Frequently Asked Questions about PEX/AL/PEX Press Fittings
Frequently Asked Questions about PEX/AL/PEX Press Fittings
Press fittings for three-layer PEX/AL/PEX pipes are currently one of the most popular ways of joining pipe distribution systems in residential and commercial installations. Although the pressed-joint technology is proven and reliable when the correct procedure is followed, in practice we encounter a number of questions – from customers doing their first floor-heating installation, as well as from experienced installers switching from other systems. This article collects the most frequent questions from our project practice and answers them with technical precision and specific figures.
If you are looking for specific types of fittings or want to understand the basic principles of selection, we also recommend our articles How to Choose PEX/AL/PEX Press Fittings: Type, Size and Compatibility and What Diameter of Press Fittings Do I Need for My System in the same Knowledge Centre.
What Are PEX/AL/PEX Press Fittings and Why Choose Them?
Before we get into specific questions, a brief technical foundation. A PEX/AL/PEX pipe is three-layered: the inner layer is cross-linked polyethylene (PEX), the middle layer is aluminium foil (AL) welded either butt-jointed or with a longitudinal seam, and the outer layer is again PEX. This combination gives the pipe shape memory (flexibility without springing back like pure PEX), heat resistance and diffusion tightness.
Press fittings for PEX/AL/PEX are specially designed so that the fitting body (usually brass or nickel-plated brass) fits inside the pipe, and the outer press ring (made of PPSU plastic or stainless steel) is deformed inward by press tongs – creating a mechanically and leak-tightly reliable joint without any natural rubber seal. In this case, sealing is achieved by the deformation of the material itself.
Are Press Fittings for PEX/AL/PEX and for Flexible PEX the Same?
This is one of the most frequent and also most dangerous questions, because customers confuse these two systems much more often than one would expect. The answer is clear: no, they are not interchangeable and must not be combined.
Flexible PEX (also referred to as PEX-B or PEX-A) is installed with an expansion or compression fitting, where the pipe itself holds the joint through plastic deformation. Press fittings for PEX/AL/PEX have a different internal profile of the body (mandrel), different insertion depth and different press ring diameter. If you were to use a fitting designed for AL/PEX on pure PEX pipe, the mandrel would likely cut into the pipe or the joint would not be tight. Always check before purchase which pipe type the fitting is certified for.
What Sizes of PEX/AL/PEX Press Fittings Are Available and How to Identify Them Correctly?
The most common sizes for residential systems are 16 mm, 20 mm and 26 mm outer pipe diameter. In commercial installations, sizes of 32 mm and, exceptionally, 40 mm are also found. The fitting size always corresponds to the outer diameter of the pipe – so if you have a 16×2 pipe (16 mm outer diameter, 2 mm wall thickness), you buy a fitting marked with the number 16.
In practice, for a larger apartment with floor heating, the following are typically combined:
- 16 mm pipe and fittings for floor heating circuits
- 20 mm for main branches from the manifold to individual circuits
- 26 mm for the primary distribution from the boiler, tank or star manifold
Reducing fittings allow a transition between two diameters without intermediate pieces. For example, the 90° reducing press elbow PEX/PEX (20x16) is a typical product used where you branch off from a 20 mm branch to a 16 mm loop – for example when connecting to a manifold or when changing the nominal diameter in the distribution system. Read more about when to reach for a reducer in the article Reducing Tees and Elbows: When and Why to Use Them in a Piping System.
Can I Mix Fittings from Different Manufacturers?
This is one of the technical-legal questions where practice and theory diverge somewhat. Technically speaking, press fittings from different manufacturers may have slightly different press ring profiles, groove shapes and tolerances. Most European manufacturers comply with the EN 1254 standard or similar for common diameters (16, 20, 26 mm), so the geometry is essentially compatible.
The problem arises with warranty claims: if a leak occurs at a joint where, for example, manufacturer A's fittings were pressed with manufacturer B's tongs, or where fittings from two different manufacturers are combined, each of them may point to the other. For this reason, we recommend sticking to a single system within one installation – fittings and tools from one manufacturer, or certified compatible combinations.
In practice, I have not encountered common mixing of fittings from two reputable European manufacturers of the same size causing a leak – but the warranty implications are real.
What Tools Do I Need for Installation and Do They Need to Be Special?
Yes, press fittings require press tongs (or a pressing jaw) designed for the given diameter. Tongs are either manual (for diameters of 16–26 mm, suitable for a smaller number of joints) or electric/battery-powered (for larger projects and larger diameters).
The jaw shape is important. For PEX/AL/PEX with a press ring made of PPSU or stainless steel, a jaw profile TH or U is used – depending on the specific system. Before buying tongs, always check which jaw profile the fittings you will be installing require. Most fitting manufacturers state this directly in the technical documentation.
In addition to press tongs, you will need:
- A pipe cutter or shears (for a clean, perpendicular cut)
- A calibrating mandrel (to restore the round cross-section of the pipe after cutting)
- An insertion depth marker (usually part of the calibrating mandrel)
- A tape measure and pencil
You can find a detailed procedure including inspection steps in our article Installing PEX Press Fittings Step by Step: Tools and Procedure.
What Is the Insertion Depth of the Pipe into the Fitting and Why Does It Matter?
Insertion depth is one of the most commonly underestimated things for beginners. For each pipe size, there is a minimum depth to which the pipe must be inserted for the joint to be functional and tight. Approximate values for common diameters:
| Pipe diameter | Min. insertion depth |
|---|---|
| 16 mm | 16 – 18 mm |
| 20 mm | 19 – 21 mm |
| 26 mm | 22 – 24 mm |
| 32 mm | 25 – 28 mm |
If the pipe is not inserted deeply enough, the press ring will not land in the correct position on the fitting body and the joint will be mechanically unstable or leaky – however, this may only become apparent after the system is filled with pressure. Therefore, it is always recommended to mark the pipe at the required insertion depth with a marker before pressing and to visually check that the pipe has reached the stop.
Are Wall-Mounted Press Elbows Suitable for Wall Installation – What Is the Procedure?
A wall-mounted press elbow (also called a "wall elbow" or "wall plate") is a fitting where one side has a press socket for the PEX/AL/PEX pipe, and the other side has an internal or external thread with a mounting flange with screw holes. This flange is fastened directly to the wall or plasterboard with screws before plastering.
Typical use: terminating a concealed (in-wall) distribution before a sink, shower or radiator, where the pipe "turns" behind the wall and the thread protrudes outward for connecting a tap or connecting pipework. An example is the wall-mounted press elbow PEX/INTERNAL THREAD (16x1/2"), which is designed exactly for such a concealed termination with a 16 mm pipe size and 1/2" thread.
When installing a wall elbow, the key points are:
- First fasten the flange part to the wall – perpendicular, at the correct height
- Check that the thread will be parallel to the wall surface (not at an angle)
- Then connect the pipe by pressing – after connection, the pipe cannot be rotated
- Final check of the rigidity of the flange before plastering or drywalling
What Is the Difference Between a Fitting with an Internal Thread and One with an External Thread?
Fittings with an internal thread (IG – Innengewinde) have the threaded part on the inside – meaning an external thread of the counterpart is screwed into them, for example a drain valve, mixing valve or ball valve. Fittings with an external thread (AG – Außengewinde) are the opposite – the thread on the fitting is male (external), and the counterpart is an internal thread with a sealing head, for example a radiator valve head or a fitting union.
In practice, the choice is determined by what is connected on the other side. As a rule of thumb: an internal thread on the fitting = an external thread on the fixture, and vice versa. For example, the 90° press elbow PEX/INTERNAL THREAD (16x1/2") is a common solution for connecting the system to a fixture with an external G 1/2" thread. If instead you are connecting to a threaded fixture with an internal thread, choose the press elbow PEX/EXTERNAL THREAD (16x1/2").
When sealing threaded joints on fittings, always use PTFE tape or hemp thread with sealing paste – pressing itself does not seal the threaded part, only the pipe socket. Read more about this type of connection in the article Pressing Fittings with Internal and External Threads: How to Correctly Connect a Distribution System.
Can I Embed a PEX/AL/PEX Press Fitting in Concrete or Hide It in the Floor?
This is a tricky question, where several things need to be distinguished. PEX/AL/PEX pipes themselves are designed to be embedded in the floor – a common situation with floor heating, where entire loops are cast in anhydrite or concrete. This is not a problem for the pipe. The problem arises with fittings.
Most manufacturers of PEX/AL/PEX press fittings do not recommend embedding in concrete or otherwise permanently concealing a fitting without access – for a simple reason: even with a correctly made joint, there is a (albeit minimal) probability that some anomaly will occur over the course of decades, and with a concealed joint, repair would mean demolition. Some systems explicitly prohibit this, and certification reflects that.
In practice, we recommend that where a fitting is located in the floor, this be addressed by either:
- Moving the fitting to an accessible location (for example, a manifold cabinet)
- Placing the fitting in an inspection chamber with a lid (so-called inspection chambers)
- Using joints only in accessible sections of the system and routing the entire loop through the floor without joints
What Pressure and Temperature Can PEX/AL/PEX Press Fittings Withstand?
Certified press fittings for PEX/AL/PEX systems commonly withstand:
- Working pressure: 10 bar (some systems up to 16 bar)
- Maximum working temperature: 95 °C (at pressure up to 6 bar)
- Maximum short-term (emergency) temperature: 110 °C
For standard residential heating systems with temperatures of 35–75 °C and pressure of 1.5–4 bar, these parameters offer a large safety margin. For industrial or technological applications with higher pressure or extreme temperature, special certified systems must be used, and this should be discussed with the manufacturer.
An interesting detail: brass fittings withstand temperatures significantly better than some plastic variants. When choosing, always check the certification sheet of the specific product – parameters may vary between manufacturers.
What Happens If I Pressed the Joint Incorrectly – Can It Be Repaired?
Unfortunately, pressing is a one-way process – once the ring has been pressed, it cannot be "unpressed" back. If a joint is made incorrectly (for example, the ring was pressed outside the groove, the pipe was not inserted deeply enough, or the tongs had the wrong jaw), the only repair is to cut off the fitting along with a section of pipe and install a new joint with a new fitting.
This naturally means you will lose a certain length of pipe. With tight routing or minimal cutting allowance, this can be a problem – we therefore always recommend leaving a slightly longer end than needed when cutting, and cutting to the final size only just before installation.
How do you recognise a faulty joint? Sometimes you see it immediately (the ring is not in the groove, the pipe protrudes too little), other times the leak only appears during the pressure test. That is why a pressure test before drywalling or plastering is absolutely essential. You can read more on this topic in the article Common Mistakes When Pressing PEX Fittings and How to Avoid Them and also in Inspection and Servicing of Pressed Joints: What to Do If You Suspect a Leak.
Is a 90° Elbow Better Than a Bent Pipe? When to Use a Fitting and When to Bend the Pipe?
PEX/AL/PEX pipe can be bent by hand (or with the help of bending springs or a pipe bender), which is one of its main advantages over steel piping. However, minimum bending radii apply when bending – for a 16×2 mm pipe, the minimum bending radius is usually around 80–100 mm (five times the outer diameter), for 20×2 mm around 100–120 mm.
If you need to change the direction of the distribution system in a confined space (for example, in a plasterboard partition, a stud, or when going around a corner in a long run), a 90° press elbow PEX/PEX (16) is a more elegant solution than trying to bend the pipe in too small a space. An elbow also creates a mechanically stronger corner that does not move.
On long runs with gradual changes of direction (for example, going around columns or staircases), it is on the other hand more advantageous to bend the pipe smoothly and save on fittings. The article Elbow, Tee or Reducer: Which Fitting Belongs Where in an Installation discusses this dilemma in more detail.
Can I Use PEX/AL/PEX Press Fittings for Cold and Hot Domestic Water?
Yes – PEX/AL/PEX press fittings are also certified for drinking water systems (both cold and hot domestic water), not only for heating systems. It is key that the product has certification according to the relevant standard for contact with drinking water (in the EU this is typically EN 1254 and the relevant hygiene certificates, in Slovakia checked by the RÚVZ or European certification institutions).
The brass from which the fitting body is made must be "low-lead brass" or surface-treated so as not to exceed the limits for lead content in water – this has been mandatory in the EU for several years. Before ordering fittings for a drinking water system, check in the manufacturer's technical documentation whether they are approved for the distribution of hygienically safe water.
How Many Fittings Will I Need? How to Estimate That?
This is a purely practical question that we deal with on every larger project. Basic rules:
- For each connection to a fixture, valve, manifold or appliance, count on at least 1–2 fittings (for example, one straight + one with an internal thread)
- For each corner turn in a wall or under plaster where you cannot bend the pipe smoothly, count one elbow
- For each T-branch (for example, a branch to a radiator from the main line), count one tee
- For each change in diameter, one reducing elbow or reducing tee
In practice, for a standard 3-room apartment with 6 radiators and floor heating in the bathroom and toilet, you will need roughly 30–50 pieces of various fittings in total. We always recommend buying 10–15% extra for possible cutting mistakes or unexpected route changes.
Why Do Some Fittings Tarnish or Turn Black?
Brass fittings may darken on contact with air (especially in humid areas) – this is a natural surface oxidation (patina) that has no effect whatsoever on the functionality or tightness of the joint. This is not corrosion in the technical sense of the word.
Actual corrosion (green or white deposits, surface etching) can occur with:
- Contact with cement or lime mortar during plastering without protective covering
- Contact of the fitting with aggressive insulation material (e.g. some foam insulations with high pH)
- The presence of stray currents in the system without cathodic protection
We recommend protecting fittings in walls or floors with a foil or PE pipe sleeves designed for this purpose before backfilling.
What Is the Difference Between a 90° Press Elbow and a Reducing Press Elbow?
A 90° press elbow with the same size on both ends (for example, the 90° press elbow PEX/PEX (16)) changes the direction of the system by 90° without changing the diameter. It is used where you simply want to turn the line at a corner without reducing the flow.
A reducing press elbow (for example 20×16) does two things at once: it changes direction by 90° and at the same time changes the pipe diameter from 20 mm to 16 mm. This is an ideal solution when branching off from a larger diameter branch (main line 20 mm) to a smaller diameter circuit (floor heating loop or radiator supply 16 mm). You save one fitting (normally you would need an elbow plus a separate reducer).
What Are Typical Real-World Scenarios Where These Fittings Are Used?
For a better idea of how PEX/AL/PEX press fittings are used in real projects, here are a few typical situations:
Scenario 1 – New family house with floor heating: The boiler is connected via a manifold, from which a main 26 mm branch runs to a star manifold in the utility room. From there, 20 mm branches run to each floor, where there are further manifolds. From these, 16 mm loops emerge, which are joint-free along their length. Fittings are located at the manifold, at the circulation connection and at the valves – a total of 60–80 pieces for a three-storey house.
Scenario 2 – Apartment renovation, replacing steel piping with PEX/AL/PEX: The old threaded steel pipes are exposed, cut and replaced. Press fittings with an external or internal thread, depending on what is on the steel side, are used to connect the PEX/AL/PEX system to the existing steel sections (e.g. to an old boiler with a threaded outlet). The rest of the distribution is routed in channels or grooves, with fittings only at the branches.
Scenario 3 – Connecting a bathroom mixer tap: A concealed 16 mm hot and cold water distribution runs in the wall. Where it passes behind the wall, two wall elbows with a 1/2" thread are fitted – one for hot, one for cold water – screwed into the partition. A mixer siphon is later mounted on the threads.
Questions and Answers (FAQ)
Can I Reuse a PEX/AL/PEX Press Fitting?
No. Pressing is a one-way, irreversible process – once the ring has been pressed, the fitting cannot be reused. After cutting off the fitting, a new one must be fitted. This applies to any repairs or modifications to the system – always keep enough spare fittings on hand.
How Do I Know That the Press Ring Was Pressed Correctly?
A correctly pressed ring (PPSU or stainless steel) is evenly deformed all the way around, sits firmly in the groove on the fitting body, and cannot be moved by hand. On some manufacturers' tongs or jaws, an indicator lights up after correct pressing. Visually, you can also see that the ring has slightly narrowed its outer surface – clearly visible under raking light.
Can I Use PEX/AL/PEX Press Fittings Outdoors or in Frost Conditions?
The fittings themselves are frost-resistant – brass and PPSU handle sub-zero temperatures without problems. The problem, however, is the PEX/AL/PEX pipe itself and especially the water inside it – frozen water expands and can damage the pipe or joint. For outdoor lines, the pipes should either be drained for winter or protected with thermal insulation, or more resistant systems (e.g. with an electric trace heating) should be used.
What Is the Maximum Angle a Press Elbow Fitting Forms?
Standard elbows are available at a 90° angle. Variants of 45° are also on the market. Other angles are not standardly manufactured – for various other direction changes, a combination of two elbows is used, or the pipe is bent manually. If you need a precise angle (e.g. 30° or 60°), this is achieved by bending the pipe with a bending spring or pipe bender.
Do PEX/AL/PEX Fittings Need to Be Lubricated or Prepared in Any Way Before Pressing?
The fitting body (mandrel) is not typically lubricated – manufacturers do not recommend lubrication, since some lubricants may affect tightness or be unsuitable for contact with drinking water. The only necessary preparation is a clean, perpendicular pipe cut, calibration (restoring roundness) and checking that the pipe end is free of shavings and dust.
Can I Combine Press Fittings with Threaded Fittings in One System?
Yes, but with care. Within one system, you can have some joints pressed and some threaded (e.g. when connecting to a threaded fixture). It is important that each joint is made correctly and corresponds to its type – and that the entire system is subjected to a pressure test before being concealed. A comparison of both technologies can be found in the article Pressing vs. Threaded Fittings: Which Is Better for PEX Systems.
Conclusion: Pressing Is Reliable If You Know What You Are Doing
PEX/AL/PEX press fittings, when used correctly, are one of the most reliable and fastest ways of joining pipe distribution systems. Most of the problems we encounter in practice do not stem from a product defect, but from insufficient pipe preparation, incorrect insertion depth, wrong choice of tong jaw, or confusion of types (e.g. pure PEX vs. PEX/AL/PEX). If you take good care of these things, the joints will last decades without any intervention.
If you are unsure about the type or size of fitting for your specific situation, take a look at other articles in our Knowledge Centre – especially How to Choose PEX/AL/PEX Press Fittings: Type, Size and Compatibility and What Diameter of Press Fittings Do I Need for My System. The entire range of PEX/AL/PEX press fittings can be found clearly listed on the page PEX/AL/PEX Press Fittings.
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