How to Properly Join Copper Pipes with Compression Fittings Without Water Leaks

We are still preparing products.

But you can look at other categories.

How to Properly Join Copper Pipes with Compression Fittings Without Water Leaks

Compression fittings are among the most commonly used types of joints on copper piping in heating and water distribution systems. This is no coincidence – they don't require soldering, don't require an open flame, and when installed correctly, provide a long-lasting, reliable, and hermetic joint. Nevertheless, this is precisely the type of joint where a great many mistakes occur in practice. Most leaks after installing a compression fitting don't arise because the joint is structurally poor – they arise because something was done incorrectly, a step was skipped, or an unsuitable part was used.

In this article, we'll take a thorough look at the whole topic: from understanding how a compression fitting works, through proper pipe preparation, a step-by-step installation procedure, typical mistakes and how to prevent them, to checking for leaks and fixing them if they occur despite your best efforts. The article is written for both plumbers and skilled DIY enthusiasts who want to get this joint really right – not just "good enough to hold for a while."

How a compression fitting works: why does it seal?

Before we get into practice, it's worth understanding what physically happens in a compression fitting. Only someone who understands the principle can correctly diagnose the problem when something goes wrong.

A compression fitting (German: Klemmverschraubung, English: compression fitting) works on the principle of radial compression of an olive (ferrule) around the pipe. The olive is a small metal ring, typically made of copper or brass, which is placed on the pipe before the compression nut. When you start tightening the nut onto the fitting body, the olive is pushed axially against the conical seat surface – and this axial force is converted into radial compression. The olive is physically plastically deformed and pressed into the outer surface of the pipe. This creates a metal-sealed joint without any sealing material – it seals metal to metal.

This is a key point: a compression fitting does not seal via a rubber gasket (unlike, for example, threaded joints with hemp/tow). It seals exclusively through deformation of the olive. All the conditions for correct installation follow from this – the pipe must be round, the surface clean, the olive correctly oriented, the cut perpendicular.

Types of compression fittings for copper pipe: what's available on the market

Not every compression fitting is the same. Two design variants are primarily used for copper pipe, and it's important to know how they differ.

Standard compression fitting for copper pipe (type EK)

This is the classic solution intended directly for hard and semi-hard copper pipe. The olive is sized for the wall thickness of the copper and the hardness of the material – it deforms in a controlled manner and reliably bites into the surface of the pipe. A typical practical example is the compression fitting for copper pipe 15×1 EK, where the number after the "x" indicates the pipe wall thickness. This fitting is designed specifically for copper, which guarantees the correct geometry of the olive and seat.

The designation "EK" comes from the German term "Einsteckverbinder Klemmverschraubung" or from the standardized labeling system. In practice, this means that fittings with this designation are certified for use in pressurized installations and their dimensions are standardized.

Adapter compression fitting for copper and PB pipe

The second type is adapter fittings, which allow you to connect copper pipe to another type of pipe – for example, polybutylene (PB) pipe or a connection on a heating manifold. An example is the 15×1 EK compression fitting for copper or PB pipe (Hepworth). These fittings are useful in renovations where older copper piping meets newer sections made of plastic or multilayer pipe.

In heating systems today, we increasingly encounter a combination of materials – copper near the boiler and main distribution, multilayer pipe in underfloor circuits or wall chases. If you're interested in where multilayer pipe is more suitable than copper and vice versa, I recommend reading the article Multilayer vs. Copper Pipe: Comparing Properties, Price, and Lifespan in our Knowledge Center.

Diameters and dimensions – what the numbers on the fitting mean

Compression fittings are sold in sizes corresponding to the outer diameter of the pipe. For copper pipe in heating, the most common diameters are 15 mm, 18 mm, and 22 mm. The number after the slash (e.g., ×1 or ×1.5) indicates the pipe wall thickness for which the olive is sized. This number is not merely formal – an olive for a 1 mm wall will deform differently than an olive for a 1.5 mm wall, and using the wrong olive can lead to leaks or even damage to the pipe.

If you're not sure what pipe diameter you need for your system, read our article What Pipe Diameter Do I Need for My Heating System? – there you'll find calculations of outputs, flow rates, and recommended diameters for typical family homes.

What you'll need before starting installation

Proper preparation is half the success. Before you start cutting and screwing, get everything you need ready at your workstation.

Tools

Pipe cutter – the ideal choice for copper pipe. It gives a clean, perpendicular cut without burrs. Never use an angle grinder or hacksaw unless you're able to perfectly remove all burrs and ensure a perpendicular cut.
Deburring tool (internal reamer) – a small tool built into most cutters or sold separately. It removes the internal burr (rolled edge) left after cutting.
Fixed open-end wrench or adjustable wrench – at least two, so you can hold the fitting while tightening the nut.
Calibrators / calibrating mandrel – for checking the roundness of the pipe before installation, especially if the pipe came wound on a coil.
Soft cloth or paper towels – for wiping dust and grease off the pipe.

Materials

A compression fitting of the correct diameter and type for copper pipe
Copper pipe of the correct outer diameter and wall thickness
Optional: thread sealant (not for the olive! – only for threads, if it's a combination fitting with a threaded end)

Pipe preparation: the most important step that's most often underestimated

From my experience with dozens of jobs, one rule holds true: more than three-quarters of leaks after installing a compression fitting arise from an error in pipe preparation, not from the actual tightening. So pay full attention to this part.

Step 1: The correct cut – perpendicular and clean

The pipe must be cut exactly perpendicular to its axis. A cut deviation of more than 1–2 degrees from perpendicular is, in practice, enough for the olive to not seat evenly around the whole circumference, causing a leak. A pipe cutter is irreplaceable here – its wheels automatically guide the cut perpendicularly. If you must use a saw for some reason, invest time in correcting the cut with a file and a protractor.

After cutting, always check the edge: it must not be split, and must have no metal shavings. Any shaving that gets into the system can later cause a valve or pump failure.

Step 2: Remove the internal burr

When cutting, the cutter typically rolls the inner edge of the pipe slightly inward. This burr (rolled edge) must be removed with a deburring tool or a fine file. If you don't remove it, the burr will increase hydraulic resistance in the joint, but more importantly – it can corrode over time due to flow and a broken-off piece can get into the system.

Step 3: Remove the outer burr and check the outer surface

Also remove the outer burr (sharp edge) with a file. The reason is different: a sharp edge can scratch the olive as it's inserted, or seat it at an angle. There must be no scratches, corrosion, or significant grooves on the outer surface of the pipe where the olive will sit. The olive presses exactly into whatever state the surface is in – including scratches through which water can leak.

Step 4: Checking roundness

Copper pipe sold in straight lengths is usually sufficiently rigid and round. But if you're working with semi-hard or soft copper pipe that was wound on a coil, it may be slightly oval. The olive assumes a perfectly round cross-section – on an oval cross-section it won't create an evenly tight seal around the whole circumference. A calibrator restores the diameter back to a round shape. If you don't have a calibrator, you can carefully round out the pipe with a mandrel.

Installing a compression fitting: step-by-step procedure

Once the pipe is properly prepared, the actual installation is fairly straightforward – but there are details that matter here too.

Step A: Correctly sliding on the nut and olive

Before inserting the pipe into the fitting, you must first slide the nut onto the pipe (threaded part facing away from the cut end, i.e., away from the fitting) and then the olive. You must remember this order – many beginners forget it until the joint is finished, and then have to disassemble the whole job.

The olive has a slight conical shape. The correct orientation of the olive is with the wider end facing toward the fitting's cone, i.e., facing into the fitting, and the narrower end facing the nut. Incorrect olive orientation is the second most common cause of leaks – if you turn the olive around, it won't seat correctly in the seat and the sealing effect will be minimal.

Practical tip: some olives have a small manufacturing notch on one side, or are slightly shaped – this notch always faces toward the fitting body.

Step B: Inserting the pipe into the fitting

Insert the pipe into the fitting body all the way – it must reach the mechanical stop (abutment surface) inside the fitting. If the pipe doesn't reach the stop, the olive won't be in the correct position and the sealed joint won't have sufficient support. Push gently but firmly. With properly prepared pipe (no burrs, no deep scratches), the pipe will easily go in all the way to the stop.

Step C: Hand-tightening the nut

Start tightening the nut by hand – without a wrench. Tightening should be easy and smooth. If the nut meets resistance or catches, stop and check whether the threads are damaged or the olive is improperly seated. Never use a wrench to force a jammed nut – you risk stripping the threads or deforming the fitting.

Step D: Tightening with a wrench – the most important moment

This is where most people make a mistake – and ironically, in both directions. Either they tighten too little (the joint leaks immediately) or too much (the joint leaks later, because the olive is over-deformed and cracked).

Correct procedure for standard copper pipe: the nut is hand-tightened to the stop, and then you make 1¼ turns with a wrench (i.e., turn by 450°). This is the value recommended by most European manufacturers of compression fittings for copper pipe with a diameter of 15–22 mm and a wall thickness of 1–1.5 mm. Some manufacturers give slightly different values – always follow the technical documentation of the specific fitting you're installing.

While tightening, hold the fitting body with a second wrench. Without a backup wrench, you transfer torque to the entire pipework or other fittings, which can loosen other joints or mechanically damage the pipe.

Step E: Checking after tightening

After tightening, check visually: the olive should be evenly bitten into the pipe around its entire circumference, and the nut should be parallel to the pipe surface with no visible asymmetry. If the olive "wobbles" or is visibly tilted, redo the joint with a new olive.

Temperatures, pressures, and conditions of use: what to watch for in heating

Compression fittings for copper pipe are designed for specific operating conditions. In heating this is key information, because heating systems operate at temperatures and pressures that are considerably higher than in cold water.

A standard brass compression fitting for copper pipe is usually certified for temperatures up to +110°C and pressure up to 10 bar (PN10) for heating installations. This is sufficient for all common boiler systems – condensing boilers typically operate at 70/50°C (flow/return) and a system pressure of 1.5–2.5 bar. If you're interested in the differences between PN10 at 70°C and at 95°C, this is covered in detail in the article Pipe Temperature and Pressure: What Do PN10, T=70°C and T=95°C Mean in Practice?

Important note: a compression fitting is not suitable for embedding in a wall or under a floor (walling in, casting into screed) unless explicitly certified for this. A standard compression fitting is designed for visible or accessible installations. For underfloor distribution, multilayer pipe without joints along the run is typically used (fittings only in inspection shafts), or special connectors with long-term certification for embedding. For interest, multilayer pipe such as Ivar Turatec 16×2, Ivar Turatec 18×2, or Ivar Turatec 20×2 is laid in joint-free sections in underfloor circuits precisely to avoid joints in the screed altogether.

Typical mistakes and why leaks occur

Over years of practice, I've seen leaking compression fittings in various forms. Here are the most common ones:

1. Pipe cut at an angle

The olive cannot compensate for an angled cut – it seats unevenly, compressed more on one side and less on the other. Result: a leak on the side with lower contact pressure. Solution: always use a pipe cutter, not a saw. If you use a saw, correct the cut with a file and protractor.

2. Incorrect olive orientation

As described above – an olive turned around won't seat correctly in the conical seat. The sealing effect is minimal or nonexistent.

3. Pipe not fully inserted to the stop

The pipe must reach the mechanical stop inside the fitting. If not, the olive is not in the correct axial position and deforms incorrectly during tightening. A common cause: an internal burr (rolled edge) that prevents full insertion to the stop. Solution: thoroughly remove the burr.

4. Insufficient or excessive tightening

Insufficient tightening – the olive isn't pressed sufficiently into the pipe surface, and the joint seals only by friction, not plastic deformation. A small pressure surge or thermal expansion is enough for the joint to start leaking.

Excessive tightening – the olive is over-deformed. In the worst case, a microcrack in the olive can cause a leak after several months of operation, once corrosion or material fatigue widens the crack. Always follow the 1¼ turn value with a wrench.

5. Damaged or dirty pipe surface at the olive location

Scratches, corrosion pits, limescale, or grease on the pipe surface prevent the olive from creating a continuous tight seal. Solution: gentle sanding with fine sandpaper and cleaning with a cloth.

6. Repeated disassembly and reassembly

A compression fitting is not designed for repeated disassembly. After the first tightening, the olive is plastically deformed – on retightening (after disassembly) it deforms further, but not correctly, and the sealing result is unpredictable. As a general rule: after disassembling a compression fitting, replace the olive (or the entire fitting) with a new one.

How to fix a leaking compression fitting

If you notice a leak at a compression fitting after filling the system, proceed as follows:

Do not start the heating until you've fixed the leak – hot water under pressure can rapidly worsen a leak.
Depressurize the system and drain the water from the relevant section.
Disassemble the joint, and visually inspect both the olive and the pipe surface.
If the olive shows signs of incorrect deformation (asymmetric, cracked, sitting at an angle), replace the olive or the entire fitting with a new one.
Check the pipe cut – if it's not perpendicular, cut off a further piece and make a new cut.
Repeat the entire installation procedure from the beginning.

Never seal a leaking compression fitting with silicone, sealing compound, or tape from the outside. Such a solution is temporary, gives a false result in a pressure test, and hides the real problem – the joint will loosen again over time, this time with greater damage to the surroundings.

Compression fitting vs. soldering: when to choose which method?

A compression fitting isn't always the best choice – even when working with copper pipe. It's important to know in which situations it's appropriate and when a soldered joint is a better choice.

A compression fitting is suitable:

For visible and accessible pipework where subsequent inspection and possible repair is possible.
For installations where the system will be regularly disassembled or expanded (e.g., a boiler with detachable circuits).
For installation without special tools and without an open flame – useful, for example, during renovations near flammable materials.
For joints between copper and another type of pipe (copper–plastic, copper–multilayer).
For places where it's unknown whether disassembly will be needed, or for a temporary connection.

Soldering is more suitable:

For long continuous runs of copper pipe in a boiler room or utility room, where the goal is a permanent, compact joint.
For places with limited space for a wrench (soldering isn't limited by the working space needed for a wrench).
When the run is long and economics favor soldering (a soldered joint is cheaper than a fitting).
For permanent installations where you never plan to disassemble the system.

In practice, these two methods are combined: long continuous sections are soldered, and compression fittings are used where connection to fittings, valves, the boiler, or a different type of pipe is needed.

Checking tightness after installation: pressure test

After completing the installation of all joints, a pressure test is mandatory before covering or backfilling any pipework. For heating, a pressure test is standardly performed with cold water at 1.5 times the operating pressure – typically at 3–4 bar, for a minimum of 30 minutes (according to STN EN 14336 and related standards).

Pressure test procedure for a residential system:

Fill the system with water and vent all air vent valves – there must be no air in the system.
Pump up a pressure tester (pressure pump) to the required pressure.
Monitor the pressure gauge for at least 30 minutes – if the pressure doesn't drop, the system is tight.
Visually check every joint – for moisture, drips, or possibly white deposits (in which case a micro-leak may have occurred and dried up in the past).

If the pressure dropped during the test but no joint visually leaks, the cause could be air absorption into the pipe or a micro-leak that will only show up later. In this case, it's advisable to extend the test and check every joint again by touching it with a paper towel – a damp towel will reveal even a minimal leak.

Differences when working with multilayer pipe instead of copper

You might be wondering – what if I use a compression fitting on multilayer pipe instead of copper? This is a situation we're encountering more and more, especially when connecting an underfloor circuit made of multilayer pipe to a copper run at the boiler. Different rules apply in this case.

Multilayer pipe – such as the aforementioned Ivar Turatec in sizes 16×2, 18×2, or 20×2 – has an aluminum layer in the middle, which gives it rigidity and prevents oxygen diffusion, but its material properties are still different from copper. For a compression fitting on multilayer pipe, it's essential to use a support sleeve (insert), which is inserted into the pipe and prevents it from collapsing when the olive is tightened. Without the insert, the olive will deform the pipe and the joint will never be reliably tight. This is a fundamental difference compared to copper pipe, where the pipe is inherently rigid enough and an insert is not needed.

If you're looking for a comprehensive guide for multilayer pipe, we recommend the article Installing Multilayer Pipe Step by Step: Tools, Fittings, and Compression Fittings – it describes the entire procedure, including the insert and calibration, in detail.

Frequently Asked Questions (FAQ)

Can I tighten a compression fitting more if it leaks a little after the first fill?

Yes, if you haven't yet exceeded the maximum recommended tightening angle – that is, if you tightened less than 1¼ turns. In that case, depressurize the system, tighten the nut by an additional ¼ turn, and check again. However, if you've already reached or exceeded 1¼ turns and the joint is still leaking, further tightening won't help – the problem lies in pipe preparation or olive orientation, and the joint needs to be disassembled and redone. Never force-tighten a persistently leaking joint.

Do I need to lubricate or treat the olive before installation?

No, not as standard. Most manufacturers supply olives that don't need lubrication – the metal-to-metal joint works correctly even without lubricant. Exceptions are specific fittings for certain types of plastic pipe, where the manufacturer specifies the use of silicone paste. For copper pipe, lubricating the olive is unnecessary and can even reduce the friction forces that keep the olive in the correct position before tightening.

How many times can I disassemble and reassemble a compression fitting?

Theoretically, only once reliably – the olive is plastically deformed after the first tightening, and reusing it doesn't guarantee an equally tight joint. In practice, it happens that plumbers disassemble and reassemble a fitting for minor adjustments, and the joint still holds – but it's a risk. The standard recommendation and good practice: replace the olive with a new one every time you disassemble the joint. The cost of an olive is negligible compared to the possible leak and damage to the building.

What type of wrench should I use – adjustable or fixed?

A fixed wrench of the correct size for the given nut is ideal – it provides more precise, controlled torque and won't slip. An adjustable wrench is an acceptable substitute, but you must set it exactly to the width of the nut and verify that it holds firmly. A slipping wrench can damage the nut or injure your hand. Never use flat-jawed pliers (combination pliers) – they bite into the edges of the nut and damage it. A damaged nut is more prone to stress corrosion cracking and later leaking.

Can I use PTFE tape or sealing paste on the olive or the nut thread?

Definitely not on the olive – the olive's seal must be purely metal-to-metal, and any material between the olive and the pipe surface will worsen the seal (the tape will be squeezed out or deform differently than metal during tightening). Lubricant or PTFE is also not needed or recommended on the threads between the nut and the fitting body – the threads of a compression fitting are designed to carry axial force without thread sealant. The exception is a combination fitting, where there's an external G thread on the other side (at the connection to the fixture) – there, thread sealing cord or PTFE tape is commonly used, but that concerns the threaded joint, not the compression joint.

How do I know if a compression fitting is suitable for the temperature in my system?

On every certified compression fitting or in its technical documentation, you'll find the maximum continuous temperature and maximum pressure (PN). For standard brass fittings and copper pipe in heating, a common certification is PN10 at temperatures up to 110°C, which covers all common boiler systems (even older ones with a flow temperature of 80–90°C). However, if your system has locations with temperatures above 110°C (for example, near a steam generator or industrial equipment), you must use special fittings with a higher certification. For family homes with a condensing boiler, this is practically never an issue.

Conclusion: a reliable joint is not a matter of chance – it's the result of a precise procedure

A compression fitting is an elegant and safe way to join copper pipes – if done correctly. The dozens of leaks I've seen in practice almost always had the same roots: rushed pipe preparation, overlooked olive orientation, the wrong type of fitting, or "I'll just tighten it a bit more, just to be sure." The correct procedure isn't complicated – but it requires patience, the right tools, and a willingness to check every step before moving on.

If you're not sure which compression fitting you need for your pipe diameter and type, take a look at the specific products in the heating pipe category – you'll find fittings for various diameters and materials there, including the directly sized compression fitting for copper

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we'll be happy to help.