Frequently Asked Questions about Couplings and Fittings

When people talk about a heating or water system, attention is usually focused on the boiler, radiators or pipework. Less attention goes to what joins these components together - couplings and fittings. Yet it is precisely this small-looking fitting that decides whether the system will stay tight and trouble-free for years, or whether a dripping joint, a damp patch on the wall, or even a bigger failure will eventually appear.

In practice, we find that customers carefully choose their boiler or radiators, but only think about fittings at the moment they are standing in the shop with a cut piece of pipe in hand, unsure which exact part they need. The aim of this article is to give a comprehensive answer to the most common questions about couplings and fittings - what the difference is between a screw joint and a press joint, what types of fittings exist, when to reach for brass and when for plastic, how to check compatibility with existing pipework, what tools you will need, and which mistakes are best avoided.

The article is designed as a clear guide you can return to at any time - whether you are planning a bathroom renovation, adding a radiator in an older flat, or just dealing with one leaking joint on a riser pipe. Alongside the general explanation you will also find specific products from our range, links to related topics from our knowledge centre, and two real-life examples that show the principles in practice.

What is a fitting and why is it an essential part of the system

A fitting, often called a coupling, is a fixture used for joining, branching, reducing diameter, or changing the direction of a piping system. It is a part that is practically invisible in the finished installation - hidden under plaster, in a service duct, or behind a radiator - yet without it the whole system could not be assembled.

A fitting in a piping system performs four basic functions.

Without the right fitting it would not be possible to connect pipework to a radiator, a tap, a manifold, or to route it around a corner of a room. In other words, fittings are just as important a part of the system as the pipe itself - while the pipe forms the "roads" along which water or the heating medium flows, it is the fittings that determine where this road leads, where it branches, and where it connects to appliances.

In practical terms, this means that when planning any piping system - whether a complete heating renovation in a family house or simply extending one branch to a new radiator - you need to account not only for metres of pipe, but also for the exact number and type of fittings at every point where direction, diameter, or connection to an appliance changes. A detailed step-by-step guide to selection can be found in the article How to Choose a Coupling or Fitting.

Screw joints vs. press joints - what is the difference and what to choose

One of the first questions to ask when choosing fittings is: screw joint or press joint? Both systems have their place, and the decision should be based on how large the installation is, what tools you have available, and whether you expect to need to return to the joint in the future.

A screw (threaded) joint is secured with a union nut that presses a sealing or retaining ring against the pipe. The biggest advantage of this solution is that it can be taken apart without special tools - ordinary pliers or a wrench are enough for both assembly and any later disassembly. This is particularly practical for maintenance, when a joint needs adjusting over time, disassembling to replace part of the system, or when work on a building site proceeds gradually and you don't want to invest in more expensive specialised tools right at the start.

A press joint, by contrast, is created with crimping pliers that permanently deform a metal sleeve around the pipe inserted into the fitting. Installation is faster when making a large number of joints at once (an experienced installer can crimp one joint in a matter of seconds), and the joint itself is stronger and cannot be taken apart. The price of this advantage is the necessary investment in crimping tools for the specific system - and this is exactly why small builders, or those making only one or two joints, usually choose the screw solution, while installation companies that carry out piping work routinely prefer crimping.

When deciding, consider above all the scope of the job. For one or a few joints in a minor repair or when connecting a radiator, it is worth using a screw fitting without having to buy crimping pliers. For a complete installation with dozens of joints, on the other hand, the investment in crimping tools quickly pays for itself in saved installation time and in a higher-quality, more durable joint. A more detailed comparison of both systems, along with further context, can be found in the article Screw vs. Press Joints.

An example of a screw solution from our range is the Screw Coupling 1/2x16 with Internal Thread - a screw transition coupling with an internal 1/2" thread for 16 mm pipe, exactly the type of fitting you will appreciate for a minor repair or connecting a single radiator without needing crimping tools.

Types of fittings - elbows, tees, reducers and wall plates

Just as important as choosing between a screw joint and a press joint is knowing what type of fitting you actually need at a given point in the system. Fittings are not only divided by their method of joining, but also by the geometric function they perform in the system.

A straight coupling joins two sections of pipe of the same diameter in one line - you would use it, for example, when you need to extend existing pipe or repair a damaged section. An elbow changes the direction of the pipe, most often by 90° or 45°, and is essential everywhere the system runs around a corner of a room or changes the height of the run. A tee branches one line into two directions - it is typically used when branching off to a radiator from the main riser pipe, when you need to "branch off" sideways from one line without interrupting the main run. A reducer, also called a transition piece, joins two sections of different diameter, which is necessary, for example, at the transition from a thicker main line to thinner pipe leading to an individual appliance.

A separate category is the wall plate - a special fitting designed to firmly anchor pipe into the wall at the point where a tap or other appliance is connected. A wall plate must be extra firmly anchored, since unlike an ordinary coupling somewhere in the middle of the system, it transfers the mechanical load from tightening and everyday use of the tap - that is, repeated tightening of hoses, slight jolts during handling, and similar stresses. This is exactly why more attention is paid to installing wall plates during assembly than to "ordinary" couplings somewhere inside a wall, where the pipe will no longer be handled afterwards.

A specific example of a wall plate from our range is the Screw Wall Plate 1/2 x 16 for Water - a screw wall plate for anchoring a tap into the wall, without the need for crimping, for 16 mm pipe. Thanks to its screw design you can install it with ordinary tools, and if needed (for example when replacing the tap with a different type) you can take it apart again.

For a more detailed overview of all fitting types, including their typical placement in a system, we recommend the article Types of Fittings - Elbows, Tees, Reducers, Wall Plates. If you are specifically interested in the topic of wall plates for connecting taps, a dedicated article covers it: Wall Plates for Taps - What They Are and How They Are Installed.

Brass or plastic - which fitting material to choose

Besides the type and method of joining, the material a fitting is made from also matters when choosing. In practice you will most often encounter two groups - brass fittings and plastic fittings (for example made of PPSU or POM) - and each has its own advantages and limitations.

Brass fittings are resistant to both temperature and pressure, which makes them suitable for heating systems as well as hot-water plumbing. They are stable over the long term, meaning they retain their properties even after years of operation with repeated temperature changes. The disadvantage is a higher price and greater weight compared to plastic alternatives.

Plastic fittings, on the other hand, are lighter and cheaper, and resistant to corrosion - an advantage especially in damp environments, where metal could corrode over time. However, they have a lower maximum operating temperature and pressure than brass, so they are more suitable for cold-water systems or lower temperatures. Before using a plastic fitting on a heating circuit, it is therefore always necessary to check the technical data sheet of the specific product - not every plastic fitting is rated for the temperatures and pressures common in a heating system.

A practical rule of thumb: if you are not sure whether a given section of the system will ever carry hot water from heating, or if it is a location with higher mechanical stress (for example a wall plate under a tap), reach for brass instead. Save plastic for cold-water systems, where temperature resistance is not critical and the cost saving and lower weight make more practical sense. A complete comparison of both materials can be found in the article Brass vs. Plastic Fittings.

A typical example of a brass solution is the Reducing Socket 1" x 1/2" FF - a brass reducing socket with 1" and 1/2" threads (the designation FF means internal/internal thread on both sides). You will appreciate this type of fitting everywhere you need to go from a larger distribution pipe to a thinner branch leading to an appliance, while also wanting to be sure the joint can handle the higher temperature of the heating medium.

Prices of three specific fittings from our range.

Fitting compatibility with pipe - what to watch out for

One of the most common causes of disappointment after buying a fitting is discovering that it "fits, but doesn't fit" - meaning the fitting can be slid onto the pipe, but the joint is not as tight or as strong as it should be. The reason is that a fitting must match the outer or inner diameter of the specific pipe and the joining method it is designed for.

It is important to realise that the same diameter designation does not automatically mean universal interchangeability across materials. A fitting designed for a screw joint on 16 mm PEX-AL-PEX pipe is not automatically compatible with copper or PP-R pipe of the same nominal diameter - the actual dimensions and joining method differ between materials. What looks at first glance like "the same" diameter may in fact have a different pipe wall thickness or a different profile that the fitting was built for.

When buying a fitting, it is therefore always necessary to check that the material, diameter and joint type match the pipe already installed in the system. We recommend not buying a fitting "by eye" based solely on the diameter number printed on the box, but comparing it directly with a piece of the pipe you have at home, or consulting the seller, or checking the product's technical specification. This seemingly trivial check can save a repeat trip to the shop as well as an unnecessarily disassembled joint. You can find more on this topic in the article Fitting Compatibility with Different Types of Pipe.

Installing fittings - what tools you will need

Basic steps for correctly installing a press joint.

The tools you need for installing a fitting depend directly on whether it is a screw joint or a press joint - and it is worth thinking this through before buying, so you don't end up holding a fitting without the right tool at home.

Installing screw fittings only requires a set of suitable wrenches, or an adjustable wrench, and sometimes a pipe cutter so the end of the pipe is straight and perpendicular - a straight cut is a prerequisite for the sealing ring to seat correctly around the whole circumference of the pipe. Practically every household has this kind of tool, or it can be obtained cheaply, which is one of the main reasons the screw solution is so popular for minor home repairs.

Press joints, on the other hand, require crimping tools compatible with the given fitting system. Here you need to watch an important detail - different manufacturers may have a different crimping jaw profile, and using the wrong crimping profile can result in a leaking or damaged joint even though the operation looks visually correct. In other words, "crimping" alone is not a guarantee of a quality joint if the pliers' jaws do not exactly match the given fitting system. Before buying crimping tools, it is therefore necessary to check that they are designed specifically for the fitting system you plan to use.

A complete installation procedure, including recommended tools for both systems, can be found in the article Installing Fittings - Procedure and Required Tools.

Common installation mistakes and how to avoid them

Most fitting leak problems don't arise because the part itself is faulty, but because one of the small yet important steps was overlooked during installation. Knowing the most common mistakes in advance is the cheapest way to avoid them.

The most common cause of leaks is an insufficiently tightened union nut on screw joints - the nut must be tightened firmly enough that the sealing or retaining ring is pressed evenly against the pipe all the way around. Another common cause is a damaged or completely missing sealing ring, which is easy to overlook if the fitting is installed in a hurry or in a cramped space. An equally common mistake is pipe inserted at an angle or not fully inserted into the fitting before crimping - if the pipe is not inserted to its full prescribed depth and straight, the crimping sleeve does not deform the way it should, and the joint remains leaky even though the crimping pliers "clicked" as they would for a correct joint.

Another common mistake is combining incompatible materials - for example brass directly with aluminium without proper separation - in a damp environment. This combination can, over time, cause electrochemical corrosion, also known as bimetallic corrosion, right at the joint. This is a gradual process that may not show any outward signs for a long time, but over time it can weaken the material and cause a leak exactly where the two different metals are in direct contact.

Practical advice: never rush the last step of installation - tightening the nut or crimping - and always check, before covering the joint (for example with plaster), that it is truly tight, ideally with a short pressure test. A detailed overview of mistakes and how to avoid them can be found in the article Common Installation Mistakes and Leaks.

Servicing and replacing fittings over the life of a system

Fittings, like other parts of a system, have a service life, and over the years a situation may arise where one needs to be replaced - for example during a bathroom renovation, replacing a tap with a different type, or simply because a leak has appeared at an old joint. This is exactly where the difference between the screw and press solutions described above becomes fully apparent.

A screw joint can be taken apart without special tools, so replacing a fitting or adjusting the system is relatively simple - just loosen the union nut, replace the damaged part (for example the sealing ring) or the whole fitting, and secure the joint again. A press joint, on the other hand, is permanent, and "replacing" it in practice means cutting off the fitting and the adjacent piece of pipe and installing a new joint from scratch, since the crimped sleeve cannot be returned to its original state.

When planning locations where you are likely to need to return to the joint in the future - typically wall plates under taps, which sometimes need adjusting when replacing sanitaryware - it is therefore worth considering a screw solution in advance, even if crimping predominates elsewhere in the system. You can find more on the topic of servicing and replacement in the article Servicing and Replacing Fittings.

Real-life examples

The scope of the job decides between a screw and a press solution.

Family house with a complete heating renovation: During the renovation of an older family house, the entire heating system was replaced, including all radiators. Since this involved dozens of metres of pipe and several dozen joints at once (elbows at every change of direction, tees at branches to individual radiators, reducers at transitions from the main line to thinner branches), the installation company chose a press system. The investment in crimping pliers quickly paid off in installation time for a job of this scale, and the resulting joints are strong and require no further maintenance. Brass wall plates were also used to connect each radiator to the wall - since these are locations with mechanical stress from connecting and disconnecting radiator valves, a plastic solution would not have been suitable here.

Panel-building flat with one bathroom tap replacement: In a flat in a panel building, only one washbasin tap needed replacing with a new one, while the original wall plate in the wall was an older type that did not match the new tap. Since this involved only one joint and the flat owner did not have crimping tools available, a screw wall plate for 16 mm pipe was chosen - installation was handled with an ordinary wrench, without needing to buy or borrow more expensive crimping pliers. Another advantage of this solution was that, in the event of a future tap replacement, the joint can again be easily taken apart without having to disturb the tiling around the wall plate again.

Frequently Asked Questions about Couplings and Fittings

What is the main difference between a screw and a press fitting?
A screw joint is secured with a union nut and can be taken apart at any time with ordinary tools (pliers, a wrench). A press joint is created with special crimping pliers that permanently deform a metal sleeve around the pipe - it is stronger, but cannot be taken apart, and requires investment in crimping tools for the specific system.

Is it worth buying crimping tools for just one or two joints?
In most cases, no. For a small number of joints (for example repairing or connecting a single radiator or tap), it is simpler and cheaper to choose a screw fitting, for which ordinary tools are enough. Investing in crimping pliers only pays off for larger jobs with dozens of joints.

How do I know whether I need an elbow, a tee, a reducer or a straight coupling?
It depends on the function the fitting needs to perform at that point. A straight coupling joins two sections of pipe of equal diameter in a straight line. An elbow changes direction (most often by 90° or 45°). A tee branches one line into two directions, for example when branching off to a radiator. A reducer (transition) joins two sections of different diameter.

Why does a wall plate under a tap need to be anchored more firmly than an ordinary coupling?
Unlike a coupling somewhere in the middle of the system, a wall plate transfers mechanical load - repeated tightening of hoses, handling of the tap and similar stresses. That is why more attention is paid to anchoring it into the wall than to a coupling that, once built into a wall or plastered over, is essentially free of further mechanical load.

Is a plastic fitting suitable for a heating circuit too?
Not always. Plastic fittings (for example made of PPSU or POM) generally have a lower maximum operating temperature and pressure than brass ones, so they are more suitable for cold-water systems or lower temperatures. Before using one on a heating circuit, always check the technical data sheet of the specific fitting.

Can I use a fitting designed for PEX-AL-PEX pipe on copper or PP-R pipe of the same diameter?
Not automatically. Even though the nominal diameter is the same, the actual dimensions and joining method differ between materials. A fitting designed for a screw joint on 16 mm PEX-AL-PEX pipe may not be compatible with copper or PP-R pipe of the same nominal diameter - always check that the material, diameter and joint type match.

What tools do I need to install a screw fitting?
A set of suitable wrenches or an adjustable wrench is enough, plus perhaps a pipe cutter so the end of the pipe is straight and perpendicular. Most households already have this kind of tool.

Why does the joint still leak after crimping?
The most common cause is using the wrong crimping jaw profile that does not match the given fitting system - different manufacturers may use different profiles. Another possible cause is pipe inserted at an angle or not fully inserted into the fitting before crimping.

Can it cause a problem if brass meets aluminium in a system?
Yes. Combining incompatible materials, for example brass directly with aluminium without proper separation, can cause electrochemical (bimetallic) corrosion right at the joint in a damp environment, which over time weakens it and can lead to a leak.

Related Topics

• How to Choose a Coupling or Fitting
• Screw vs. Press Joints
• Types of Fittings - Elbows, Tees, Reducers, Wall Plates
• Brass vs. Plastic Fittings
• Fitting Compatibility with Different Types of Pipe
• Installing Fittings - Procedure and Required Tools
• Common Installation Mistakes and Leaks
• Wall Plates for Taps - What They Are and How They Are Installed
• Servicing and Replacing Fittings

Have a question about couplings or fittings?

Not sure what to decide, or dealing with a specific situation in your home? Write to us - we're happy to help.

Full name Do not fill in this field:

Email

Message