Step-by-step installation of a coupling with ferrule on copper pipe

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Installation of a compression fitting on copper pipe step by step

Copper pipe belongs to the proven and long-term reliable solution in central heating systems. However, working with it requires precision – especially when installing compression fittings, where pressed or soldered copper pipe meets a threaded connection to a valve, pump, or other device. From my own experience, I know that compression fittings are the most common place where mistakes occur: insufficient sealing, twisted threads, insufficient tightening of the nut connection, or incorrect measurements. The result is a leaking joint, interrupted heating operation, and unnecessary work to fix the issue.

In this article, we will go through the entire installation process from the first measurement to the pressure test – as detailed as I would explain it to a skilled customer or an installer who is working with copper pipe for the first time. In addition to the actual procedure, I will also cover common mistakes, the selection of the right tools, and situations I have seen dozens of times in real customer projects.

What is a compression fitting and where is it used

A compression fitting (also known as a union fitting, depending on the type) is a fitting that allows you to connect a copper pipe with a threaded end of a valve or another fitting without the need for soldering. It usually consists of three basic parts: the body of the fitting with a thread, a union nut, and a sealing olive (sometimes also a spherical washer or a flat sealing surface, depending on the type).

Typical places of use in practice:

Connecting copper pipe to a circulation pump with a threaded neck
Connecting a ball valve or balancing valve to a copper line
Transition between copper pipe and a threaded elbow, T-piece, or straight coupling
Connecting a copper heating line to a brass manifold
A service point where it is necessary to disconnect the line without destructive intervention

In the category compression fittings, you will find fittings for the most common combinations of copper pipe diameters and threads. The most commonly used are, for example, Compression fitting 1/2" x 15 for standard copper pipe DN15 with a 1/2" external thread, or Compression fitting 3/4" x 22 for larger diameters. Choosing the correct size is the first step we will focus on.

Tools and materials: what you will need before starting the installation

Professional installation is characterized by preparation. Before making any cut in the pipe, prepare the following:

Copper pipe cutter – always prefer a pipe cutter over a saw. The cut will be straight and without burrs.
Reamer – most pipe cutters have an integrated one. It is used to remove the internal edge after cutting.
Adjustable wrench or combination wrench – for common sizes 1/2" to 1", a 24–32 mm wrench is sufficient
Second wrench (spreader wrench) – essential for proper tightening without rotating the body of the fitting
Sealing material – Teflon tape (PTFE) or hemp fiber with paste (Fermit, Loxeal, etc.)
Clean cloth or paper towel
Tape measure and marker
Caliper – for accurate measurement of the external diameter of the pipe in case of doubts

I do not recommend using pipe wrenches on the body of the compression fitting – it risks damaging the surface and distorting the sealing surface. For working with brass or copper fittings, a flat or open-end wrench is always better.

Selecting the correct size of the fitting – the foundation on which everything stands

The most common mistake I see among customers is confusing the external diameter of the copper pipe with the nominal diameter of the thread. These are two completely different things, and mixing them up will result in the fitting simply not fitting. Copper pipe is marked by its external diameter in millimeters (e.g., 15 mm, 22 mm), while the thread is marked in inches according to the BSP standard (e.g., G 1/2", G 3/4", etc.).

Approximate table of the most common combinations:

External diameter of copper pipe	Common BSP thread	Typical application
12 mm	G 3/8"	Thermostatic valves, small fittings
15 mm	G 1/2"	Radiator outlets, circulation pumps
22 mm	G 3/4"	Manifold lines, larger valves
28 mm	G 1"	Main branch, pump groups

If you are unsure about the dimensions, also read another article in our Knowledge Center: Dimensions of compression fittings: how to correctly measure pipe diameter, where you will find a detailed guide on measuring with a caliper and without it. The topic is also addressed in more detail in the article How to choose the right compression fitting for heating.

Step 1: Pipe preparation – cutting and deburring

A properly cut pipe end is the basis for a tight joint. Use a copper pipe cutter only – never an iron saw, even in an emergency. A saw will create an irregular cut with fine metal chips that will get into the system, damage the circulation pump, and cause leaks under the olive.

Cutting procedure:

Measure and mark the cut location with a pencil or marker.
Mount the cutter and tighten the wheel so that the blade just lightly grips the pipe – not by force, only by touch.
Rotate the cutter around the pipe 2–3 times, then slightly tighten the wheel and repeat.
Do not try to make deep cuts in one go – you will create a wavy cut and deform the pipe.
After cutting, deburr the inner edge using an integrated reamer or deburring tool – this step is absolutely mandatory, not optional.

Why is deburring so important? After cutting, a sharp edge forms on the inner edge of the pipe, which can cause turbulent flow, noise, and in the long term, material erosion under full flow. Moreover, if the edge falls into the pipe before installation, it travels through the entire system all the way to the pump.

Step 2: Sliding the nut and olive onto the pipe

This step is simple, but it is precisely here that people most often forget. Before any further handling, first slide the union nut onto the pipe – with the thread facing outward, i.e., toward you. Then slide on the olive. Only after that proceed to mounting onto the fitting body.

Why do I emphasize this? Because in practice it happens – even with experienced plumbers – that they forget to slide on the nut before inserting the pipe into the fitting body. Result: you have to disassemble the entire joint, sometimes even cut the pipe, because the nut cannot go back through the olive.

Order of sliding onto the pipe:

Union nut (thread facing away from the end of the pipe)
Olive (cone facing toward the end of the pipe, toward the fitting body)

In some types of fittings (e.g., with a flat sealing surface and rubber gasket), the olive is not part of the joint. In such cases, the nut plus the rubber gasket directly in the nut or in the body is sufficient. Always read the documentation for the specific product or view a disassembled sample before installation.

Step 3: Sealing the threaded joint – PTFE or hemp

Sealing the threaded part is a separate topic, which is addressed in detail in the article How to seal compression fittings for heating: teflon vs. hemp. Here I summarize what is practically important for installing a compression fitting:

PTFE tape (Teflon tape) is a quick and clean solution suitable for most standard joints in heating. Wind it in the direction of the thread (with the thread, not against it), 3–5 layers for 1/2" threads, 4–6 layers for 3/4" and larger. Stretch the tape slightly so that it fills the thread grooves well.

Hemp fiber with paste (Fermit, Loxeal 55-03 or similar) is a traditional method and, when applied correctly, equally reliable. Wind the fiber in the direction of the thread, in a thin, even layer, and then apply the paste. The advantage is better filling of the thread in case of irregularities and greater resistance to vibrations.

What not to use: silicone sealants, mounting foam or improvised materials. Heating systems operate at temperatures up to 80–90 °C and pressures of 1.5–3 bar, which most other sealants cannot withstand without degradation.

Wind the sealing material only on the thread – that is, on the threaded part of the fitting body or on the external thread of the valve to which you are connecting the fitting. Never wind sealing material on the smooth sealing surface of the olive joint – the sealing there is ensured by the deformation of the olive, not by the sealing material.

Step 4: Inserting the pipe into the fitting body and pre-tightening

Insert the pipe into the fitting body perpendicularly, all the way to the stop – you must feel that it rests on a stop ring inside the body (also called a seat edge). This insertion must be done without force, only with smooth movements – if you have to twist or force it, something is wrong (deformed end, foreign object, incorrect dimensions).

After inserting to the stop, tighten the nut by hand onto the fitting body – turn the nut, not the fitting body. At this stage, tightening by hand plus about 1–1.5 turns with a wrench is sufficient. The goal is for the olive to start seating on the conical surface of the fitting body, but it should not yet be fully compressed – that will happen during the final tightening.

Step 5: Final tightening with a wrench – without prejudice, without excessive force

This is the phase where the quality of the joint is decided. First, clearly determine what you are tightening: the nut (union nut) is turned towards the body of the fitting, while holding the fitting body firmly with the other wrench. Never turn the fitting body itself – you will deform the sealing surface on the threaded end and risk damaging the valve or pump to which the body is connected.

Recommended tightening procedure:

Tighten the nut by hand until it stops.
Add 1 full turn with a wrench (360°).
Check if the pipe can still be moved axially – if yes, continue.
Add another 1/4 to 1/2 turn – this final movement compresses the olive definitively.
Stop. Do not continue with force – a cracked olive or broken nut body is an expensive mistake.

The total tightening angle from the hand-tight position to the final position is usually 1.25 to 1.75 turns (for standard compression fittings on copper pipe 15–22 mm). Larger diameters (28 mm, 35 mm) require a slightly higher torque, but the principle remains the same.

Practical experience: When tightening suddenly becomes significantly stiff and the pipe stops moving, the olive is properly compressed. From this point, add only 1/4 turn and stop. If you continue, you may crack the olive and ruin the entire joint.

Step 6: Sealing the threaded end – connection to the valve

The fitting body connects on the other end via a thread to the valve, manifold, or other device. The same rules apply here as for any threaded joint in heating – PTFE tape or hemp fiber with paste, wound in the direction of the thread, 3–6 layers depending on the size.

For example, when using Compression fitting with union 3/4" x 15, the external thread G 3/4" BSP on the fitting body is sealed and tightened into the internal thread G 3/4" in the valve body. Tightening is done with a wrench – about 3–4 turns from the hand-tight position, while listening for a uniform increase in resistance. If the thread "jumps" or you go around without increasing resistance, something is wrong (damaged thread, incorrect size, dirt in the thread).

For transitions between different systems (e.g., copper to brass, copper to plastic), read the article Compatibility of Hepworth fittings with other pipe systems and materials – there you will also find warnings about galvanic corrosion when combining different metals.

If you need to connect a copper pipe directly to a device with an internal thread (e.g., a boiler with a G 3/4" internal neck), you can use Direct transition 3/4" x 22, internal thread – this is a fitting with a compression connection to a 22 mm copper pipe on one side and an internal thread G 3/4" on the other side. Unlike a union fitting, there is no union (detachable union), but a solid transition. For the differences between these types, read more in the article Difference between a union fitting and a direct transition.

Leak test and first heating start-up

After installation, a leak test is carried out before the first start-up. The procedure I recommend:

Visual inspection – check that all nuts are tightened, no olive is protruding or misaligned, and the sealing material is not sticking out of the thread.
Filling the system with water – slowly open the filling valve and let the system fill. Monitor each joint.
Pressure test – for a standard installation, a working pressure of 1.5 bar is sufficient. For a new system, I recommend a test at 1.5× the working pressure (e.g., 2.5 bar) for 30 minutes. This pressure is usually set with an expansion tank or a manual pump.
Inspection after 30 minutes – check each joint with a dry finger or a piece of paper. Any moisture indicates a leak.
First start-up with hot water – after heating the system (first heating to operating temperature), check the joints again, as materials have expanded and slight re-tightening may be necessary.

If a leak appears, read the article Why a union fitting is leaking and how to fix it – there you will find diagnostics and a repair procedure without the need for complete disassembly in many cases.

Common mistakes from practice and how to avoid them

Over the years of working on custom installations, I have seen the same mistakes repeated over and over. Here are the most common ones, listed according to the seriousness of their consequences:

1. Forgotten union nut before installation – a classic. Solution: always plan the order of installation before cutting, and physically slide the nut (and olive) onto the pipe before the actual cut, if you know where the joint will be.

2. Not deburring the cut – two problems: debris in the system and leakage under the olive. Solution: deburring is a mandatory step, not a shortcut.

3. Over-tightening – cracked olive, deformed nut or broken valve body. Solution: follow the rule "hand + 1.5 turns", not instinct.

4. Sealing in the wrong place – PTFE tape on the olive surface (where it has no function) or, conversely, missing on the threaded joint. Solution: thread = PTFE or hemp; olive joint = only olive.

5. Using a damaged olive – olives are single-use. If you disassemble an old joint, the olive is deformed and must not be reused. Solution: always have spare olives of the appropriate size available.

6. Swapping external and internal threads – a union with compression fitting has an external thread on one end and a compression connection on the other. If you need the opposite thread orientation, look at the Straight adapter 1/2" x 15, internal thread – it has an internal thread on the threaded side instead. A more detailed explanation can be found in the article Internal vs. external thread: which type of adapter do you need.

7. Twisted pipe during tightening – if you do not hold the body of the union with a wrench, you are rotating the entire valve or pump, which can crack the casting or damage the pump bearing.

Repeated assembly and disassembly: when and how

Unions with compression fittings are designed to be removable – this is their main advantage over soldered connections. However, there is a limit: each olive can withstand a maximum of 1–2 compression cycles. After the third and subsequent assembly cycles, the olive loses its correct shape and the connection no longer seals properly.

If you expect to disassemble the connection in the future (e.g., for a pump that you service regularly), consider using ball valves with union fittings instead of a standard compression olive connection – these use a rubber O-ring seal instead of an olive, and can withstand many more cycles.

During annual heating system maintenance, always visually inspect all unions – possible micro-leaks will appear as a white lime coating or corrosion on the surface of the nut. Further tips on preventive inspection can be found in the article Inspection and tightening of unions during annual heating service.

Special situations: tight spaces, hidden installations and vertical positioning

In practice, you do not always have ideal conditions. Here are solutions for the most common complicated scenarios:

Tight space with no access for wrenches: Use a combination wrench with short arms or a special pipe wrench with articulated movement. If there is still not enough space, consider rearranging the pipe route or using crimped fittings with push-fit technology, where a wrench is not needed.

Vertical pipe: The nut has a tendency to slip during installation. A solution is masking tape – temporarily tape the olive to the pipe in the correct position until you start tightening.

Hidden installation (behind drywall, in a drop ceiling): Compression joints in hidden spaces are technically acceptable, but generally not recommended without an inspection opening. In the case of any leak, the damage to the building structure is significantly greater than the cost of an inspection hatch.

Old installation with oxidized pipe: If the surface of the copper pipe is greenish in color (patina), do not sand it with sandpaper before installation – just wipe it with a damp cloth and let it dry. Surface oxidation of copper does not affect the seal of the olive connection. Sanding, on the other hand, can damage the roundness of the cross-section.

Most frequently asked questions (FAQ)

Can I use a union with compression fitting on plastic or aluminum pipe?

Standard compression unions designed for copper pipe (Hepworth system) are not directly intended for plastic or aluminum pipes, because the olive calibrated for the hardness of copper would crack or not grip the soft plastic properly. For plastic pipes (PEX, MLCP), there are special compression or crimped fittings available. For multilayer pipes with an aluminum layer, compression fittings with an insert are intended. More on this is covered in the article Compatibility of Hepworth fittings with other pipe systems and materials.

How many layers of PTFE tape are appropriate for a G 1/2" thread?

We recommend 3–5 layers of PTFE tape wound in the direction of the thread (the same direction as you tighten the thread). Less than 3 layers is insufficient, while more than 7 layers will make tightening difficult and may cause undesirable stress on the valve body. Wind the tape with slight tension, not loosely.

Can I reuse the olive after disassembling the connection?

The general recommendation from manufacturers is: no. The olive is permanently deformed during the first tightening and does not guarantee a perfect seal when reused. In practice, the olive is sometimes reused in emergency repairs, but always with the risk of a leak. The correct procedure is to replace the olive with every disassembly. Olives are inexpensive spare parts, so you can avoid problems this way.

How can I tell if I have over-tightened the olive?

An over-tightened olive can be recognized either directly (audible crack during tightening, visible deformation or crack on the olive after disassembly) or indirectly – the pipe cannot be pulled out even when the nut is completely unscrewed, because the olive is so compressed that it holds the pipe directly. In such cases, it is usually necessary to cut the pipe and use a new olive and union body.

Is there a difference between a union with compression fitting and a straight adapter with a thread?

Yes, and it is a significant difference. A union with compression fitting has a union nut – the connection is removable without tools on the pipe side (you only need to unscrew the nut). A straight adapter with a thread (e.g., Straight adapter 3/4" x 22, internal thread) is a fixed fitting without a union – once installed, the adapter cannot be disconnected from the device without unscrewing the entire body of the fitting from the thread. For devices that you will service or replace, a union with compression fitting is always better.

What to do if the union starts to leak slightly after a few days?

A small leak that appears after the system is heated is quite common – materials expand and the PTFE seal settles. First, try tightening the nut by 1/8 to 1/4 of a turn (of course, the system must be depressurized and cooled). If that does not help, disassembly, replacement of the seal and reinstallation are necessary. Never tighten the connection under pressure or while it is hot – you risk injury and greater damage. The full diagnosis can be found in the article Why is the union leaking and how to fix it.

Conclusion: Precision and patience always pay off

Installing a union on copper pipe is not a complicated task – but it requires discipline at every step. Correctly cut and deburred pipe, properly wound sealing tape, the olive in the correct position and tightening without excessive force. These four principles cover 95% of all situations you will encounter in practice.

Always verify the dimensions before installation – the outer diameter of the pipe and the size of the thread. Unnecessary trips caused by confusing 1/2" and 3/4" are still common in practice. If you are working with standard dimensions in heating, for a 15 mm copper pipe, choose the Union with compression fitting 1/2" x 15, for a 22 mm pipe, choose the Union with compression fitting 3/4" x 22. These are the dimensions that cover the vast majority of home installations.

And remember: unions with compression fittings are designed for decades of operation – but only if they are installed correctly from the very first day.

Do you have a question about this topic?

Not sure what to choose or dealing with a specific situation in your home? Write to us – we are happy to help.