How to choose the right straight reducing coupling for heating

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How to choose the right straight reducing coupling for heating

The straight reducing coupling is one of those parts that most people overlook – until they start looking for it. There comes a moment when you have two pipes of different diameters in front of you and need to connect them in such a way that the joint lasts for decades, does not leak water, and the entire system behaves normally. That is exactly when it becomes clear how much it matters whether you have chosen the right piece.

This article will explain in detail what a straight reducing coupling actually is, why it is not enough to just take the first piece from the shelf, what to pay attention to when choosing a specific size, and how to avoid mistakes that are commonly repeated in jobs ranging from single-family homes to apartment buildings with central heating distribution.

What is a straight reducing coupling and where is it used

A straight reducing coupling is a fitting that has a different internal diameter on one end than on the other. Its purpose is to connect two pipes of different nominal bores into one tight, rigid joint. Unlike a reducing sleeve (nipple), which is used only to change the diameter without simultaneously connecting sections, the coupling does both at once – it forms a shape-sealed joint and at the same time solves the transition between two diameters.

In heating practice, these parts appear most often in the following situations:

supplying radiators from a distribution manifold, when the supply is larger than the connection to the radiator
connecting old steel pipe with current copper or plastic piping during renovations
supplying the boiler circuit, where the outlet from the tank has a different diameter than the manifold
branches, where the distribution pipe narrows towards the last radiators in the loop
connecting a circulation pump or expansion tank

In short: everywhere where two pipes of different diameters must meet and where there is neither space nor reason to solve the transition with two separate pieces (i.e., a reducing sleeve and a separate coupling).

Key parameter: correctly determining both diameters

The most common mistake when choosing a straight reducing coupling is confusing the outer and inner diameter of the pipe. When we talk about a copper pipe of 22 mm, we mean the outer diameter. When we talk about a threaded steel pipe DN15, we mean the nominal bore, which differs from the outer diameter by the wall thickness. And when we talk about plastic piping such as PEX or PB, manufacturers sometimes also list the inner diameter as the main dimension.

For heating systems made of copper (such as the Hepworth system and similar crimpable or solderable systems), a simple rule applies: the dimension in the name of the coupling = the outer diameter of the pipe. So:

Straight reducing coupling 22 x 15 accepts on the larger end a copper pipe with an outer diameter of ⌀22 mm and on the smaller end a pipe with an outer diameter of ⌀15 mm.
Straight reducing coupling 28 x 22 connects a pipe with ⌀28 mm to a pipe with ⌀22 mm.

This sounds simple, but practice shows that customers repeatedly come with complaints because they have mixed up the sides (they put the larger end where the smaller one should be), or they tried to fit a 22×15 coupling onto a steel pipe DN20 – which does not work, because DN20 has an outer diameter of 26.9 mm.

If you are unsure about the dimensions of your piping, read our guide article Dimensions of straight reducing couplings: which size do you need before ordering – there you will find a table of corresponding dimensions for copper, steel, PEX and PB.

Hepworth system: what distinguishes it from cheaper alternatives

Hepworth is a British manufacturer with a long history in the field of pressure installations. Their crimpable and solderable fittings for copper piping are designed according to the EN 1254-1 standard and are subject to strict dimensional tolerances. What does this mean in practice?

Exact insertion depth: Each Hepworth coupling has a precisely defined depth to which the pipe must be inserted. If the insertion depth is too small (as with some cheaper fittings), the capillary gap during soldering is not sufficient and the joint does not seal reliably in the long term.
Consistent wall thickness of the fitting: Cheap fittings from uncontrolled sources sometimes have thinner walls, which reduces the maximum working pressure or shortens the lifespan under thermal cycles.
Compatibility with standard copper pipes according to EN 1057: Hepworth fittings are designed for standard copper pipes, so if the dimensions are followed, you do not have problems with clearance or jamming during installation.

More about the differences between systems can be found in the article Hepworth couplings vs. other brands: what to watch out for when choosing. Here we just summarize: saving a few cents on a fitting can cost you more when repairing a leak under a ceiling or under the floor. Practical experience confirms this again and again.

Material and pressure parameters: what a reducer must withstand

Direct reduced couplings for central heating are standardly manufactured from soft copper (CW024A according to EN 12163). Copper is an ideal material for heating systems for several reasons:

Temperature resistance: Copper fittings easily withstand the operating temperature of the heating medium (typically 70–90 °C in open systems, 55–75 °C in low-temperature systems), as well as short-term peaks during expansion tank failure.
Pressure parameters: Standard copper solderable fittings are dimensioned for a maximum operating pressure of 16 bar at 110 °C. A typical central heating system in a family house operates at 1–2.5 bar, so you have a large safety margin.
Corrosion resistance: Copper does not corrode as quickly in heating systems as black steel, especially in closed systems with deaerated water.
Solderability: Tubes with smooth interiors are designed for soft soldering (capillary joint), which is one of the most reliable methods of joining metal piping when done properly.

More technical parameters including pressure-temperature curves can be found in the article Material and pressure parameters of direct reduced couplings for central heating.

When is a direct reduced coupling the right choice – and when not

A reduced coupling is an excellent choice whenever you need to connect two straight pipe sections of different diameters in one line. It is a compact solution that does not require any intermediate fitting or unnecessary bend. However, it also has its limitations:

When yes

Direct transition between two diameters on a straight run
Renovation, where an existing pipe is extended and the new section has a different diameter
Feeding a radiator with a smaller connection from a larger distribution branch
Connecting two boiler circuits with different bore sizes (e.g. 28 mm primary → 22 mm secondary)

When better to choose another fitting

Change of direction + change of diameter at the same time: In this case, use a reduced elbow or a T-piece with a reduction – a direct coupling does not change direction.
Connecting metal and plastic pipe: A transition fitting with an internal thread or a special crimp fitting for combined materials is required, not a standard copper capillary coupling.
If you need the option to disassemble: A coupling is a permanent connection. If you expect to open the connection later (for service access to a valve), use a removable connection (sleeve coupling).
Very large diameter difference: If the difference is more than two size steps (e.g. 35→15), it is better to use two couplings with an intermediate section or a special multi-reduction fitting if available – from a hydraulic point of view, a too abrupt reduction is a source of local pressure losses and noise.

If you are unsure, read the article When to use a reduced coupling and when to choose another type of reducing fitting – there you will also find discussions of edge cases from practice.

Practical scenarios from everyday customer practice

Scenario 1: Apartment renovation, old steel piping, new copper circuit to radiators

One of the most common cases: core renovation of an apartment, where the original rising distribution is made of black steel pipe DN25 (external diameter ~33.7 mm). From this, a new copper pipe circuit ⌀22 mm is branched off to the radiators in the apartment. Here, a direct reduced coupling of the capillary system is not sufficient – you must use a transition fitting with a thread (external or internal thread 1" on the steel pipe) and on the other side a solderable copper neck ⌀22 mm. Only from this transition piece can you continue with standard capillary fittings Hepworth, including reduced couplings. For example, if you want to reduce the diameter from 22 mm to 15 mm before the last radiator further from the manifold – there you will use exactly the Direct reduced coupling 22 x 15.

Scenario 2: New boiler room, connecting boiler and TÜV tank

A standard condensing boiler has an outlet port DN25 or DN32. A TÜV tank may have connections G1" (equivalent DN25) or G¾" (DN20). The primary circuit to the tank is often made in copper pipe ⌀28 mm. If the tank has a G¾" connection with an external thread and you connect a copper pipe ⌀22 mm, you will need a transition fitting and possibly a 28×22 coupling if the primary line narrows on the secondary circuit. The Direct reduced coupling 28 x 22 finds application here, for example, when transitioning from the main supply pipe ⌀28 to a distribution branch ⌀22 before the floor heating manifold.

Scenario 3: Family house, floor heating + radiators combined

In practice, floor heating (with low-temperature circuits ⌀16 or ⌀20 PEX) is often combined with radiators on the upper floor. The distribution pipe from the manifold to the radiators on the floor is in copper pipe ⌀22 mm. At the last radiator at the end of the circuit, it is not necessary to run the full thickness – it can be reduced to ⌀15 mm. Here, the reduced coupling 22×15 again comes into play, solving this transition with one piece, without unnecessary delay in the route.

Scenario 4: Replacing an old radiator with a new one with a different connection

The old cast iron radiator had connections G¾" (corresponding to an external diameter of ~26.4 mm). The new flat steel radiator has connections G½" (external diameter ~21.3 mm). The existing copper piping is ⌀22 mm. In such a case, when connecting to the new radiator, a combination of a transition fitting G½" → ⌀15 mm and possibly a reduced coupling 22×15 appears, if the pipe routing also changes. Such situations arise in every second radiator replacement in older properties.

Proper Installation: The Foundation of a Tight Connection

Even the best fitting will fail if the installation is poor. A capillary soldered joint has simple physics – when soldering is done correctly, the molten solder is drawn into the capillary space between the tube and the fitting by its own force. Key conditions:

Surface cleanliness: The copper tube must be cut straight at the end (not at an angle), stripped of burrs and thoroughly sanded with sandpaper or a special cleaner. The same applies to the inside of the fitting.
Flux: Before inserting the tube into the fitting, apply a thin layer of flux to both surfaces. Flux chemically cleans the surface when heated and allows the solder to flow freely.
Correct insertion depth: The tube must be inserted all the way to the stop at the neck of the fitting. Half a millimeter less and the capillary gap is not uniform – the result may be a porous joint.
Heating the fitting, not the tube: The flame (or induction soldering tool) should primarily focus on the fitting, not the tube. When the fitting is sufficiently hot, the solder will be pushed into the gap by itself – it is not enough to just press it from above without heating.
Lead-free solder: A requirement for potable water, a recommendation for heating – lead-free solder (e.g. Sn97Cu3) has similar properties to classic Sn60Pb40, but without health risks.

A detailed step-by-step installation procedure with a description of each phase can be found in the article Installing a direct reduced fitting step by step. For completeness: if you are working with a compression system (press-fitting), the procedure is completely different – there is no open flame, but a mechanical compression tool.

Tightness and Long-Term Reliability: What Affects the Lifespan of a Joint

A direct capillary joint made of copper, when properly executed, is practically eternal – in old buildings, there are functioning copper pipe systems 50–60 years old with no single leak. However, there are factors that shorten the lifespan:

Thermal cycling: Systems where water temperature fluctuates by more than 60 °C (e.g. when the boiler is quickly started and cooled), mechanically stress the joints more. On average, this is not a problem, but with poor installation, it may lead to a slow leak.
Corrosive water: Hard water with a high chloride content or acidic water (pH below 7) can corrode copper from the inside. A solution is water treatment or the use of a different pipe material.
Presence of oxygen: Open systems (expansion tank open to the atmosphere) allow oxygen to enter the water. Oxygen accelerates the corrosion of steel parts and can form corrosive products that indirectly damage copper joints. A closed system with a membrane expansion tank is much more gentle.
Vibrations: If the fitting is near a pump or compressor and is not sufficiently supported, vibrations can loosen the joint over time. The pipe layout should be supported at sufficient intervals (every 1–1.5 m for ⌀22, every 0.8–1 m for ⌀15).

More on the prevention and solutions for tightness problems can be found in the articles Tightness of fittings: common problems and how to avoid them and Maintenance and inspection of fittings in heating systems.

How to Properly Connect Pipes of Different Diameters: Practical Tips

Connecting pipes of different diameters is not only about the fitting itself. It is about the overall logic of the layout. Here are a few things experienced installers do not forget:

Reduce towards the periphery, not the other way around: Larger diameter near the boiler or manifold, smaller towards individual radiators. Increasing the diameter away from the boiler has no hydraulic sense and means unnecessarily high material and installation costs.
Do not forget about hydraulic balancing: A reduction in diameter increases the flow speed and local resistances. When there are more radiators on one circuit, do not forget about balancing valves to ensure that distant radiators receive enough flow.
Only connect compatible materials: Copper fitting for copper tube. Transition fitting when combining copper + plastic or copper + steel. The article How to properly connect pipes of different diameters without water leakage addresses precisely these scenarios.
Account for expansion: Copper expands by about 1.2 mm per meter at 70 °C compared to the cold state. At long straight sections, you must compensate for this with expansion bends or compensators.
Mark flow directions: Especially with reduced fittings in more complex pipe systems, mark which end is the supply and which is the return – this saves time and nerves during renovations.

Most Common Mistakes in the Selection and Installation of Reduced Fittings

From decades of customer experience, these are the most common mistakes that repeat regardless of whether an experienced installer or a DIY enthusiast is working:

Mixing up the ends of the fitting: The larger end is used where the smaller one should go. The tube either cannot be inserted (too large for the smaller end) or fits too loosely (too small for the larger end). Solution: always check the fit at room temperature before heating.
Insufficient surface cleaning: An oxide layer on copper prevents proper fusion of the solder with the surface. Just five seconds of sanding with sandpaper is enough to achieve a shiny surface.
Too much flux: Excess flux when heated can get into the inside of the system and may cause corrosive effects in the long term. A thin, even layer is sufficient.
Heating only the tube, not the fitting: The result is that the solder solidifies on the tube, but does not penetrate the capillary. The joint looks good, but lacks the necessary strength.
Moving the joint before the solder solidifies: After applying the solder, the joint must not be moved for at least 30 seconds. Interrupting the crystalline structure of the solder during cooling = a porous joint.
Ordering the wrong size due to mixing up systems: The customer has a plastic pipe 20×2 mm (outer diameter 20 mm) and orders a Hepworth 22×15 fitting with the idea that ⌀22 "should fit". It does not – the systems are not mutually compatible without a transition fitting.

Selecting the correct size: overview of the most commonly used combinations

The following logic applies to the selection of the size of a reducing coupling for standard residential heating:

28 × 22: Transition from the main supply pipe (⌀28 mm) to distribution branches (⌀22 mm). Typically behind the boiler or distributor. See Straight reducing coupling 28 x 22.
22 × 15: The most commonly used reducing coupling in heating practice. Transition from a distribution branch (⌀22) to a radiator connection or the last branch of a circuit (⌀15). See Straight reducing coupling 22 x 15.
35 × 28: Primary circuits in larger boiler rooms, industrial piping, and higher flow rates.
15 × 12: Less common, used in refrigeration micro-distribution or for very small consumers.

Frequently asked questions (FAQ)

Can I use a Hepworth straight reducing coupling for cold and hot water?

Yes. Copper capillary fittings in the Hepworth system are certified for potable water (cold and hot), central heating, and solar thermal systems – always within the permitted pressure and temperature parameters of the respective system. For solar circuits with higher temperatures (up to 200 °C in stagnation), it is necessary to verify whether the specific fitting can withstand these conditions.

What if I have PEX plastic pipe and want to connect it to a copper Hepworth coupling?

This does not work directly. PEX plastic pipe cannot be soldered. You need a transition fitting – for example, a crimp or threaded transition fitting for copper/PEX, which has a capillary copper socket on one side and a crimp or threaded connection for the plastic pipe on the other. Pay special attention to the compatibility of the outer diameter – PEX 20 mm is not the same as a copper pipe with ⌀22 mm.

Can I install a reducing coupling myself, or is it a job for a professional?

Capillary soldering is a skill that can be learned, but it requires the right tools (a gas torch or induction soldering unit, the correct solder, and flux) and patience for the first attempts. If you are installing in concealed piping (behind drywall, in the floor), we strongly recommend leaving the work to a certified installer. If the joint is accessible and the system can be easily drained and refilled, an experienced DIY enthusiast can manage it. Always test the tightness before covering it up.

What is the difference between a "reducing coupling" and a "reduction" (nipple)?

Functionally, it is a subtle difference, but practically important. A reduction (nipple) is a short piece with two different diameters, but it is used only for changing the diameter – it may not be able to connect two full pipe sections (it has a shorter socket). A straight reducing coupling has sufficiently deep sockets on both sides for standard insertion of the pipe and is specifically designed for connecting two pipe sections. In practice, these terms are sometimes used interchangeably, but when purchasing, always check the insertion depth.

How many of these couplings will I need for the entire piping system?

This depends on the layout of the piping. In a typical family home with 6–8 radiators and a branch distribution system (not flow circuits), you can expect to need 2–4 pieces of 28×22 coupling (for the transition from the boiler to the branches) and 6–12 pieces of 22×15 coupling (one per radiator, for the transition from the distribution branch to the connection). In a star distributor system, the number of reducing couplings is lower, as the reduction occurs directly in the distributor. The exact quantity is based on the project documentation or actual measurements of the piping system.

What should I do if the pipe feels too loose when inserted into the coupling?

If the pipe "wobbles" in the socket of the coupling, it may be the wrong size (e.g., you used a 22 coupling for a 15 pipe), or it may be from a different material system. A copper pipe with ⌀22 mm should fit into the 22 socket with slight friction, without play. If the play is greater than 0.3–0.4 mm, the capillary joint may not be reliable. Replace the coupling with the correct size – do not try to compensate for the play with extra solder.

Conclusion

Selecting the correct straight reducing coupling for heating is not complicated, but it requires attention to three key points: accurately determining both diameters (the outer diameter of the copper pipe), choosing a quality system (e.g., Hepworth with proven dimensional tolerances), and proper installation with all steps of capillary soldering followed precisely.

The entire category of straight reducing couplings is designed so that you can find the right combination quickly and without unnecessary doubts. If you are unsure which size you need, check out other articles in the Knowledge Center – from dimensional tables to step-by-step installation instructions. Each one is based on real customer practice and written to save you time, money, and worries about repairs.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.