How to choose the right crimp fittings for the ALPEX PRESS system
How to choose the right press fittings for the ALPEX PRESS system – a complete guide for installers and builders
Press fittings for the ALPEX PRESS system are today the standard in modern installations of floor heating, heating distribution and plumbing systems in residential and commercial construction. Despite their simple appearance and relatively quick installation, selecting the right fitting is not a trivial matter. It is enough to mix up the dimensions, forget to check the condition of the O-ring or use an incompatible fitting from another manufacturer – and the result is a leaky connection, flooded floor structures or the need to open embedded floors. From my own experience, I know that most complaints and malfunctions in PRESS installations do not stem from a tool error or poor quality material, but from a wrong choice of fitting right from the start.
This article provides a systematic overview of everything you need to know before ordering press fittings for the ALPEX PRESS system – from basic dimensions through material properties, selection of transitions and special fittings, up to typical errors that can be easily avoided. The article is primarily intended for plumbers and heating engineers, but it can also be useful for a builder who wants to check what is being installed on the construction site.
What is the ALPEX PRESS system and why it has its own fittings
ALPEX is a trade name for a multi-layer pipe with a PE-Al-PE structure – polyethylene, aluminum, polyethylene. The outer and inner layers are made of polyethylene (in most cases PE-Xb or PE-Xc), and the middle layer is a welded aluminum core. This construction gives the pipe excellent thermal insulation properties, the ability to cold bending and maintaining shape without springing, and at the same time impermeability to oxygen – which is crucial for closed heating systems.
The PRESS system uses pressing as a joining technology: the fitting is inserted into the pipe (or the pipe into the fitting), and special pliers compress (press) the fitting sleeve around the pipe. A hermetic connection is created without threads, without glue and without dependence on the human factor during tightening. This deterministic installation is the reason why PRESS fittings are preferred in hidden installations – behind drywall, in anhydrite screeds or in base concrete slabs.
Each manufacturer of ALPEX pipes has its own fitting geometry – the diameter and length of the sleeve, the profile of the groove for the O-ring, the depth of pipe insertion. ALPEX PRESS fittings from the company Giacomini (distributed under the ALPEX brand, or as OEM for other resellers) are dimensioned precisely for ALPEX pipes and are not compatible without verification with other multi-layer pipes on the market. Combining incompatible parts is one of the most common causes of leaks, about which you will learn more in a separate article Leakage of PRESS fittings – common causes and how to avoid them.
Overview of available diameters and their applications
In the ALPEX PRESS range, three basic pipe and fitting sizes are commonly available: 16 mm, 20 mm and 26 mm. Each of these sizes has its typical application and choosing the wrong diameter can significantly affect the hydraulics of the entire system.
16 mm pipe and fittings
The smallest standard size is mainly intended for floor heating loops, where long pipe loops are embedded in anhydrite or concrete screeds. The recommended maximum length of a single loop at a diameter of 16 mm is usually 80–100 meters (depending on thermal load and flow), with pressure loss increasing quadratically with increasing flow velocity. 16 mm fittings are also common in plumbing installations in apartments – for supply to the sink, bidet, WC cistern.
20 mm pipe and fittings
The medium size is universal: it is used as a distribution pipe for several floor heating loops, as a riser in a smaller apartment building, or as horizontal distribution in a family house. In heating systems with a temperature drop of 45/35 °C and a power of up to about 5–6 kW, 20 mm is usually a sufficient diameter for the entire branch. 20 mm fittings are also a common "intermediate" size when transitioning from a larger to a smaller diameter – for example, at a branch from the main distribution.
26 mm pipe and fittings
The largest of the three is intended for main distribution lines, risers in multi-storey buildings and connection to larger equipment (boilers, storage heaters, larger floor heating distributors). In floor heating, 26 mm is commonly used only for the supply and return pipes to the distributor. 26 mm fittings are significantly larger, their installation requires a pressing tool of corresponding jaw size – don't forget this when planning the rental or purchase of tools. A separate article on this topic can be found in What diameter of PRESS fitting do I need – 16, 20 or 26 mm?
Types of PRESS fittings and their use in practice
The range of crimp fittings for ALPEX PRESS includes several basic types of fittings. Each has its specific place in the installation and the substitution of fittings (for example, a T-piece instead of an elbow) is sometimes a solution in an emergency, but is not the correct practice. Let's look at the individual types:
Straight couplings (sleeves)
The basic fitting for extending a straight section of pipe or connecting two pipes of the same diameter. In practice, they are used when repairing a damaged section (cut out the damaged piece and insert a new one with two sleeves) or when connecting a coil to a straight bar. Couplings are also available in a reduced version – for example 20×16 mm – which allows a smooth transition between the distribution branch and a branch to a smaller circuit.
Elbows
A crimp fitting in the form of an angle, standard 90°. It is used when changing the direction of the pipe, for example when connecting to a manifold, when transitioning from a vertical rising pipe to horizontal distribution or when bypassing an architectural obstacle. In space-restricted installations (distribution cabinet, technical room), an elbow is irreplaceable.
T-pieces (branch fittings)
A three-way fitting for creating a branch. Available in the same or reduced version (e.g. 20×16×20 mm). A T-piece is typically installed when supplying each branch from the main distribution. In larger installations with many branches, it is more advantageous to design the installation with a central manifold, where T-pieces are replaced by one collector block.
Threaded adapters
These are probably the most sold fittings in the entire range, because they allow the connection of the plastic ALPEX pipe with metal threaded components – valves, thermostatic valves, boiler, tanks, etc. Adapters are available with an external thread (for an internal thread of the device) and with an internal thread (for an external thread of the device).
A specific example is Press adapter – external thread 1/2"M × 16, which is used when connecting a thermostatic valve body to a radiator or when installing a corner valve for sanitation. For larger distributions when connecting a manifold block or a larger valve, Press adapter – external thread 3/4"M × 20 is used, or Press adapter – external thread 3/4"M × 26 when connecting from a 26 mm pipe to a 3/4-inch output of a boiler or tank.
The choice between an external and internal thread on the adapter depends on what kind of thread the device you are connecting to has. If the boiler has an external thread at the output (which is common), you need an adapter with an internal thread. If the valve has an internal thread, you need an adapter with an external thread. It seems trivial, but the substitution is a fairly common mistake for beginners. More about the topic of adapters can be found in the article Press adapter with thread – how to connect an ALPEX pipe to a threaded system.
Plug (cap, cover) fittings and other special fittings
The range also includes plugs for temporary sealing of the end of the pipe during pressure testing, plug couplings for angled transitions through the wall, clamps for pipe fastening and other auxiliary fittings. For a larger order, I always recommend completing the order including these "small items" – on the site you will search for them for two hours and there are not enough of them in the nearest supply.
The key role of the O-ring – why you must not underestimate it
The O-ring is an apparently insignificant part, but it is precisely this that ensures the tightness of each PRESS connection. The O-ring made of EPDM rubber sits in the groove of the fitting and is compressed between the fitting and the surface of the pipe during crimping – an elastic seal is created that is resistant to water, heat transfer media and temperature cycles.
ALPEX PRESS fittings are supplied with an O-ring built into the groove. The problem arises when the O-ring is damaged – for example, it is caught when inserting the pipe, or the fitting has been lying on the site for a long time and the O-ring has slipped, cracked or hardened. In such a case, it is necessary to replace the O-ring before installation. Spare O-rings for PRESS fittings (brass) – for example 26×3 are available separately and every responsible installer should have them in the toolbox. A detailed guide on replacing and checking the O-ring can be found in the article How to properly use the O-ring with PRESS fittings – selection and installation.
Practical experience: fittings stored for longer than 18–24 months, or in unsuitable conditions (direct sunlight, frost, chemically aggressive environment), have degraded O-rings. Always physically check each fitting before installation – the O-ring must be soft, elastic, without cracks and properly seated in the groove.
Material of fittings – brass or stainless steel?
ALPEX PRESS fittings are commonly manufactured from brass (CW617N or CW614N) or from stamped stainless steel. Each material has its advantages and typical use.
Brass fittings
Brass is a traditional and proven material for heating and plumbing fittings. It resists corrosion well in closed heating circuits, is easy to machine, and has good thermal conductivity. The disadvantage is dezincification when in long-term contact with aggressive water (low pH, high chlorine content) – therefore brass fittings are not suitable for drinking water with an aggressive character. In standard heating circuits and comfort plumbing installations, brass is a problem-free and economically advantageous choice.
Stainless steel fittings
Stainless steel fittings are more resistant to aggressive media and are required in some specific applications – for example in installations for solar thermal collectors (aggressive media, high temperatures), or in areas with soft and aggressive water, where brass cannot withstand long-term exposure. However, they are more expensive and require different (sometimes special) pliers jaws.
In practice, brass PRESS fittings are sufficient for most single-family homes and apartment construction. If you are dealing with a solar circuit or have a laboratory water analysis with low pH (below 7) and high hardness, consult the material before ordering.
Tool compatibility and selection of the correct jaws
One of the most common problems on construction sites: the installer has a plier, but does not have the correct jaws for the given diameter. Or they have the correct jaws, but the plier is from a different manufacturer and the jaw geometry is not compatible with the fitting. ALPEX PRESS fittings are dimensioned for jaws with TH (Thermo), V-profile or M-profile – depending on the specific fitting design and manufacturer. Always check before purchasing fittings what jaws your plier has and what profile of pressing contour the fittings require.
There are separate jaws for each diameter (16, 20, 26 mm). Diameter 16 mm usually has the smallest jaws, diameter 26 mm the largest. When using rented tools (which is common for one-off jobs), check whether the jaws for all three diameters you will need on the job are included with the plier. If not – order additional jaws or rent another tool. A more detailed installation procedure is described in the article Installation of PRESS fittings step by step – procedure and required tools.
Mounting template – a special aid for wall mounting
When installing wall-mounted devices (radiators, heating bodies, bathroom fixture sets), the Mounting template for wall mounting of PRESS – 100–150 mm is used. This simple, but very practical aid ensures that the outlet fittings (usually threaded adapters) are placed precisely at the correct distance and in a mutual position corresponding to the axis of the device connections.
In practice, it looks like this: before embedding or casting the installation into the wall, you insert the template between both adapter fittings. The template holds them at a mutual distance (100 or 150 mm depending on the version used) and prevents them from shifting during the hardening of the material. After hardening, remove the template and both fittings are exactly where they should be – without measuring, without improvised wooden templates from waste, and without the risk that one adapter will be 5 mm off the other and the radiator does not fit.
The template is a small detail, but on a job where you install 20 wall-mounted units, it will save you at least an hour and prevent complaints. More about it can be found in a separate article Mounting template for wall mounting of PRESS – what it is for and how to use it.
Typical project scenarios and selection of fittings
Let's look at a few common situations from practice where you need to choose PRESS fittings:
Scenario 1: Floor heating in a single-family home
A standard single-family home with an area of 120–150 m² and 6–8 floor heating loops. Loops are made of 16 mm pipe, the manifold is supplied from a 20 or 26 mm pipe. In this scenario you need: 16 mm fittings (elbows and couplings for loop distribution), 20 mm fittings (elbows, T-pieces, adapters for the manifold) and threaded adapters for connecting to the manifold, boiler and safety assembly. If the boiler has 3/4-inch outputs, you will need adapters 3/4"M × 20 or 3/4"M × 26.
Scenario 2: Bathroom renovation – replacing a radiator with a towel dryer
Existing wall pipes are made of copper or old steel pipe. The new dryer has G1/2" connections. The solution is a PRESS adapter 1/2"M × 16 or 1/2"M × 20 (depending on which ALPEX pipe you run to the connection point), an adapter is needed before it to connect to the existing pipes. The wall mounting template will ensure the correct spacing of the connections.
Scenario 3: New apartment unit – pipe distribution from a sub-meter
From the apartment distribution panel, you run the main pipe (26 mm) through a corridor sleeve to the apartment core, where it branches via T-pieces to the bathroom (20 mm) and kitchen (20 mm), and further to individual outlets (16 mm). You need a complete range of all three diameters, including reduced couplings (26×20 mm, 20×16 mm) and adapters for connecting to fixtures.
Scenario 4: Repair of a leaking connection in an existing installation
This is a specific case where you usually buy a minimal amount of fittings. It is critical to: correctly identify the diameter of the existing pipe (measure the outer diameter with a caliper), check whether it is an ALPEX system (the cross-section should show the PE-Al-PE layering), and order exactly matching fittings. When repairing with the need to shorten the pipe section, you will need two couplings and a piece of new pipe, or a special repair sleeve. More in the article Leak in PRESS fitting – common causes and how to avoid them.
How to correctly determine the quantity of fittings for an order
Mistake number two after mixing up the dimensions: ordering an insufficient number of fittings. Practical rule from practice: always order at least 10–15 % more fittings than the technical calculation from the floor plans. There are several reasons for this:
- When measuring from floor plans, we underestimate the number of direction changes (each elbow = 1 fitting).
- Unexpected obstacles arise on the construction site (blocked passage, route change).
- One or two fittings get damaged due to incorrect pressing – for example, if the jaws slip or the fitting is pressed without the tube.
- Reordering by post costs time and transportation expenses; having a stock of fittings is a cheap buffer.
For larger orders (apartment buildings, industrial facilities), it is advisable to prepare a detailed quantity survey from the project documentation and add a 15 % reserve. For smaller orders (single-family homes), an experienced installer can estimate the stock based on experience.
Differences between PRESS fittings and threaded fittings
Even today, the question occasionally arises on some projects: why use a pressing system instead of traditional threaded (compression) fittings. Both systems have their place, but for modern installations there are reasons why the PRESS system dominates:
- Installation speed: One PRESS joint takes 5–10 seconds (insertion + pressing). A compression fitting requires tightening the nut with a wrench, which is slower and more prone to human error.
- Reproducibility: A PRESS joint is either correctly pressed or not – there is no such thing as "tighten a little". A compression fitting depends on the strength and experience of the installer.
- Hidden installations: A PRESS joint is approved for embedding, pouring into slabs, and placement in drop ceilings. Compression fittings are generally not recommended for hidden installations.
- Reliability over time: A PRESS joint does not loosen even under thermal expansion. Compression fittings may gradually lose tightness under repeated thermal stress.
A more detailed comparison can be found in the article PRESS fittings vs. threaded fittings – when to use which?
Storage and handling of fittings before installation
This is a topic that is rarely covered in manuals, but from practice I know that poor storage is a direct cause of problems. PRESS fittings are delivered in foil or cardboard with protective packaging. During the entire storage period, they must be:
- Protected from direct sunlight (UV degrades O-rings).
- Stored at room temperature or in a slightly cool environment (not in a freezer, not near a heat source).
- Without contact with oils, chemicals, and organic solvents (they damage EPDM O-rings).
- With covered openings – no dust, sand, or dirt must get into the cavity of the fitting.
On the construction site, it is common for fittings to lie freely on building materials for several weeks. Always check them before installation – visually and by touch (the O-ring must be soft and in place), and clean the cavity of the fitting from dust with a cloth. This extra minute can save you hours of searching for the cause of a leak.
Pressure test – a step that must not be skipped
Every PRESS installation must be subjected to a pressure test before being covered. This is also the moment when possible errors in pressing, damaged O-rings, or incorrectly used fittings are revealed. Standard procedure for a pressure test for the ALPEX system:
- Fill the system with water and vent it.
- Increase the pressure to 1.5 times the maximum operating pressure (minimum 6 bar for standard heating circuits).
- Maintain the pressure for at least 30 minutes, monitoring the drop on the manometer.
- Visually inspect all joints – no drops, no condensation.
- Record the results of the pressure test in a protocol (necessary for handing over the work and in the case of an insurance incident).
Although this is a simple rule, in practice the pressure test is often skipped when there is pressure to hand over the building. This is an error that inevitably manifests itself after the floor is poured or the walls are plastered – and then the repair becomes several times more expensive. More on this topic can be found in the article Inspection and maintenance of PRESS joints – what to check during a service inspection.
Most frequently asked questions (FAQ)
Can I use ALPEX PRESS fittings on a different type of multi-layer pipe (e.g. from another manufacturer)?
We strongly advise against combining fittings and pipes from different manufacturers without explicit confirmation of compatibility from the fitting manufacturer. Each manufacturer designs fittings for the specific outer geometry of their own pipe – a difference of a millimeter in the outer diameter can mean insufficient compression of the O-ring and a leaky joint. Always consult the technical department of the manufacturer or supplier.
What happens if a fitting is pressed without the inserted pipe or too shallowly?
A fitting pressed without the pipe or with insufficient insertion depth is unusable – the sleeve is permanently deformed and the fitting cannot be reused. Similarly, a fitting with the pipe inserted too shallowly (the pipe does not reach the stop of the fitting) will have insufficient tensile strength of the joint and there is a risk of pipe extrusion under pressure surges. Each fitting has an indicated insertion depth – always follow it.
Is it possible to disassemble a PRESS fitting and use it again?
No. A PRESS joint is non-disassemblable. After pressing, the sleeve is permanently deformed and the fitting cannot be disassembled or reused. If it is necessary to disconnect the joint (e.g. for repair), the pipe section with the fitting is cut out and replaced with a new fitting and possibly a piece of new pipe. Therefore, each fitting is single-use.
How do I know if the joint is correctly pressed?
A correctly pressed joint has a regular, symmetrical deformation of the sleeve around the entire circumference. Two parallel grooves (or another profile according to the system) should be visible on the sleeve. The pressing tool should emit a characteristic sound and resistance should be felt during pressing. Visually check that the sleeve is not deformed only on one side (slanted) – this is a sign of incorrect positioning of the jaws or an under-pressed joint. Always replace any suspicious joint before covering it.
What are the maximum operating parameters of ALPEX PRESS fittings?
Brass ALPEX PRESS fittings are standardly certified for operating pressure up to 10 bar at temperatures up to 95 °C (long-term) and a short-term maximum temperature of 110 °C. Certification for potable water is required separately (EN 1254). Always verify the specific parameters from the manufacturer’s technical sheet for your product – our fittings have parameter sheets available at atria.sk.
Should I forget about threaded sealing materials for PRESS transitions?
No. The threaded part of the PRESS transition (where it is screwed to the equipment or valve) must be sealed with standard threaded sealing material – Teflon tape, hemp fiber with paste, or special threaded sealant (e.g. Loctite 577 or similar). The pressing side (on the pipe) does not require sealing – here the O-ring seals. Confusing the sides where the sealant is applied is a fairly common mistake among beginners.
Conclusion: a systematic selection always pays off
Selecting the correct pressing fittings for the ALPEX PRESS system is not a complicated science, but it requires a systematic approach. Key steps are: correctly determining the pipe diameter and corresponding fitting, selecting the correct type of fitting for each position in the installation, checking the O-rings before installation, using a compatible press and jaws, following the pipe insertion depth, and performing a final pressure test before covering the installation.
If you have doubts about compatibility, dimensions, or materials – always ask before purchasing rather than after installation. In the category press fittings and couplings for ALPEX PRESS you will find a complete range with detailed technical descriptions, and further practical topics are covered in other articles in the Knowledge Center, such as Common questions about PRESS fittings for ALPEX or Installation of PRESS fittings step by step – procedure and required tools.
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