How to Solder a T-Piece onto a Copper Pipe: Step by Step

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Installation of a T-junction on a copper pipe: soldering step by step

Soldering copper T-junctions is among the basic plumbing skills that still require precision, patience, and the right tools today. Despite the increasing availability of compression or threaded connections on the market, a soldered joint on copper remains the gold standard in the field of central heating – and for good reason. A well-executed soldered joint will last for decades without any maintenance, withstand working pressures typical in heating systems (usually 2.5 to 3 bar, sometimes up to 6 bar), and does not require any special tools for inspection or tightening.

In this article, I will walk you through the entire process from A to Z – from preparing the tools, through preparing the pipe and T-junction, to the actual soldering and checking the joint. We will be talking about specific dimensions, specific temperatures, and specific materials. Nothing from the air – only what really works in practice, whether you are doing your first or tenth similar installation.

What you will need: tools and materials

Before you reach for the torch, you must have everything you need within reach. Experience shows that most problems arise precisely because the plumber starts working without complete equipment and then improvises on the fly – and improvisation when soldering pays back.

Tools

Pipe cutter (tube cutter) – never cut a copper pipe with a hacksaw unless you have experience. A tube cutter provides a clean, perpendicular cut without burrs or deformation of the cross-section.
De-greaser and cleaning paste – flux is not a substitute for cleaning. These are two different things.
Brush for cleaning the inside of the fitting – there are specific round brushes for diameters of 15, 22, and 28 mm. Invest in them.
Emery cloth or cleaning cloth for copper – for the outer surface of the pipe.
Propane-butane or acetylene torch – for diameters of 15 and 22 mm, propane-butane is sufficient; for 28 mm and larger, acetylene or oxygen-propane is more advantageous due to faster heating.
Solder and flux – we will go into detail about these below.
Damp cloth or joint cooler – for cooling adjacent joints, if you are working near an existing installation.
Teflon tape or heat-resistant cloth – for protecting the surrounding area from sparks and heat.
Loupe or inspection mirror – for checking the joint in hard-to-reach places.

Material: solder and flux

Many people make a mistake here. There are two basic types of soft soldering for copper:

Soft solder (up to 450 °C) – tin and copper alloys, or tin and silver (e.g., S-Sn97Cu3 or S-Sn97Ag3). Working temperature approximately 200–250 °C. Suitable for water and heating systems at standard pressures up to 6 bar.
Hard solder (above 450 °C) – silver solders, e.g., AG 45 (45 % silver). Working temperature 650–750 °C. Used for refrigeration systems, gas installations, and areas with high mechanical demands.

For domestic central heating with Hepworth T-junctions, the soft solder S-Sn97Cu3 or S-Sn97Ag3 is the standard choice. The flux must be compatible with the solder – for soft solders, flux based on fluorides or chlorides is used (e.g., type F-SW 21 or F-SW 26 according to EN 29454). Note: after soldering, the remaining flux must be wiped off with a damp cloth, as it is corrosive to copper with prolonged contact.

Step 1: Measuring and cutting the pipe

The first step may seem trivial, but it is precisely here that mistakes are made in practical work, which are then carried through the entire piping system. Always measure the pipe with regard to the depth of insertion into the T-junction. For the 15 × 15 × 15 T-junction, the insertion depth (so-called "stop" position) is usually 13–14 mm on each branch. For the 22 × 22 × 22 T-junction, it is usually 16–17 mm, and for the 28 × 28 × 28 T-junction, it is approximately 18–20 mm. Always verify these values directly on the fitting – insert the pipe dry and measure where it stops.

Cut the pipe with a tube cutter. Procedure:

Set the cutter at the cut location and tighten it gently – not forcefully.
Rotate the cutter around the pipe once or twice and gradually increase the pressure using the tightening screw.
Never tighten too much at once – the cutting wheel may deform the pipe instead of cutting it.
After completing the cut, remove the internal burr using a special deburring tool, which is part of most cutters.

Internal burrs are critical: if not removed, they reduce the flow cross-section and at higher flow speeds cause cavitation noise. In practice, I have seen installations where leftover burrs caused constant tapping in the piping, and the owner could not figure out what it was for three years.

Step 2: Cleaning the pipe and fitting

This is, without exaggeration, the most important step. Soldering stands or falls on the cleanliness of the surface. Copper oxidizes very quickly in the air – a thin layer of copper oxide is visible as darkening or reddening. This layer prevents the formation of a metallurgical bond between the solder and the copper. Flux chemically dissolves this oxide, but only to a certain extent. If the oxide layer is thick or if there is grease on the pipe (even from your hands), the flux cannot handle it on its own and the joint will be porous or leaky.

Cleaning procedure for the outer surface of the tube (insertion part)

Using fine sandpaper (grit 120–180) or a special copper cleaning cloth, gently abrade the surface of the tube for a length corresponding to the insertion depth + approximately 5 mm extra.
Do not sand aggressively, only gently – the goal is to remove oxides, not to abrade the surface deeply.
Do not touch the cleaned surface with bare hands – skin oil is a problem.

Cleaning procedure for the inside of the T-piece

Use a wire round brush of the correct size – insert it into each branch of the fitting and rotate it 3–5 times.
The inside of the fitting must be shiny, silvery-gray, without dark stains.
Blow out or wipe away any cleaning residue with a clean dry cloth.

Step 3: Applying flux

Apply flux as a thin, even film to the outer surface of the tube in the insertion area and to the inner surface of the T-piece branch. Be careful to avoid two common mistakes I see repeatedly:

Too much flux – excess runs into the pipe interior, carbonizes when heated, and causes long-term internal corrosion. A thin, even layer is sufficient.
Flux applied only on one side – it must be on both surfaces – on the tube and inside the fitting. The capillary effect that pulls the solder into the joint only works if the flux is evenly distributed around the entire circumference.

After applying the flux, immediately insert the tube into the T-piece all the way to the stop position. Rotate the tube by 90° to evenly distribute the flux around the entire circumference. Now you can begin soldering – but ideally within 10–15 minutes, as flux degrades in air and loses its effectiveness.

Step 4: Preparing the work area and safety

Soldering is a task involving open flame. Copper pipe conducts heat very well, so it is essential to protect the surrounding area. In practice, this means:

All flammable materials (wood, thermal insulation, plastic pipes) must be protected with ceramic plates, aluminized foil, or damp cloth.
If soldering near wooden structures (in wooden buildings or when the pipe passes through a wooden beam), use special protective pads made of ceramic wool.
Always have a bucket of water or a fire extinguisher within reach.
Ensure proper ventilation – flux emits vapors when heated, which are not health-neutral.
Protect adjacent existing joints with damp cloth – heat from the torch spreads several decimeters along the copper pipe and can unsolder existing joints.

Step 5: The soldering process – step by step

This is the core of the entire process. Here, experience and technical understanding of what is happening are decisive. Soldering is not "burning" – it is heating copper to the correct temperature, at which the solder capillary flows into the gap between the pipe and the fitting.

5.1 Preheating

Set the torch to medium intensity. Direct the flame onto the fitting (T-piece), not directly onto the tube. The T-piece has a higher thermal mass and requires more heat. Move the flame smoothly along the joint circumference – never stay in one place. The goal is even heating of the entire joint, not localized overheating of one side.

Estimated preheating times (propane-butane, standard intensity):

DN15 (15 mm): 20–30 seconds
DN22 (22 mm): 35–50 seconds
DN28 (28 mm): 50–70 seconds

5.2 Temperature check

You can recognize the correct temperature as follows:

Flux begins to bubble and becomes watery – this is a sign that the correct temperature is approaching.
Flux begins to turn brown to dark – be careful, this is a sign of mild overheating. Move the flame away from the edge and continue more gently.
If the flux hardens or carbonizes (black mass), the temperature was too high. The joint must be disassembled, cleaned, and the process started again.

Practical test: touch the solder rod to the edge of the joint (not directly into the flame). If it begins to melt and flow into the joint by itself – you are at the correct temperature. If the solder only melts in the flame, but not when touching the fitting, the fitting is still cold.

5.3 Applying the solder

Touch the solder rod to the edge of the fitting branch (where the tube is inserted) and slowly add it. Capillary force pulls the solder into the gap around the entire circumference. Apply it carefully – for a 15 mm diameter, about 8–10 mm of solder rod is sufficient for one joint, for 22 mm about 15 mm, and for 28 mm about 20–22 mm.

Never apply an excessive amount of solder – excess runs down, forms droplets, and although it may look "robust," it actually indicates poor control. A correct joint is smooth, with a minimal solder bead visible around the entire edge of the branch.

5.4 Finishing and cooling

After applying the solder, move the torch away and let the joint cool naturally for 30–60 seconds. During this time, the joint must not be moved – any movement will break the crystalline structure of the solid solder and create micro-cracks. The joint may look tight, but under pressure it will leak.

After cooling (when the joint is warm, not hot), wipe off any remaining flux with a damp cloth. This is important in the long term – residual fluoridic fluxes corrode copper.

Step 6: Inspection of the joint after cooling

A well-soldered joint is easily recognizable by a smooth, shiny solder line around the entire circumference of the pipe fitting. If you see gaps, areas where the solder is missing, or if the solder is applied unevenly on only one side – the joint is not tight and must be redone.

Visual inspection is basic. However, the only truly reliable inspection is a pressure test. After completing the entire piping system and connecting the system, perform a pressure test using cold water (not air – air pressure testing is dangerous) at 1.5 times the working pressure for at least 30 minutes. Any pressure drop indicates a leak.

In practice, for a simple home installation job, the following is sufficient: after filling the system with water and bleeding the air, let the system stand at working pressure for 1–2 hours and visually inspect each joint. Wet, damp, or dewy areas are signs of a problem.

Special situations and practical problems from practice

Soldering in vertical position

T-fittings are often installed with a vertical branch in heating systems. When soldering a vertical joint, one rule applies: apply the solder from bottom to top, not from the top. Melted solder flows with gravity, so if you start at the top, the solder runs down and does not cover the upper part of the capillary gap. Start at the bottom of the joint, work your way up, and gravity will be your ally.

Soldering in confined spaces

In areas with limited access (behind bathroom cabinets, in installation shafts), it is not possible to evenly move the torch around the entire circumference. Solution: use a reflective sheet (e.g., a piece of aluminum sheet) that reflects the heat from the torch to the opposite side of the joint. Monitor the temperature visually via the solder applied to the accessible side.

Redoing a leaky joint

If you need to redo a joint (which will happen in practice), it is annoying but technically solvable. Procedure:

Heat the joint with the torch until the solder melts again.
Use pliers to gently separate the pipe from the fitting – never pull, just gently slide.
While the solder is still molten, wipe off the remaining solder from both surfaces with a clean cloth.
Let it cool, thoroughly clean both surfaces of oxides and flux and solder residues.
Apply new flux and repeat the entire procedure.

Soldering on an existing filled installation

If you are making a branch on an existing pipe (a typical case when adding a T-fitting), you must completely drain the system. Soldering a wet pipe does not work – steam from the water cools the metal too quickly, so you cannot reach the required temperature. In addition, steam causes unwanted pores in the joint. This is a rule without exception.

Most common mistakes and how to avoid them

Over the years of contract work, I have seen the same mistakes being repeated. I discuss them in more detail in the article Common mistakes when installing T-fittings and how to avoid them, but here is a summary:

Insufficient cleaning – already mentioned, the most common cause of leaks.
Too much flux – causes internal corrosion.
Moving the joint during solidification – micro-cracks, immediate or delayed leak.
Solder applied directly into the flame – the solder melts in the fire, but does not wet the cold metal. The joint is only surface-level and will leak under pressure.
Overheating the fitting – when overheated, the flux decomposes, copper is strongly oxidized, and the solder "does not hold". An overheated joint is recognizable by the blue or purple color of the copper around the joint.
Wrong size of T-fitting – if you are unsure of the dimensions, read the article How to choose the right T-fitting for copper pipe: diameters 15, 22 and 28 mm.

Specifications according to diameter: 15, 22 and 28 mm

Each diameter has its own specifications, which stem from the physics of thermal conductivity and material mass. This is not just theory – in practice, it means adjustments in working with the torch.

DN15 (15 mm) – The smallest diameter, heats up the fastest, but also overheats the fastest. The torch flame must be at medium or low intensity. Move the torch smoothly, never hold it in one place for more than 2–3 seconds. The insertion depth into the T-fitting 15 × 15 × 15 is usually 13–14 mm – always verify this specifically.

DN22 (22 mm) – The medium diameter, standard in family homes for heating branches to radiators. Requires a longer preheating time, but is more forgiving than DN15. For the T-fitting 22 × 22 × 22, expect an insertion depth of about 16–17 mm and a solder amount of about 15 mm of rod per joint.

DN28 (28 mm) – Used for main branches and distribution piping. Requires the longest preheating time and a larger torch. For the T-fitting 28 × 28 × 28, I recommend an oxygen-propane or acetylene-oxygen torch instead of regular propane-butane, to achieve sufficient thermal intensity without overheating at one point. Solder amount: 20–22 mm of rod per joint.

Pressure test and commissioning

After completing all soldered joints, the system is filled and a pressure test is carried out. Correct procedure:

Fill the system with cold water slowly, while bleeding air from all bleed valves.
Increase the pressure to 1.5 times the maximum working pressure (e.g., at a working pressure of 2.5 bar, test at 3.75 bar).
Let it stand for at least 30 minutes, ideally 2 hours.
Visually inspect each joint – dry, without condensation.
Check the pressure gauge – the pressure must not drop.

If everything is correct, you can put the system into operation. During the first heating (to a working temperature of 60–80 °C), inspect the joints again – the thermal expansion of copper is 0.017 mm/(m·K), and when heated from 20 to 70 °C, a 1 m long pipe will elongate by 0.85 mm. If the joints are well done, this is not a problem. More about long-term maintenance can be found in the article Maintenance and inspection of T-fitting joints in central heating systems.

Most Frequently Asked Questions (FAQ)

Can I solder a T-junction without flux if the copper is completely clean?

Theoretically yes – if you could guarantee absolute cleanliness and a zero-thickness oxide layer. In practice, this is not possible. During the heating process itself, copper oxidizes immediately, and flux is necessary precisely to chemically reduce this oxide at the moment when the solder flows into the joint. Never skip flux when soldering copper.

Can I use leaded solder for heating pipe systems?

No. Leaded solders (e.g., old Sn60Pb40) have been banned in the EU for potable water since 2006 and are not recommended for heating systems since 2013. Use lead-free solders only – S-Sn97Cu3 or S-Sn97Ag3 are the standard for residential buildings.

Why is the solder dripping down and not flowing into the joint?

The most common cause: the fitting is too cold. The solder melts from the heat of the torch, but the metal is not sufficiently heated to allow capillary force to draw the solder in. Move the torch away from the solder, reheat the fitting for 10–15 seconds, and repeat the test. Another possibility: the flux is contaminated or has exceeded its usable time after application.

What is the lifespan of a properly soldered joint?

When using soft soldering of copper in a heating system with lead-free solder and proper flux, the realistic lifespan of the joint is 40–60 years under operating conditions (max. pressure, max. temperature). There are installations from the 1950s and 1960s where the original soldered copper joints are still working without any problems.

Can I solder a T-junction when it is cold outside and the pipes are cold?

Yes, but preheating will take longer. In addition, at low temperatures, flux solidifies more quickly and is less effective. If you are working in an unheated boiler room or on the north side of a house in winter, protect the joint from wind (which prevents even heating) and allow for a longer preheating time.

Where can I find more information about T-junction dimensions and how to choose them?

In our Knowledge Centre, you will find detailed articles focused precisely on these topics: What do T-junction dimensions mean and how to read them correctly, Same vs. reduced T-junction: when to use which type, and How to divide heating pipes using a T-junction without losing pressure. These articles will help you choose the right fitting even before installation.

Conclusion: what to remember

Soldering copper T-junctions is not rocket science, but it does require a systematic approach. Each step – measuring, cutting, cleaning, applying flux, preheating, applying solder, and cooling – is important. Skipping or neglecting any step will eventually catch up with you, sometimes during a pressure test, sometimes after years of operation in the form of corrosion or leakage.

If you are doing a T-junction for the first time, I recommend practicing the whole process first on a cut-off pipe and an unused fitting – to get used to the movement of the torch, find the right temperature, and see how the flux behaves. This "dry run" will save you a lot of trouble during the actual installation.

And don't forget: choosing the correct size of the T-junction is just as important as the installation itself. Always choose a fitting that corresponds to the actual outer diameter of the pipe – for the most common diameters, we offer T-junction 15 × 15 × 15, T-junction 22 × 22 × 22, and T-junction 28 × 28 × 28. The entire range of fittings is neatly sorted in the section Hepworth T-junctions.

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.