Step-by-step installation of a direct reducing coupling

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Step-by-step installation of a straight reducing coupling

A straight reducing coupling is one of the seemingly simple pipe fittings – yet surprisingly many errors occur during its installation in practice, which later cause leaks, corrosion, and in the worst case, damage to the entire piping system. This article will guide you through the entire installation process from preparing tools and materials to the final pressure test, with a focus on specific values, practical experience, and common scenarios from heating system jobs.

If you are still considering which size of reducing coupling you need, read the topic Sizes of straight reducing couplings: which size do I need? or How to choose the right straight reducing coupling for heating first – these texts will help you get oriented before you reach for the tools.

What is a straight reducing coupling and where is it used

A straight reducing coupling is a pipe fitting that connects two pipes of different diameters on the same axis – without changing direction. It has a larger opening on one side (e.g. 22 mm) and a smaller opening on the other side (e.g. 15 mm). It is used wherever the pipe diameter changes: when branching off from the main line to a radiator circuit, when feeding underfloor heating from a manifold, when connecting a boiler branch to a distribution pipe, or simply when repairing an existing system, where a damaged section of pipe of a different diameter needs to be replaced.

In the Hepworth range, which you can find at atria.sk in the category of straight reducing couplings, these fittings are made of copper or hard-drawn brass with standardized dimensions according to the EN 1254 standard. This means they are compatible with all standard copper pipes EN 1057 and their dimensions are accurate to tenths of a millimeter – which is crucial for soldered or crimped joints.

Tools and materials needed before starting the installation

A good installation starts with good equipment. From experience, I know that unsuccessful joints are caused by two reasons: poor surface preparation and inattention when setting up the tool. So before starting the work, prepare the following:

Tools

Pipe cutter – rotating, for copper, diameter according to the pipes being cut (15 mm or 22 mm). Cutting with a hacksaw results in a worse and often poor-quality end.
Deburring tool (reamer) – integrated in the pipe cutter or separate. Do not skip this step.
Emery cloth or copper joint cleaner – grit P120 to P180 for the outer surface of the pipe.
Brush for cleaning the inside of the coupling – small wire brush (size 15 or 22 mm according to the opening).
Soldering lamp (for hard or soft soldering) or crimping pliers for crimped joints.
Flux – essential for soldered joints.
Solder – lead-free Sn97Cu3 for potable water and heating, or Ag-solder for higher temperatures.
Damp cloth for wiping off excess flux immediately after soldering.
Tape measure / sliding caliper – to check the depth of pipe insertion into the coupling.
Marker or scribe – to mark the insertion depth.
Protective gloves and goggles – thermal protection when working with the lamp.

Materials

Correctly sized straight reducing coupling – e.g. straight reducing coupling 22 x 15 for the transition from the main line to a radiator branch, or straight reducing coupling 28 x 22 for feeding from a boiler connection.
Copper pipe of the appropriate diameter – EN 1057, hard or semi-hard.
Cleaning agent for degreasing the surface (isopropyl alcohol or special cleaner).

Step 1 – Cutting the pipe to the correct length

The first step may seem trivial, but in practice we see a huge number of problems exactly here. Always cut the pipe with a rotating pipe cutter, never with a hacksaw. The reason is simple: a hacksaw creates an uneven cut with notches, splinters and deformation of the pipe end, which prevents proper insertion into the fitting and the creation of a watertight joint.

Procedure: Mark the length of the pipe with a marker. Place the cutter perpendicular to the pipe, tighten the screw slightly so that the wheel just touches the surface – not cutting yet. Rotate the tool around the pipe (not the pipe in the tool!) and after each rotation slightly tighten the screw. Never tighten too much – you might flatten the pipe. Repeat the process until the cut goes through the entire diameter.

Result: The cut must be perpendicular to the axis of the pipe. The deviation from a right angle should not exceed 1°. If the cut is not perpendicular, the fitting will not insert correctly to the depth and the soldered area will be asymmetric – which leads to leaks under operating pressure.

Step 2 – Deburring and cleaning the surface

This is a step that amateurs most often skip. And it is precisely here that most complaints arise. After cutting the pipe with a rotating pipe cutter, a sharp internal burr (which we deal with using a reamer) appears on the inner edge and a small edge on the outer edge. Both must be removed.

Reamer (internal deburring)

Insert the reamer into the cut pipe opening and turn it a few times. The goal is to remove the sharp inner edge and slightly bevel the edge. Be careful not to remove too much material – a bevel of 1–2 mm at an angle of about 30° is sufficient. Metal chips that arise during this process must be blown out of the pipe – they must not remain inside and later get into the boiler or pump.

Cleaning the outer surface of the pipe

Use sandpaper or a special cleaning pad to abrade the outer surface of the pipe at the joint location (this means the length of insertion into the fitting plus an additional 5 mm). The copper pipe EN 1057 has a thin layer of copper oxide and other impurities from production and storage on the surface. Solder and flux do not properly bond to such a surface. After abrading, the surface must shine with a bright copper color – not dark brown or black.

Cleaning the inside of the fitting

It is equally important to clean the inside of the fitting with a wire brush of the appropriate diameter. Rotate the brush inside the fitting's neck 3–5 times in each direction. Then shake or blow out the fitting with compressed air. The rule is simple: if you touch any part with your fingers after cleaning, it is necessary to clean it again – fingerprints leave grease that impairs soldering.

Step 3 – Applying the flux

Flux is a chemical compound that, when heated, removes oxides from the copper surface and allows the solder to flow smoothly into the capillary gap between the pipe and the fitting. Without flux, soldering does not work – the solder balls on the surface and does not penetrate the joint.

Apply a thin and even layer of flux with a brush or spatula. Apply it to the outer surface of the pipe at the insertion point into the fitting, as well as to the inner surface of the fitting's neck. Excessive amounts of flux are harmful – when heated, the flux can run along the pipe and have a corrosive effect in the long term. The golden rule applies: a thin, even layer, not a smear.

Important warning: flux is not a sealing material. Flux does not replace solder or any other sealing material. Its role is preparatory. More on this topic can also be found in the article Sealing of fittings: common problems and how to avoid them, where the relationship between flux, capillary gap and the sealing function of the solder is explained in detail.

Step 4 – Inserting the fitting onto the pipe and checking the depth

Before inserting the fitting, measure the insertion depth – that is, how deep the pipe must enter the fitting's neck. Mark this depth on the pipe with a marker (a ring around the circumference). For standard diameters, the following approximate values apply:

Ø 15 mm – insertion depth approx. 13–14 mm
Ø 22 mm – insertion depth approx. 16–17 mm
Ø 28 mm – insertion depth approx. 18–19 mm

These values are for soft soldering with Sn97Cu3 solder and correspond to standard copper fittings Hepworth. Always verify the actual depth of the given piece with a measuring tool before installation – manufacturers can differ by a millimeter or two.

Insert the pipe into the fitting smoothly until it reaches the stop. Check that the mark on the pipe aligns with the edge of the fitting. If the pipe does not go in easily, do not hammer it with a hammer. Either you did not clean it sufficiently, or the pipe is deformed. Adjust the fitting to the correct position (the axes must align), if it is a situation where you will later fix the pipes to the wall.

Step 5 – Soldering: heating and applying the solder

This is the heart of the entire process and also the most common source of problems for inexperienced installers. Several essential rules apply:

Correct temperature and even heating

When soft soldering with copper solder Sn97Cu3, the joint is heated to a temperature of approximately 220–250 °C. Heat the body of the fitting with a torch, not the solder wire itself. Direct the flame alternately to both sides of the joint to ensure even heating of the entire neck. The correct temperature is reached when the flux begins to turn dark brown (before that it bubbles and foams – this is normal) and when the applied solder begins to melt on its own without direct contact with the flame.

Common mistakes: heating the solder itself (not the joint), too strong a flame for a small joint, heating only one side. The result is either a cold joint (cold joint) with insufficient solder penetration, or overheating with surface oxidation.

Applying the solder

Apply the solder to the edge of the joint – where the pipe enters the fitting's neck. The capillary effect itself draws the molten solder into the gap between the pipe and the fitting. The capillary gap for soft soldering of copper pipes should be 0.05–0.2 mm – which is exactly what occurs with a properly cut and inserted pipe in a standard EN 1254 fitting.

The solder should fill the entire circumference of the joint and slightly flow out – this is a sign of a well-executed joint. If the solder runs down the pipe or balls up into beads, the joint is too cold or the surface is not clean.

Estimated solder consumption for pipes of standard diameters:

Ø 15 mm: approx. 1.5–2 g per socket
Ø 22 mm: approx. 2.5–3.5 g per socket
Ø 28 mm: approx. 4–5 g per socket

Special note on soldering a direct reducing coupling

With a reducing coupling, you have two sockets of different diameters, and the larger socket requires more heat. I recommend soldering the smaller socket first (less heat, shorter time), letting it cool slightly, and then soldering the larger socket. Reason: if you heat the large side too aggressively, the heat can transfer to the small side and overheat the joint or degrade the flux before you have a chance to fill it with solder.

Step 6 – Cleaning the joint after soldering

Right after soldering is completed (while the joint is still warm, but the solder is already solid – about 30–60 seconds after finishing), wipe the entire joint with a damp cloth. This removes any remaining flux, which could corrode the copper over time. Caution: apply the cloth gently, do not press on the joint – the solder is still soft and you could damage the joint.

After cooling, visually inspect the joint. A properly soldered joint should show a visible, even, slightly shiny solder bead along the edge of the socket. If you see gaps, cold spots, or areas where the solder is missing, it is necessary to reheat the joint and add more solder – but only if you have not yet performed the pressure test.

Step 7 – Cooling, fixing, and further installation

Let the joint cool naturally. Do not speed up the cooling with water or air – this could cause thermal shock and microcracks in the joint. The natural cooling time to touch temperature is about 3–5 minutes for a Ø 22 mm joint. During cooling, do not move the pipes or press on the joint.

If you are installing multiple joints in sequence (which is common in distribution systems), work systematically: always from the most distant point towards the heat source, or proceed in such a way that new soldering does not overheat adjacent joints. In dense clusters of joints, use wet cloths as thermal protection for adjacent joints.

Step 8 – Pressure test

The pressure test is a mandatory final step for every heating installation. For heating systems, the following rules apply according to the standard EN 14336:

Test pressure = 1.5 times the maximum operating pressure
For standard heating (max. operating pressure 3 bar): test pressure 4.5 bar
Duration of the test: at least 30 minutes at test pressure
Result: maximum pressure drop of 0.1 bar over the entire duration of the test

Fill the system with cold water (not hot!), bleed the air and increase the pressure using a manual pressure pump. Visually inspect all joints – even the smallest drop of moisture is a reason to repair the joint before putting the system into operation.

More detailed information about pressure parameters can be found in the article Material and pressure parameters of direct reducing couplings for central heating.

Mounting specifics for various practical scenarios

Scenario 1: Connecting a boiler DN28 to a distribution DN22

This is one of the most common cases in the renovation of older boiler rooms, where the boiler is replaced with a modern condensing one. The new boiler has Ø 28 mm connections, while the existing distribution is Ø 22 mm. In this case, you will use a direct reducing coupling 28 x 22 directly after the boiler's ball valve. Pay attention to the orientation – the larger opening must face the boiler connection, the smaller one towards the existing distribution. Since you are working close to the boiler, where temperatures are higher, I recommend using silver solder (Ag5) or at least solder Sn97Cu3 certified for operating temperatures up to 120 °C.

Scenario 2: Transition from main distribution Ø 22 mm to radiator branch Ø 15 mm

A typical situation when installing a new radiator or renovating a radiator circuit. The solution here is a direct reducing coupling 22 x 15, connected between a T-fitting on the main distribution and the radiator branch. Since the radiator branch is shorter and has a smaller diameter, pressure losses are higher – therefore, ensure that the branch route is as short as possible and without unnecessary bends.

Scenario 3: Repair of a damaged section in an existing distribution

When repairing a broken pipe, it sometimes happens that the cut length does not match the available replacement pipe. If you need to connect two pipes of different diameters that arose during the repair, a reducing coupling is an elegant solution. Here, it is especially important to carefully clean the old pipe – there are often deposits, oxide layers, and corrosion on the surface that hinder quality soldering. A copper chemical cleaner (available in specialized stores) is essential here.

About when a reducing coupling is the right choice and when it is better to reach for another type of reducing fitting (e.g., a threaded reducing piece), you will read in the article When to use a reducing coupling and when another type of reducing fitting.

Common installation errors and how to avoid them

Insufficient surface cleaning – the most common cause of leaks. Solution: always use emery cloth and a wire brush before every joint, without exception.
Too much flux – the flux runs into the pipe and corrodes the metal over time. Solution: apply a thin, even layer with a brush.
Insufficient heating of the joint – the solder does not penetrate the capillary gap. Solution: heat the joint longer with a lamp, check the temperature by touching the solder.
Moving the joint during solder solidification – microcracks that only appear during the pressure test. Solution: fix the pipes or do not touch them during cooling.
Wrong coupling size – 22×15 must not be confused with 22×18 or 28×22. Solution: always verify the dimensions with a caliper before installation.
Soldering in a vertical position downward – the solder may run out before the entire joint is filled. Solution: solder in a horizontal position if possible, or work more carefully with the lamp, using shorter heating intervals.

For more information on this topic, see the topics Joint tightness: common problems and how to avoid them and How to properly connect pipes of different diameters without water leakage.

Maintenance and inspection of the joint after installation

A properly made soldered copper joint is practically maintenance-free for the entire lifetime of the installation—up to 30–50 years—under normal operating conditions (temperature up to 110 °C, pressure up to 10 bar, water pH 7–9). Nevertheless, it is advisable to visually inspect visible joints once a year during boiler service: look for wet stains, salt crystals on the surface of the joint (a sign of micro-leakage), or a corroded surface.

If a joint starts to act up, the first step is to check the operating pressure and water pH. Aggressive water (pH below 7) attacks copper joints and can cause seepage within a few years. Likewise, an excessively high oxygen content in the water accelerates corrosion. In case of doubts about water quality, have a water analysis performed and consider installing a water treatment unit.

More practical tips can be found in the article Maintenance and inspection of joints in heating distribution systems.

Most frequently asked questions (FAQ)

Can I use a direct reduced joint without soldering, for example with sealing paste?

No. Hepworth copper fittings are intended for soldered (capillary) joints according to EN 1254-1. Sealing paste is not a substitute for solder—it cannot withstand operating temperatures or pressures of heating. There are special press-fit joints or threaded reductions that work on a different principle, but a standard copper fitting without soldering is not watertight or pressure-resistant.

Do I have to drain the entire system before installation, or is it enough to close the appropriate shut-off valves?

Before any soldering, the section to be soldered must be completely dry and free of water. Even a minimal amount of water in the pipe causes steam to form when heated, which cools the joint from the inside and prevents reaching the correct soldering temperature. It is sufficient to close the appropriate shut-off valves and drain the section of the pipe—there is no need to drain the entire system, provided the installation is properly designed with the necessary shut-off valves.

What is the difference between soft and hard soldering when installing a reduced joint?

Soft soldering (Sn97Cu3 solder, temperature around 220–250 °C) is standard for heating and is sufficient for operating temperatures up to 110 °C. Hard soldering (silver solder Ag5 or more, temperature 600–800 °C) is used for higher pressures and temperatures, for example, when connecting directly to hot water sources. For a standard residential heating installation with a condensing boiler, soft soldering is sufficient. Always follow the manufacturer's technical data sheet for the fitting and the parameters of your installation when selecting solder.

Can I use a reduced joint with floor heating, where the pipe is plastic (PEX or PE-RT)?

Hepworth copper fittings are intended for copper pipe. For plastic, other fittings are used—brass or polypropylene with the appropriate connection system (pressing, welding, threaded connections). The transition from copper to plastic is handled with special Cu-plastic transition pieces, not with a direct reduced joint. If you need to connect two diameters in the copper section of the distribution before the floor heating manifold, a reduced joint is the correct choice for the copper section—the plastic system continues after the manifold.

How can I determine if a joint was properly made if I cannot visually inspect it (it is embedded or covered)?

That is precisely why a pressure test is carried out before embedding or covering the installation. This is a mandatory procedure—never cover the pipes without a successful pressure test first. A pressure test at 4.5 bar for 30 minutes is sufficiently reliable for detecting even micro-leakages. A pressure test report should be part of the documentation for every installation.

What if I need to connect a copper pipe to a threaded piece on one side?

A direct reduced joint (capillary × capillary) is not sufficient in this case. You need a transition fitting with one capillary side and one threaded side (external or internal thread). These parts are also available in the Hepworth range and solve the transition between copper and threaded systems. More on selecting the correct fitting can be found in the article When to use a reduced joint and when to use another type of reducing fitting.

Conclusion

Installing a direct reduced joint is not a complex task, but it requires careful preparation, the right tools, and respect for each step. The most important message from thousands of customer cases we have seen: most leaks do not occur during soldering, but before it—due to insufficient cleaning and surface preparation. When both surfaces are perfectly clean and the joint is properly heated, the solder will find its way into the capillary gap on its own.

For reliable and long-term results, choose proven components—direct reduced joint 22 x 15 and direct reduced joint 28 x 22 from the Hepworth category are industrially tested parts with dimensional accuracy according to EN 1254, which will last the entire lifetime of the heating installation without any intervention when installed correctly.

Do you have a question about this topic?

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