Installing Fittings – Procedure and Required Tools

A fitting is, at first glance, a small and unremarkable component – a piece of brass or plastic that fits in the palm of your hand. In reality, though, it is the point where it is most often decided whether a heating or plumbing system will work trouble-free for decades, or whether a wet stain will appear on the wall or ceiling within a few months. While a straight section of pipe just carries water from point A to point B, a fitting is a point of change – a change of direction, diameter, material, or number of branches. And it is precisely at these points of change that the joint experiences the most mechanical and thermal stress.

In this article we take a detailed look at how fittings work, what types are commonly used, the difference between a threaded and a press joint, when to reach for brass and when plastic will do, what matters when combining a fitting with a specific pipe type, what tools you will need for installation, and what to watch out for to avoid the most common mistakes that lead to leaks. The article also includes two real-world examples and answers to the questions we most often get from customers on this topic.

This article is intended equally for experienced installers who want to check or refine their procedure, and for DIYers about to replace a wall plate under a basin or connect a radiator who want to be sure the joint will hold. If you're not sure exactly which fitting you need, we recommend first reading the article How to choose a coupling or fitting, where we cover the basic selection criteria.

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

A fitting (coupling) is a component used to join, branch, reduce the diameter of, or change the direction of a piping run. Without the right fitting it would be impossible to connect pipe to a radiator, a tap, a manifold, or to route it around the corner of a room – fittings are therefore just as important a part of the system as the pipe itself.

It's quite common that when planning a bathroom renovation or a boiler replacement, most people first think of the pipe, the radiators, or the heat source itself, and view fittings merely as a minor supplementary item bought at the last minute. In practice it's exactly the opposite – while good pipe lasts practically maintenance-free for decades, it is precisely the joints where any installation inaccuracy most often shows up. Experienced installers therefore know that time spent choosing the right fitting and installing it correctly always pays off in the form of peace of mind from future complaints.

Fittings appear in a system at practically every transition or change – where pipe enters the wall, where it branches off toward individual radiators, where the diameter changes between the main run and the connection to an appliance, and where an obstacle needs to be avoided or the direction changed by 90 degrees. In an ordinary family house, a single heating circuit can therefore use several dozen fittings of various types – and each one is a potential leak point if not correctly chosen or installed.

Both joint types are reliable; they differ in installation method and detachability.

Threaded vs. press joints – what is the difference between them

When choosing a fitting you almost always also decide between two basic connection types – threaded and press. Both have their place, and both, if done correctly, provide a reliable, long-term tight joint. The difference lies in the installation method, the tools needed, and whether the joint should be detachable in the future.

A threaded (screw) joint is secured with a compression nut that presses a sealing or grip ring against the pipe. The advantage is that it can be taken apart without special tools – pliers or a wrench are enough – which is practical especially for maintenance or a situation where the joint needs adjusting over time, taking apart to repair another part of the system, or simply relocating. Threaded joints are therefore popular especially for connections to fixtures, taps, manifolds, and everywhere it is likely that the location will need to be accessed again in the future.

A press joint is made with press 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, the joint is stronger and non-detachable, but it requires investment in press tools for the specific system. That's exactly why press joints are most often used for larger installations, where a professional installer or company already owns the tools and can therefore complete dozens of joints quickly and with consistently high quality.

A more detailed comparison of both systems, including recommendations on when each type is more advantageous, can be found in the separate article Threaded vs. Press Fittings. In general, a simple rule applies – if you plan to take the joint apart again in the future, or it's a one-off home repair where buying press pliers for just a few joints doesn't make sense, reach for the threaded variant. If, on the other hand, you're installing a complete new system with dozens of joints and you have (or can borrow) the necessary press tools, a press system pays off thanks to faster installation and greater joint strength.

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

Fittings are not a single uniform category – based on their function in the system, they are divided into several basic types, each solving a different situation along the pipe route.

A straight coupling joins two sections of pipe of the same diameter in a single line. Used everywhere pipe simply continues on, but for some reason needs to be interrupted and rejoined – for example when connecting to an older section of the system or repairing a damaged part.

An elbow

Elbows are made in two of the most common angles depending on the required change in the pipe route.

changes the direction of the pipe, most often by 90° or 45°. Elbows are essential everywhere pipe needs to run along a wall, go around the corner of a room, or drop vertically down to an appliance. The choice of angle depends on the specific route – 90° elbows are used for right-angle direction changes, while 45° elbows are suitable where a smoother change of direction or less space for the bend is needed.

A tee branches a single run into two directions. Used, for example, when branching off to a radiator from the main riser – the main run continues on to further radiators, while the branch through the tee delivers water to one specific radiator.

A reducer (transition fitting) joins two sections of different diameter. It is essential wherever pipe dimension changes – for example from a larger-diameter main run to thinner branches leading to individual radiators, or when connecting an appliance with a thread of a different size than the pipe itself.

A wall plate is a special fitting used to firmly anchor pipe to the wall at the point where a tap or other appliance connects to it. It must be extra firmly anchored, since it transfers mechanical load from tightening and using the tap – every time the tap lever is turned, or during cleaning, a certain force acts on the wall plate, and if it isn't firmly anchored in the wall, it can loosen over time. That is exactly why we cover wall plates in more detail in the separate article Tap wall plates – what they are and how they are installed, where we also discuss the correct spacing and embedment depth into the wall.

A complete overview of all fitting types, including recommendations on when to use which, can be found in the article Types of Fittings – Elbows, Tees, Reducers, Wall Plates.

Brass or plastic? Choosing the fitting material

Besides connection type and fitting shape, the decision about the material a fitting is made from is equally important. On the market you will most often encounter brass and plastic fittings, each with its own advantages and limitations.

Brass fittings are resistant to both temperature and pressure, suitable for both heating and hot-water plumbing systems, stable in the long term, but more expensive and heavier. Thanks to their weight and strength, brass fittings are well suited precisely to locations with higher mechanical stress – for example wall plates under taps or radiator connections, where long-term operation without the need for replacement is expected.

Plastic fittings, for example made of PPSU or POM materials, are lighter and cheaper, resistant to corrosion, but have a lower maximum operating temperature and pressure than brass – so they are more suitable for cold-water systems or lower temperatures. Their advantage is also that, unlike metal, they don't corrode and are not prone to electrochemical reactions with other materials in the system. Before using a plastic fitting on a heating circuit, however, it's always necessary to check the technical data sheet of the specific product – not every plastic fitting is rated for the temperatures and pressures typical of heating systems.

A more detailed comparison of both materials, including specific recommendations by system type, can be found in the article Brass vs. plastic fittings. In general, whenever in doubt, it is safer to choose a brass fitting – its margin of resistance to temperature and pressure is higher, and the risk of it turning out inadequate in the future is lower.

Fitting compatibility with pipe type – what to watch out for before buying

One of the most common mistakes when buying fittings is not a poor choice of type or material, but a wrong assumption about compatibility with the existing pipe. A fitting must match the outer or inner diameter of the specific pipe and the connection method it is designed for.

A fitting designed for a threaded connection on 16 mm PEX-AL-PEX pipe is not automatically compatible with copper or PP-R pipe of the same nominal diameter, since the actual dimensions and connection method differ between materials. So although two fittings may appear to be the same "sixteen" size, in reality they may have a different inner diameter or a different sealing profile that simply won't seal on a different pipe type.

When buying a fitting it is therefore always necessary to check that the material, diameter and joint type match the existing pipe in the system. If you're not sure what type of pipe you have installed at home or on-site, the most reliable approach is to take a sample or precisely measure the outer diameter and material of the original pipe before ordering fittings – this avoids a situation where the fitting arrives but turns out not to fit on-site.

The same caution applies when combining older and newer parts of a system – if you're renovating only part of a house or apartment and connecting to original pipe from a time when different materials and dimensions were used, it's worth checking in advance whether a suitable transition fitting exists, or whether a longer section of pipe will need to be replaced to keep the whole system compatible. We cover this topic in detail in the article Fitting compatibility with different pipe types.

Tools needed to install fittings

The tools you will need for installation depend primarily on whether it is a threaded or a press joint.

Installing threaded fittings requires only a set of suitable wrenches (or an adjustable wrench) and sometimes a pipe cutter, so the end of the pipe is straight and square. A straight, square cut matters more than it might seem for threaded joints – an angled or unevenly cut pipe end may not seat properly in the sealing ring, which can cause a leak even with a properly tightened compression nut. For ordinary home installation of a couple of wall plates or couplings, an adjustable wrench, or two wrenches of the right size (one to hold the fitting, one to tighten the nut), plus a sharp pipe cutter, will be enough.

For press joints, on the other hand, press tools compatible with the given fitting system are essential. Different manufacturers may use a different press jaw profile, and using the wrong press profile can lead to a leaking or damaged joint even though the action was visually performed correctly – the joint may look fine, but the sleeve may not be deformed exactly into the shape that ensures a perfect seal. Before buying or borrowing press pliers, it is therefore essential to check that their jaw profile matches exactly the fittings you plan to use.

Besides the wrenches themselves, it's worth having a few extra supplies on hand when installing fittings – PTFE tape or hemp sealant for threaded joints that require it, a cloth to immediately wipe up any drop of water during a test pressurization, and, for a larger renovation, a suitable container to drain residual water from the original system before taking old joints apart.

Step-by-step installation procedure

Installing a threaded joint consists of five simple steps.

The installation procedure itself differs by joint type, but several common principles apply to both.

With a threaded joint, the first step is preparing the end of the pipe – it is cut straight and square to the pipe axis with a cutter, and the edge is deburred or slightly chamfered as needed, so the seal inside the fitting isn't damaged when inserting the pipe. The compression nut and the sealing or grip ring are then slid onto the pipe in the order they will appear in the finished joint – it's important not to forget this before inserting the pipe into the fitting, since fitting the nut afterward onto already-inserted pipe is often not possible. The pipe is pushed into the body of the fitting all the way to the stop, the nut is brought up to the thread and screwed on by hand, then tightened with a wrench to the specified torque (or, if the manufacturer doesn't state a torque, to firm resistance without excessive force). After tightening, it's recommended to check the joint with a test pressurization of the system and verify visually and by touch that not even the slightest moisture appears at the joint.

With a pressed joint, the pipe is likewise first cut straight and deburred, then inserted into the fitting all the way to the stop – it's important to make sure the pipe has actually reached the stop inside the fitting, not just up to the edge of the sleeve. Press pliers with the correct jaw profile are placed exactly on the sleeve location specified by the manufacturer (often marked on the fitting with color or a line), and a single smooth squeeze creates the permanent press deformation. Unlike a threaded joint, there's no room here for further tightening afterward – if the pressing was done incorrectly, the only solution is to replace both the fitting and the piece of pipe and redo the joint with new material.

With both joint types, once installation is finished it's a good idea to pressurize the whole system before it is concealed in a wall or floor, and leave it under pressure for several hours (ideally overnight), so that any slow leak shows up before the location becomes inaccessible. This simple principle can save significant costs on additional demolition work if a leak were to show up only after concealment.

The most common installation mistakes and how leaks happen

Most fitting leaks are not the result of a faulty product, but of installation inaccuracy. Knowing the most common mistakes is therefore the most effective prevention.

The most common cause of leaks is an insufficiently tightened compression nut on threaded joints, a damaged or missing sealing ring, or pipe inserted at an angle or not fully inserted into the fitting before pressing. All three causes have in common that they can be prevented fairly easily with a thorough check before finishing the joint – a visual check of the presence and condition of the sealing ring, checking that the pipe is truly inserted all the way to the stop, and, for threaded joints, checking the tightening torque.

Another common mistake is combining incompatible materials, for example brass directly with aluminum without proper separation, in a damp environment, which can, over time, cause electrochemical corrosion (bimetallic corrosion) at the joint. This type of corrosion isn't visible right away – it shows up only after months or years of operation, once the material at the point of contact between two different metals, in the presence of moisture, gradually breaks down. Prevention consists either of carefully following the manufacturer's recommendations regarding material combinations, or using suitable insulating inserts or transition fittings designed exactly for this purpose.

Other common mistakes include using an unsuitable type of sealant (for example an old or incorrectly sized seal left over from disassembling the original joint), over-tightening the nut, which can damage the sealing ring just as much as under-tightening, or installing a fitting onto pipe that wasn't cleaned of dirt and metal shavings from cutting before installation. Even a small metal shaving caught on the sealing surface can cause a slow but persistent leak that only shows up after some time in operation.

A complete overview of mistakes and ways to avoid them can be found in the article Common installation mistakes and leaks. If, despite all precautions, a leak does appear, or if a fitting shows signs of wear after years of operation (greenish stains on brass, cracks on plastic), we recommend also reading the article Servicing and replacing fittings, where we cover how to recognize it's time for a replacement and how to go about it.

Real-world examples

Example 1: Family house with a boiler replacement and connection to an older heating system. When replacing an older boiler with a new condensing type, the new heat source had to be connected to the original copper pipe system, which had been installed in the house when it was built. The new boiler had connection ports with different thread dimensions than the original system was sized for, and the pipe route also had to be changed to go around the new, larger boiler, positioned at a different spot on the wall than the original one. The solution was a combination of several fitting types – a reducer between the boiler's larger connection port and the smaller diameter of the original pipe, two elbows to route around the obstacle on the changed path, and finally a threaded joint at the connection point to the old system, so the joint could easily be taken apart in the future if further adjustment were needed there. Thanks to correctly chosen reducers and a thorough seal check at every joint, installation went ahead with no leaks whatsoever, even after several weeks of operation at full heating output.

Example 2: Apartment in a block of flats with a wall plate replacement under a kitchen tap. After years of operation, slight wobbling appeared at the kitchen tap – the tap could be gently moved at the point where it was anchored to the wall, signaling that the original wall plate had lost its firm anchoring. Instead of a complicated intervention into the whole system, the situation was resolved by replacing just the wall plate itself with a new threaded type, which made the repair possible without press tools and without disturbing the rest of the system behind the wall. During the replacement, it was crucial to exactly match the spacing and embedment depth of the original wall plate, so the new tap would sit in place without strain, and to thoroughly tighten the wall plate's mounting mechanism into the wall, since insufficient anchoring was exactly what caused the original problem. After the replacement and reconnection of the tap, the mechanical load from everyday use is transferred evenly into the wall, without unnecessary stress on the joint with the pipe itself.

Both examples show a common principle – correctly choosing the fitting type and size, combined with careful installation, can resolve even a situation that looks complicated at first glance without extensive work on the system. Conversely, underestimating any of these steps, whether it's a wrongly chosen reducer or an insufficiently anchored wall plate, will sooner or later show up as a visible problem.

Summary of the most important answers from the Frequently Asked Questions about Installing Fittings section.

Frequently asked questions about installing fittings

Does a fitting always have to be the same material as the pipe?
Not necessarily; dimensional and joint-type compatibility matter more. Nevertheless, it's advisable to avoid direct contact between incompatible metals (for example brass and aluminum) in a damp environment, where electrochemical corrosion can occur. If you're not sure about the combination of materials in your system, it's better to consult someone or use a fitting from the same manufacturer as the rest of the system.

Can a press joint be taken apart afterward if something needs repairing?
No, a press joint is designed by construction to be permanently non-detachable – the sleeve is permanently deformed once pressed. If the joint may need to be taken apart in the future, choose a threaded fitting type during installation, which is specifically designed for repeated disassembly without damaging the components.

What tools do I need for a simple wall plate replacement under a tap?
For a threaded wall plate, an adjustable wrench, or two wrenches of the right size, and a pipe cutter, if the pipe needs shortening or squaring, are enough. Press tools are not needed for a threaded type.

Why does the joint still leak slightly after tightening the fitting?
The most common cause is a damaged, missing, or incorrectly seated sealing ring, or pipe inserted into the fitting at an angle. Check that the seal is in place and in good condition, and that the pipe is inserted straight and all the way to the stop, or slightly tighten the nut without excessive force.

Can I use a plastic fitting on a heating circuit with higher water temperature too?
Not every plastic fitting is rated for that – plastic fittings generally have a lower maximum operating temperature and pressure than brass ones. Before using one on a heating circuit, always check the technical data sheet of the specific fitting to see whether it matches the temperatures and pressures in your system.

Do I need special press pliers for every brand of fittings?
Different manufacturers may use a different press jaw profile, so universal pliers may not fit all systems. Before buying or borrowing press tools, always check that their jaw profile is compatible with the specific fittings you plan to press.

How do I recognize whether I have copper, PEX-AL-PEX, or PP-R pipe at home?
The materials differ in color, weight, and surface – copper pipe is metallic with a characteristic reddish tint, PEX-AL-PEX is multilayer pipe with an aluminum layer between plastic layers, and PP-R is solid plastic. If in doubt, the most reliable approach is to take a short sample or precisely measure the outer diameter and consult before buying fittings.

Do I need to leave the system pressurized after installing a fitting, even if the joint seems fine?
Yes, we always recommend it, especially before concealing the joint in a wall or floor. A visually flawless joint can have a hidden, slowly developing leak that only shows up after several hours under pressure – checking before concealment saves potential additional demolition work.

What is the difference between a wall plate and an ordinary coupling?
A wall plate is designed for firm mechanical anchoring into the wall and must withstand repeated load from using the connected tap or appliance, while an ordinary coupling only serves to join two sections of pipe with no requirement for mechanical anchoring. That's why wall plates generally have a more robust construction and wall-mounting elements that an ordinary straight coupling doesn't have.

More answers to frequent questions can be found in the separate overview Frequently asked questions about couplings and fittings.

Related topics

• How to choose a coupling or fitting
• Threaded vs. Press Fittings
• Types of Fittings – Elbows, Tees, Reducers, Wall Plates
• Brass vs. plastic fittings
• Fitting compatibility with different pipe types
• Common installation mistakes and leaks
• Tap wall plates – what they are and how they are installed
• Servicing and replacing fittings
• Frequently asked questions about couplings and fittings

Do you 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.

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