it to extend a run, repair a broken pipe, or as a reducer when transitioning between two diameters (e.g. from 20 mm to 16 mm). In practice, straight couplings are most common when installing underfloor heating, where a loop needs to be extended, or when repairing an existing distribution system without replacing a larger section.

90° elbows

An elbow is one of the most commonly used fittings - it allows a 90° change of direction without bending the pipe. Although PEX/AL/PEX pipe has shape memory and can be bent by hand or with a spring bending tool, in plant rooms, manifold cabinets and connections to fittings, an elbow is more practical and reliable than a bend. Elbows come in several versions:

PEX/PEX: both ends are press type - for example 90° press elbow PEX/PEX (16) for a 16 mm diameter. Used purely within the piping without a threaded connection.
PEX/female thread: one end is press type, the other is threaded with a female thread (FT). An example is 90° press elbow PEX/FEMALE THREAD (16x1/2") - ideal for connecting a thermostatic valve, drain valve, or other component with a male thread of 1/2".
PEX/male thread: the other end has a male thread (MT), e.g. Press elbow PEX/MALE THREAD (16x1/2") - used for connecting to nuts, couplings or fittings with a female thread.
Wall-mounted elbow: has a fixed mounting flange for attaching to a wall (typically behind an installation board), the other end is press type or threaded. For example Wall-mounted press elbow PEX/FEMALE THREAD (16x1/2") - a classic choice when installing a shower or washbasin mixer behind tiling.
Reducing elbow: changes direction and diameter at the same time, e.g. 90° reducing press elbow PEX/PEX (20x16) - used where the main run is 20 mm and the branching leg needs to be 16 mm.

Tees and reducing tees

A tee creates a branch from the main run. It can be equal on all three ends (straight tee), or reduced on one or two ends. Reducing tees are especially practical in two-pipe heating systems, where individual 16 mm circuits branch off from a 25 mm or 20 mm main supply. Read more about this in the article Reducing tees and elbows: when and why to use them in a piping system.

Threaded adapters and transitions

Fittings with a male or female thread are used to connect the press piping system to conventional threaded fittings, boilers, pumps and manifolds. The topic of threaded transitions is covered in more detail in the article Pressing fittings with female and male threads: how to correctly connect a piping system.

How to correctly determine the fitting size

This is where the most common mistake tends to happen - a customer measures the outer diameter of the pipe with a calliper and wonders why it fits "differently" than what was stated on the fitting's packaging. The explanation is simple: PEX/AL/PEX pipe dimensions are given according to the outer diameter in millimetres, but the standards system includes several nominal size ranges and different wall thickness tolerances. Common diameters in practice are:

16 mm - the most common diameter for underfloor heating, radiator connections in smaller apartments, hot/cold water distribution to outlet points
20 mm - main underfloor heating circuits, distribution in apartments with higher capacity, connecting the manifold to the heat source
25 mm - primary distribution lines, connecting the circulation pump group, distribution for family houses with a higher heat load
32 mm - larger installations, apartment buildings, technical/process piping

When choosing a fitting, always go by the outer diameter of the pipe, not by its labelling on another manufacturer's packaging (since different manufacturers may have slightly different tolerances). If you're unsure, read the more detailed guide in the article What diameter of press fittings do I need for my piping system.

Wall thickness and pressure classes

PEX/AL/PEX pipes with the same outer diameter may differ in wall thickness, which directly affects the inner diameter and flow parameters. A standard 16 mm pipe usually has a wall thickness of 2 mm (inner diameter 12 mm), while some products have a 2.2 mm wall. This difference affects whether the fitting's mandrel seats correctly inside the pipe. Always check which wall thickness the fitting is certified for - a good manufacturer states this in the technical documentation.

Thread sizes

Fittings with a female or male thread usually have a thread according to the BSP (British Standard Pipe) standard - i.e. an imperial thread labelled as 1/2", 3/4", 1", etc. This is the European standard for most plumbing and heating fittings. The most common combinations in practice:

16 mm pipe + 1/2" thread (FT or MT) - connecting a thermostatic valve, mixer, drain valve
20 mm pipe + 3/4" thread - connecting a manifold, collector, larger-capacity ball valve
25 mm pipe + 1" thread - primary distribution lines, pump groups

System compatibility: why you can't just mix manufacturers

This is a chapter that many installers skip, but it's precisely where the biggest problems occur during inspections and warranty claims. PEX/AL/PEX press systems are not universally compatible across manufacturers - and the reason is technical, not marketing-driven.

Press groove profile (contour)

Each system has its own shape of pressing groove (called a contour or profile). The most widespread profiles are the U-profile and V-profile, although proprietary profiles from specific manufacturers also exist. Press tools (pressing tongs) come with jaws for a specific profile and diameter. If you use U-profile jaws on a V-profile fitting, the joint will not be pressed correctly - it will be either under-pressed (leaking) or damaged.

Sleeve (ferrule) dimensions

The sleeve (ferrule) is a metal collar that holds the pipe on the fitting's mandrel after pressing. Different manufacturers may have sleeves with slightly different outer diameter or length - this affects whether the press jaws seat correctly. Manufacturers certify the system (pipe + fitting + press tool + jaws) as a whole, and guarantee tightness only within this system.

Certification and liability

In practice, this means the following: if you install manufacturer A's fittings with jaws for manufacturer B, and the joint later leaks, manufacturer A disclaims liability (and rightly so - the combination was never tested). In contract work, where you issue an inspection report and the customer has property insurance, this is a very serious matter.

Practical recommendation: stick to one system throughout the job, and if you switch manufacturers (for example due to availability), check the technical documentation for both to see whether they have the same groove profile and whether they are mutually certified. Some systems have cross-certification (manufacturer B states that its fittings can be pressed with jaws for system A), but this must be documented in writing.

Compatibility with press tools (jaws vs. diameter)

Pressing tongs (electric, battery-powered or manual) are used with interchangeable jaws for specific diameters: 16, 20, 25, 32 mm. Jaws are not universal - jaws for 16 mm cannot be used on a 20 mm pipe. On larger jobs, it's common for an installer to have one press tool with a complete set of jaws, or several different press tools for different diameter ranges.

Fitting materials: brass, stainless steel, gunmetal

PEX/AL/PEX press fittings are manufactured from various metal materials, which affect their price, lifespan and suitability for a specific medium.

Brass (CW617N, dezincification-resistant): The most widespread material. Dezincification-resistant brass (also labelled DZR or CR) is certified for drinking water. Not suitable for aggressive environments with acidic water or a high chloride content.
Stainless steel (AISI 304, AISI 316): Used in more aggressive environments, in brine circuits of heat pumps, and in some industrial applications. Higher price, but excellent corrosion resistance.
Gunmetal (red alloy): High copper, tin and zinc content - excellent resistance to dezincification, historically the preferred material for drinking water installations in the UK and some other countries.

For standard heating and sanitary distribution systems, dezincification-resistant brass is the standard and fully sufficient. For special applications (salt de-icing circuits, pool piping, brine systems), consult the system supplier.

Practical scenarios from installation practice

Theory is good, but on real jobs specific situations decide. Here are a few typical examples of how fittings are chosen in real projects:

Scenario 1: Complete underfloor heating system in a family house

The owner built a 180 m² family house, and the designer planned underfloor heating with 8 circuits. The main run from the boiler to the manifold runs in 25 mm pipe, while the individual underfloor heating circuits are in 16 mm pipe. To connect 25 mm pipes to ball valves and the manifold, we use press fittings with a male or female 1" thread (depending on the manifold outlet). Where the direction of the 25 mm pipe run needs to change in the plant room, we use a 90° press elbow PEX/PEX (25). To extend the 16 mm circuits at panel joints, we use straight press couplings PEX/PEX (16). The system stays with one manufacturer, and the press tool jaws are interchangeable for 16 and 25 mm.

Scenario 2: Apartment renovation - connecting sanitary fittings

Renovation of an older apartment, new hot and cold water distribution in PEX/AL/PEX 16 mm. In the bathroom, a shower enclosure with a built-in mixer needs to be connected to the wall. We use a wall-mounted press elbow PEX/FEMALE THREAD (16x1/2") - one end is pressed onto the 16 mm pipe, the other (1/2" female thread) is connected directly to the built-in mixer outlet or secured behind the tiling board using a mounting flange. For a standard washbasin with conventional fittings, we use a 90° press elbow PEX/FEMALE THREAD (16x1/2") with a flexible connecting hose.

Scenario 3: Repairing a fault in an existing system

Corrosion was found in an old distribution system at a point where the original joint was made with a screwed fitting. A section of 20 mm pipe needs to be cut out and replaced. We use two straight press couplings PEX/PEX (20) - one at each end of the new pipe section. If it's also necessary to route around an obstacle, we add two 90° press elbows PEX/PEX (20) and a straight section. We make sure that the new fittings are of the same system as the original ones (same groove profile) - otherwise we need to have jaws for both systems available.

Scenario 4: Reducing the diameter on a branch of a distribution system

Main branch 20 mm, from which a 16 mm radiator branch splits off. Instead of two fittings (a straight tee plus a reducer), we use a single fitting - a reducing tee 20x20x16, or a 90° reducing press elbow PEX/PEX (20x16) if the direction also needs to change at the same time. Fewer joints means fewer potential leaks and faster installation.

What to watch out for when ordering and purchasing fittings

From experience, problems when ordering fittings arise fairly often - mostly not from a lack of knowledge of the whole technology, but from small oversights. Here are the most common pitfalls:

Fitting diameter depends on pipe diameter, not on the system designation

Some systems (for example Rehau, Uponor, Giacomini and others) use the same outer pipe diameters but differ in wall thickness. However, fittings are always sized for the outer diameter - so when ordering, always state the outer diameter of the pipe, not the pipe manufacturer's internal designation.

Confusing female and male threads

A female thread (FT, also labelled Rp) and a male thread (MT, also labelled R) are opposites - you screw a male-threaded fitting into a female thread. If you order the wrong combination (e.g. a female thread on the fitting, but the fitting it connects to also has a female thread), installation will not be possible. Before ordering, always check what type of thread the fitting you'll be connecting to has.

Confusion between imperial and metric dimensions

A pipe has an outer diameter in millimetres (16, 20, 25...), but the threads on fittings are in inches (1/2", 3/4", 1"...). This isn't a mistake, it's a system - you just need to know that "1/2 inch" in the BSP thread system doesn't mean the outer diameter of the thread will be exactly 12.7 mm (0.5 inch). BSP 1/2" actually has an outer thread diameter of about 20.9 mm - this is a historical convention. So always state both parameters: pipe diameter in mm + thread size in inches.

Ordering without checking jaw availability

If you're not sure whether you have the right jaws available for the system you're ordering, check this before purchasing. Alternatively, read the article Pressing vs. screwed fittings: which is better for PEX piping systems, where you'll also find a comparison in terms of tools and operating costs.

Most common mistakes when choosing fittings (from practice)

Besides mistakes made during the actual pressing (more on that in the article Common mistakes when pressing PEX fittings and how to avoid them), choosing the wrong fitting is itself a source of problems:

Using a fitting for PEX (without aluminium) on a PEX/AL/PEX pipe: The mandrel has a different thickness, the sleeve has different tolerances - the joint will either leak or won't be possible to install at all.
Buying fittings without certification for drinking water contact: For drinking water distribution, fittings must be certified according to DVGW W 270 (or a locally equivalent standard). This is not mandatory for heating, but it's good to know what you're working with.
Combining different pressing systems without checking compatibility: See above - this is a source of warranty disputes and claims.
Choosing the wrong type of elbow for concealed applications: A classic elbow (not wall-mounted) installed behind tiling or in a wall complicates repairs. A wall-mounted elbow allows the fitting to be secured and fixed in position before tiling.
Underestimating reducing fittings: Using one fitting (a reducing tee) instead of two (a tee plus a reducer) - fewer joints, less risk, faster installation.

Storage, handling and lifespan of fittings

Fittings made of dezincification-resistant brass are robust, but a few basic rules should be followed. Don't store them long-term in a damp environment without protective packaging - an oxide layer on the surface of the brass is normal, but in a damp environment surface corrosion can occur, which makes pressing more difficult. Never store fittings in contact with aggressive chemicals (solvents, acids).

The lifespan of a correctly pressed PEX/AL/PEX fitting under normal operating conditions (media temperature up to 95°C, pressure up to 10 bar) is guaranteed by most manufacturers for 50 years - which is realistic with a quality installation. The condition, of course, is the correct choice of system, correct pressing, and adherence to operating parameters. Read more about checking and diagnosing joints in the article Checking and servicing pressed joints: what to do if you suspect a leak.

Frequently asked questions (FAQ)

Can I use PEX/AL/PEX press fittings on plain PEX without an aluminium layer?

No. Fittings for PEX/AL/PEX are designed for the outer diameter and wall thickness of the five-layer pipe with an aluminium layer. Plain PEX pipe without aluminium has different stiffness, different dimensions, and behaves differently under pressing. The joint would not be reliable. Special pressing systems exist for plain PEX (e.g. with different mandrels and greater pipe deformation during pressing).

What thread size should I order to connect to a standard thermostatic valve?

The vast majority of thermostatic radiator valves on the European market have a 1/2" BSP male thread (RAL). For a 16 mm pipe, therefore order a 16x1/2" fitting with a female thread (FT). Before ordering, however, check the specific valve - some valves for designer radiators or underfloor convectors may have a 3/4" or special thread.

Can a pressed fitting be disassembled and reused?

No - pressing creates a one-time, permanent joint. Once pressed, the sleeve (ferrule) cannot be returned to its original state. If the joint needs to be taken apart (for repair or renovation), you cut the pipe, discard the fitting, and install a new one. That's precisely why it's important to do the pressing correctly the first time - more on mistakes and prevention in the article Common mistakes when pressing PEX fittings and how to avoid them.

Is a 90° elbow always better than manually bending the pipe?

Not always. In an open installation (plant room, without embedding or covering), bending the pipe is faster and cheaper - fewer fittings, fewer joints. An elbow is more suitable where the pipe passes through a structure or where bending the pipe is mechanically demanding (small bend radius, stiff pipe at low temperature). In an embedded floor installation, the rule applies: fewer joints in the embedded zone is better, so preferring a pipe bend over an elbow is justified.

What does a "reducing elbow" mean and when do I actually need it?

A reducing elbow (e.g. 90° reducing press elbow PEX/PEX (20x16)) combines a 90° change of direction and a diameter reduction (here from 20 to 16 mm) in a single fitting. You need it when, in corners or bends of a distribution system, you simultaneously need to change the pipe dimension - instead of installing two fittings (elbow + reducer), you use just one. Saving on fittings and joints is a real advantage, especially in tight plant rooms.

How do I find out whether my fittings are compatible with the pressing tool I have available?

Check two parameters: (1) the groove profile - the fitting manufacturer always states which groove profile (U, V, or proprietary) the fittings are designed for; and (2) the diameter - the jaws must match the outer diameter of the fitting/sleeve. If in doubt, contact the fitting manufacturer's technical support or the tool supplier, or read the relevant chapter in the article Installing PEX press fittings step by step: tools and procedure.

Conclusion: a systematic approach pays off

Choosing PEX/AL/PEX press fittings isn't rocket science - but it does require a systematic approach. First determine the type of fitting (coupling, elbow, tee, reducer, threaded adapter), then the diameter (always based on the outer diameter of the pipe), then the threaded version (if needed), and finally verify system compatibility with the pipes and available pressing tools. If you stick to one proven system and consistently follow the correct pressing procedure, you'll get a distribution system that will hold up worry-free for decades.

You'll find the entire product category clearly arranged on the page PEX/AL/PEX press fittings. You'll also find more practical advice, dimensions, diagrams and answers to specific questions in other articles of the Knowledge Center - for example in Frequently asked questions about PEX/AL/PEX press fittings or in Elbow, tee or reducer: which fitting belongs where in an installation.

Do you have a question on this topic?

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