How to choose brass fittings for polyethylene pipes – what to pay attention to

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How to choose brass fittings for polyethylene pipes – what to pay attention to

Selecting the right brass fitting for polyethylene pipes may seem trivial at first glance – after all, it's just a piece of brass with a nut, right? The truth is different. From practical experience, I know that a poor choice of fitting, whether in terms of dimensions, type, material, or installation technology, is among the most common causes of leaks in underfloor heating systems and other PE installations. One unsuitable piece in a 200-meter pipe run is enough to cause problems that are only discovered after the floor has been concreted.

In this article, we will go through everything you need to know before choosing a brass fitting for PE pipes – from basic types through correct dimensions, material properties, compatibility with pipe types, to practical experiences from installations where mistakes are most commonly repeated.

What is a brass fitting for PE pipes and why is it used

A brass fitting for polyethylene pipes is a connecting element that allows a firm, tight, and repeatedly dismountable connection between a PE pipe and other system components – distributors, valves, ball valves, air vents, or transitions to other pipes. Unlike crimped or welded connections, fittings are mechanical, meaning they can be disassembled and replaced if needed.

Brass is used for these applications for several reasons:

Corrosion resistance – brass does not rust and withstands moisture and minerals in the circulating water well
Mechanical strength – brass is much more durable than plastic, can withstand lateral loads, vibrations, and thermal expansion
Thermal resistance – high-quality brass fittings perform reliably even at temperatures of 90–95 °C, which PE pipes usually do not reach, but distributors do
Long service life – with proper installation and a quality product, we are talking about a service life of 30+ years
Compatibility with most fittings – standardized threads G ½″, G ¾″, and G 1″ simplify the entire system design

If you are interested in a comparison with plastic alternatives, there is a separate article Brass vs. Plastic Fittings for PE Pipes – Which Are More Reliable, where you will also find specific test values and scenarios when considering plastic makes sense.

Types of brass fittings – what actually exists on the market

Before we dive into the selection criteria, it is important to understand what types of fittings we are actually talking about. There are several construction solutions on the market, each with different uses and installation requirements.

Compression (clamping) fittings

The most common type. The principle is that the PE pipe is inserted through the nut and a sealing olive (conical or cylindrical ring), and tightening the nut deforms the olive to mechanically grip the pipe. The advantage is simple installation without special tools. The disadvantage is a higher tendency to loosen over long-term thermal expansion, if the olive is not of sufficient quality or if the nut is not tightened to the correct torque.

Fittings with a brass insert (insert fitting)

A more modern approach, where a brass reinforcing insert is first inserted into the PE pipe. This insert supports the pipe from the inside, preventing deformation during nut tightening. This type is significantly more reliable, resistant to vibrations, and is preferred in professional underfloor heating systems and for larger diameters. For PE-X (cross-linked polyethylene) pipes, the insert is practically essential.

Eurocone (24° cone fitting)

A specific type mainly used when connecting to hydraulic distributors and valves. Sealing is done by a cone directly between the fitting and the opposite surface – not by an olive. It requires precise tightening to ensure the cone does not deform. In practice, you will encounter it in supply/return hoses for underfloor heating loops.

Straight, elbow (90°), and T-piece fittings

In addition to straight transitions, you will also find elbow fittings that allow a change of direction without using an additional elbow. For tight spaces in distributor cabinets, this is very practical. T-piece fittings are used for branches to a secondary loop.

Key selection parameters – what you need to know before ordering

This is the core of the topic. Five basic parameters you need to be clear about before choosing a specific product:

1. Outer diameter of PE pipe

This is the most basic parameter. PE pipes are specified by their outer diameter (abbreviation de or OD – outer diameter). The most common dimensions in floor heating installations are de 16 mm and de 20 mm. For larger distribution systems, de 25 mm, de 32 mm and de 40 mm are used. The compression fitting must fit precisely to the outer diameter of the pipe – confusing de 16 with de 20 is optically small, but the compression fitting will simply not fit or will have insufficient grip.

Attention: the pipe diameter does not automatically relate to the thread diameter of the compression fitting. One compression fitting de 16 mm × G ½″ and another de 16 mm × G ¾″ have the same grip on the pipe, but a different output thread. This is important when feeding distributors with different port thread sizes.

A detailed overview of combinations can be found in the article Dimensions and diameters of brass compression fittings for PE pipes – dimension overview.

2. Type of polyethylene pipe

Not all PE pipes are the same and not every compression fitting is suitable for every type:

PE-X (cross-linked polyethylene) – pipes with memory effect. After deformation, they try to return to their original shape. With a compression fitting without an insert, there is a risk that the pipe will "decompress" over time and leak. For PE-X, brass inserts are clearly suitable.
PE-RT (polyethylene with increased thermal resistance) – no memory effect, but it is softer than PE-X, so an insert is also suitable. A high-quality compression fitting can work if it is properly dimensioned.
HDPE (high-density polyethylene) – a stiffer material, more common in garden distribution, water supply and pressure connections. Here, compression fittings without inserts are common, but it depends on the pressure and temperature of the application.
PE-Xa, PE-Xb, PE-Xc – different cross-linking methods affect the rigidity of the pipe. PE-Xa (Engel method) is the hardest, PE-Xb (silane method) is softer. For PE-Xa, inserts are especially important.

The article Compatibility of brass compression fittings with different types of PE pipes (PE-X, PE-RT, HDPE) deals with this topic in much more detail including compatibility tables.

3. Type and size of thread

The thread on the output side of the compression fitting must match the opposite fitting. In Europe, the following are used by standard:

G ½″ (external or internal) – most common for individual distributor connections, valves, air vents
G ¾″ – larger circuits, main distribution, ball valve connections
G 1″ – primary distribution, boiler connections, larger distributors
G 1¼″ and G 1½″ – industrial applications, less common in residential heating

It is important to distinguish between external thread (AG – Außengewinde) and internal thread (IG – Innengewinde). This is a very common mistake – the customer orders a compression fitting with an external thread, but the opposite distributor also has an external thread. The result is either the need for a reducer or returning the product.

4. Maximum working pressure and temperature

In heating applications, you usually work with pressure 1.5–3 bar and temperature 35–80 °C (floor heating up to 55 °C, radiator circuits up to 90 °C). High-quality brass compression fittings for heating applications usually handle:

Working pressure: 10–25 bar (depends on the manufacturer and size)
Maximum temperature: 90–110 °C
Minimum temperature: -20 °C (important for garden installations where freezing is a risk)

In practice, the problem does not arise from the operating pressure itself, but from hydraulic shocks and test pressures. The test pressure according to the STN EN 1264 standard is 1.5 times the operating pressure, so for a system with an operating pressure of 4 bar, you test at 6 bar. The compression fitting must be able to withstand this pressure without problems.

5. Application – heating, DHW or garden

A compression fitting for a heating system, a compression fitting for domestic hot water and a compression fitting for garden irrigation are three different worlds. In heating, you have constant thermal cycles, deoxygenated or softened water with additives, a closed circuit. In DHW, you have potable water and must meet hygiene requirements (WRAS or ACS/DIN EN 15664 certification). In the garden, you have seasonal draining, UV radiation and mechanical loading. These differences influence the choice of sealing type (EPDM, FKM/Viton, NBR), surface treatments and the overall design of the compression fitting.

Material of brass and additives – not every brass is the same

Brass is an alloy of copper and zinc, and the ratio of these components significantly affects the properties of the final product. In water and heating applications, we most often encounter the following alloys:

CuZn39Pb3 (MS 58) – the most common type, contains lead (Pb) for better machinability. Therefore, it is not recommended in the EU for potable water supply (directive 98/83/EC), but is fully acceptable for heating.
CuZn21Si3P (CW724R) – lead-free brass, suitable for potable water and DHW. Excessive for heating, but certified for all applications.
CuZn36Pb2As – dezincification-resistant brass, contains arsenic (As) precisely to suppress dezincification. Suitable for soft water with low pH.

Dezincification is a phenomenon in which aggressive water leaches zinc from the structure of the brass, causing porosity and cracking. In heating systems, the water is usually chemically treated and the problem of dezincification is smaller, but in garden installations connected to a local source with soft water (pH below 7) it can be a serious problem.

How to avoid this problem and other forms of corrosion is described in the article How to prevent corrosion of brass compression fittings in floor heating systems.

What to watch out for when buying – warning signs of poor-quality compression fittings

Over the years of working with customers, I have seen compression fittings from all corners of the world. Some last for decades, others crack during the first pressure test. Here are specific warning signs:

Too low a price

Quality brass compression fittings de 16 × G ½″ from a reputable manufacturer (Herz, Watts, Uponor, Pettinaroli, Giacomini) cost realistically 2–5 € in 2024. A compression fitting for 0.40 € is made from a different alloy, with worse hole tolerance and likely a seal of questionable quality. You won't notice it on a single piece. But on a 200-loop distribution with 200 compression fittings, you will feel it during the first winter season.

Missing certification

A serious manufacturer specifies the following for the product: material specification (e.g. CW617N), pressure-temperature diagram, certification for the intended application (DVGW for water, CE for pressure equipment), and a product code allowing identification of the batch. If these details are missing or vague ("brass, suitable for heating"), it is a warning sign.

Poor thread quality

The thread on the compression fitting should screw into the opposite thread by at least 3–4 turns by hand, without tools and without resistance. If the thread "heats up" from the first turn, the tolerances are not correct and during installation you risk breaking the thread or improperly seating the seal.

Inappropriate type of olive or seal

A soft NBR (nitrile rubber) seal is suitable for water up to about 70 °C. For higher temperatures (80–110 °C), EPDM or FKM (Viton) is required. Many cheap compression fittings come with an NBR seal even for products declared for heating, which is a mistake.

Missing insert or a plastic insert instead of brass

Some kits include a plastic insert instead of a brass one. A plastic insert is less resistant to thermal stress and may crack during long-term use at higher temperatures. Always prefer a brass insert for heating systems.

Dimensions and tolerances – why detail determines sealing

The brain of the entire compression fitting–pipe system is the precise geometry. PE pipes have a defined outer diameter according to ISO 4427 and EN 12201 with tolerances, for example:

Nominal diameter	Outer diameter (de)	Tolerance	Typical SDR
DN12 / 16 mm	16.0 mm	+0.3 / 0 mm	SDR 6 (wall thickness 2.7 mm)
DN16 / 20 mm	20.0 mm	+0.3 / 0 mm	SDR 6 (wall thickness 3.4 mm)
DN20 / 25 mm	25.0 mm	+0.3 / 0 mm	SDR 6 / SDR 7.4
DN25 / 32 mm	32.0 mm	+0.4 / 0 mm	SDR 7.4 / SDR 11
DN32 / 40 mm	40.0 mm	+0.4 / 0 mm	SDR 11

The compression fitting must be designed so that it still guarantees sealing at the maximum pipe tolerance (de +0.3 mm). Cheap products do not meet this tolerance – they are designed for an ideal pipe with zero deviations, which does not exist in practice.

This is also related to the pipe wall thickness (SDR = ratio of outer diameter to wall thickness). A pipe de 16 SDR 6 has a wall thickness of 2.7 mm, while a de 16 SDR 11 has a wall thickness of only 1.5 mm. With insert compression fittings, this can affect sealing if the insert is not properly dimensioned for the given wall thickness.

Where mistakes are most commonly made – from customer practice

Allow me to be a bit more informal now – these are scenarios I have seen repeatedly:

Scenario 1: Garden hose instead of PE-X

The customer buys a compression fitting for PE pipes and installs it on a regular garden PE hose (LDPE, low density). The outer diameter fits, but the wall is too soft. The compression fitting appears to seal, but at 3 bar garden pressure during summer irrigation, it starts to leak. Solution: always verify not only the diameter, but also the type and SDR of the pipe.

Scenario 2: Insert forgotten during installation

The installer, in a rush, installed 15 compression fittings with inserts, but forgot to insert the insert in three of them before tightening the nut. Visually, everything looks fine, but the pipe is not supported. The first pressure test goes smoothly (PE at room temperature is stiffer), but after the first heating season, when the PE-X warms up and softens, these three fittings start to leak. Detectable only by a full pressure test at elevated temperature. Solution: always visually verify the insert before tightening the nut and keep an installation protocol.

Scenario 3: Confusing G ½″ AG and G ½″ IG

The designer specified a compression fitting with an external thread G ½″ for connection to a manifold with internal threaded ports G ½″. The customer mistakenly ordered compression fittings with an internal thread G ½″. The result: the installer discovers it on site, three hours and two truckloads are lost. Solution: always explicitly specify AG (external) or IG (internal) when ordering.

Scenario 4: Mixing manufacturers

In one building, brass inserts from manufacturer A and nuts from manufacturer B were used. The outer dimensions matched, but the olive angle was slightly different. The result was occasional leaks at random locations, which were hard to detect. Never mix parts of compression fittings from different manufacturers.

In detail, the installation procedure including torque values and typical errors can be found in the article Installation of brass compression fittings on polyethylene pipes – step-by-step procedure. The topic of tightness and sealing of the thread is addressed in the article Tightness of PE pipe connections – how to properly seal brass compression fittings.

System compatibility – compression fittings as part of the whole

You never buy a compression fitting separately – it is part of a system. Therefore, you should be aware of several system-related connections:

Compatibility with manifolds

If you have a floor heating manifold with an external G ¾″ port thread, you need a compression fitting with an internal G ¾″ thread. If the manifold uses a 24° eurocone, the compression fitting must have the same conical surface. Always check the manifold documentation before purchasing compression fittings.

Compatibility with pipe system manufacturer

System manufacturers such as Uponor, Rehau, Purmo, or Wavin recommend (and sometimes require for warranty purposes) the use of their own compression fittings. Their fittings are precisely dimensioned for the tolerances of their pipes. Using a "universal" compression fitting from another manufacturer on pipes with system warranty certification may jeopardize the warranty claim.

Thermal expansion of PE pipes

PE pipes have a significantly higher coefficient of thermal expansion than brass: PE-X approx. 0.15–0.2 mm/(m·K), brass approx. 0.018 mm/(m·K). This means that when the system is heated from 20 °C to 55 °C, a 10-meter PE pipe will extend by about 50 mm, while the brass compression fitting at the end will practically not move at all. In rigid pipe layouts without expansion joints, this force can exert axial pressure on the compression fitting. Therefore, expansion loops or U-shaped compensators are important before each fixed point – including the compression fitting – in longer straight sections.

Maintenance and inspection during operation

Brass compression fittings on PE pipes are not "install and forget" devices. As part of regular inspection of the heating system (at least once every two years is recommended), you should:

Visually inspect all accessible compression fittings on the distribution (especially at manifolds in boxes) – look for drops, salt deposits, or copper corrosion (greenish discoloration indicates a water leak)
Test tightness with a pressure test (filled with water, pressure 1.5 times the operating pressure, monitored for 30 minutes)
Check the tightening of nuts – not "tighten again with force," but check whether the nut has loosened on its own (which may indicate vibrations or creep of the PE pipe)
Check the condition of seals at accessible locations during disassembly for other purposes

This topic is addressed in detail in the article Maintenance and inspection of brass compression fittings in heating systems. Problems that can occur during operation are listed in Common faults of brass compression fittings on PE pipes – causes and solutions.

Where to buy and what to avoid

On the Slovak market, there are compression fittings in three price ranges:

Premium segment (Uponor, Rehau, Giacomini, Herz, Watts, Pettinaroli) – certified products with complete documentation, long-term warranties, system compatibility. Price is higher, but for professional installation in concrete or walls, it is the clear choice.
Middle segment (well-known Central European and Italian brands) – good price-to-quality ratio for smaller projects and applications where the compression fitting is accessible during operation.
Low segment (no-brand products, marketplace imports) – suitable only for uncovered, low-pressure, low-temperature applications such as garden irrigation. For heating systems embedded in structures, I categorically do not recommend them.

In the category Brass compression fittings for polyethylene pipes on atria.sk, you will find products from verified sources with available technical parameters, so you can verify all key data before purchase.

Practical selection process – how to do it step by step

Summing it up into a practical algorithm, the selection of a brass compression fitting should proceed as follows:

Determine the outer diameter of the pipe – de 16, de 20, de 25, de 32, or de 40 mm. Measure with a caliper if you are unsure.
Determine the type of pipe – PE-X, PE-RT, or HDPE. For PE-X and PE-RT, choose a compression fitting with a brass insert.
Check the thread of the opposite fitting – external or internal? What size? G ½″, G ¾″, or G 1″?
Check pressure and temperature parameters – what is the operating pressure and temperature of your application? The compression fitting must be dimensioned for at least 1.5 times the value.
Select the type of sealing – EPDM for heating (up to 110 °C), NBR for cold water, FKM (Viton) for aggressive media.
Check system compatibility – if you have a branded pipe system, check whether the manufacturer requires the use of their own compression fittings.
Select a manufacturer with certification and documentation – do not buy "anonymous" compression fittings without PN (rated pressure) and maximum temperature.

Most frequently asked questions (FAQ)

Can I use the same compression fitting for PE-X and PE-RT pipes?

In most cases, yes, if the compression fitting is designed for both types of pipes (which is the case for most quality products for heating). The key is that the compression fitting contains a brass insert and that the tolerances of the hole match the outer diameter of the pipe. Always check the manufacturer's data – some compression fittings without inserts are certified only for PE-RT, not PE-X.

What is the difference between G ½″ and G ¾″ compression fittings – can they be interchanged?

No. G ½″ and G ¾″ are different thread sizes. G ½″ has an outer thread diameter of approx. 20.9 mm, G ¾″ approx. 26.4 mm. They cannot be physically mounted on the same opposite fitting. Interchange is only possible via a reducer, which is not recommended for heating systems due to hydraulic losses and an additional connection.

Do I need to use sealing tape or Loctite with a brass compression fitting?

It depends on the construction of the compression fitting. Compression fittings with O-ring sealing (circular rubber seal in a groove) are installed without sealing tape – sealing tape would damage the seal. Compression fittings with threaded sealing (conical threads) require Teflon tape or Teflon paste. Always read the manufacturer's installation instructions. A common mistake is using Loctite on connections that are supposed to be removable – Loctite is intended for securing screw joints, not for sealing water supply lines.

How long do brass compression fittings in floor heating embedded in concrete last?

Quality brass compression fittings from a reputable manufacturer, properly installed and used in a closed heating system with chemically treated water (correct pH 7.5–9.0, low oxygen content, corrosion inhibitors) can last 30–50 years. The decisive factors are: water quality in the system, absence of oxygen in the circulation, proper installation with an insert, and absence of mechanical damage during concrete pouring. Most faults occur in the first two years – if the system survives the first two heating seasons without problems, the prognosis is very good.

What if I have a PE pipe de 20 and the compression fitting won't fit – is the pipe thicker?

There are three possible explanations: the pipe is not actually de 20 (measure with a caliper), the pipe is de 20 but with a large tolerance (older manufacturer or non-standard production), or the compression fitting is too tight (lower production tolerance). In such a case, do not bend or cut the pipe – damage may occur. Check the dimensions of the pipe and compression fitting, or try another piece of compression fitting from the same batch. If the problem persists, contact the supplier.

Can I use a brass compression fitting for PE pipes on copper pipes?

No, brass compression fittings for PE pipes are designed for plastic pipes with a precisely defined outer geometry and sealing on the outer

Do you have a question about this topic?

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