Dimensions of straight reducing couplings: which size do I need
Dimensions of straight reducing couplings: the complete guide to selecting the correct size
When renovating a heating system or during a new installation, you often need to connect two pipes of different diameters and reach for a straight reducing coupling. It sounds simple – yet many plumbers and even experienced tradespeople make mistakes when selecting the correct dimensions, which only become apparent during pressure testing or, in the worst case, after years of operation. In this article, we will take a detailed look at how the dimensions of straight reducing couplings work, what the numbers in the designation mean, how to determine which size you actually need, and where people most commonly make mistakes when selecting them.
What the numbers in the designation of a straight reducing coupling mean
A straight reducing coupling is always marked with a pair of numbers – for example, 22 x 15 or 28 x 22. The first number is always the larger diameter, the second is the smaller one. Both numbers indicate the outer diameter of the pipe in millimeters for which the coupling is intended. It is important to understand this from the very beginning, as in the case of copper or plastic pipe, this dimension can be easily verified by direct measurement.
The Hepworth system (and most European manufacturers) follows the EN 1254-1 standard for copper, where it is defined that the outer diameter of the pipe is the nominal value – that is, a pipe marked as DN 15 has an outer diameter of 15 mm, DN 22 has 22 mm, DN 28 has 28 mm, and so on. There is no mystery here – the number in the coupling name corresponds directly to the outer diameter of the pipe that will be inserted into it.
The internal diameter of the coupling (i.e., the opening into which the pipe enters) is slightly larger than the outer diameter of the pipe, as it must accept the pipe and at the same time ensure a seal. In the case of compression or press-fit couplings, the manufacturing tolerance is very precise and ranges in tenths of a millimeter.
Standard dimensions of straight reducing couplings in heating
In practice, in heating pipe systems, you encounter several common combinations of diameters. These are not random numbers – they stem from the hydraulic design of the system, where the distribution lines from the boiler towards the circuit loops gradually narrow according to the flow rate.
The most commonly used dimensions of reducing couplings in heating:
- 15 x 12 mm – a less common dimension, used in very small circuits or for connecting capillary distribution lines
- 22 x 15 mm – by far the most common dimension; reduction from a distribution line to a radiator or floor heating circuit connection
- 28 x 15 mm – a larger step, used when reducing the main line to a side branch circuit
- 28 x 22 mm – very common for reducing the boiler or storage tank primary circuit to the distribution line
- 35 x 22 mm – used in larger systems with boilers over 25 kW
- 42 x 28 mm and 42 x 35 mm – used in commercial installations and larger boiler rooms
In this category, you will find for example the Straight reducing coupling 22 x 15, which is by far the most sold in the entire range – and for good reason. Almost every radiator in a standard family home is supplied via a 15 mm copper pipe, which connects to a 22 mm distribution line. This combination forms the backbone of most heating pipe systems.
For larger systems, where the flow is reduced from the primary to the secondary circuit, the key is the Straight reducing coupling 28 x 22. You typically see this when connecting a boiler (DN 28 or 1" connection) to the rest of the distribution system (DN 22).
How to actually measure the diameter of your pipe
This is the point where customers most often make mistakes. They come with the phrase "I have a normal pipe at home" or remember a number from memory, but practice shows that memory is misleading and the eye measures inaccurately.
Measuring the outer diameter with a caliper
The correct procedure is to measure with a caliper (sliding gauge). Place the caliper jaws perpendicular to the pipe axis and measure the outer diameter at a clean, undamaged location. Round the result to the nearest whole millimeters – if it comes out to 21.8 mm, it is DN 22. If it comes out to 14.9 mm, it is DN 15.
Watch out for a few pitfalls:
- Corrosion and deposits – old copper may have an oxide layer or limescale on the surface, which can distort the measurement by 0.5–1 mm. Clean the pipe before measuring
- Ovality – a damaged or old pipe may not be perfectly round. Measure at several places and use the average
- Steel threaded pipe – a different logic applies here: threaded pipe (DN/G) has completely different diameters and for connecting to copper or plastic pipe, adapters are used, not direct couplings
- Plastic pipe – some plastic pipes (e.g., PPR or PE-X) may have the same outer diameter as copper, others not. Always verify the type and manufacturer
Quick verification with a tape or string
If you don't have a caliper at hand, the circumference of the pipe can be roughly measured with a tape (tailor's tape) or a piece of string. Divide the circumference by the number π (3.1416) – the result is the outer diameter. For DN 22, the circumference is about 69 mm, for DN 28 it is about 88 mm, and for DN 15 it is about 47 mm. This method is less accurate, but sufficient for orientation.
Typical situations from practice: which size of coupling do you need
Theoretical tables are nice, but practice is more specific. Here are real scenarios we encounter repeatedly:
Scenario 1: Adding a new radiator to an existing system
You have an old DN 22 system in the hallway and want to connect a new panel radiator. Radiator valves (thermostatic valve and lockshield) usually have a 1/2" or direct copper DN 15 connection. You therefore need to reduce from DN 22 (distribution pipe) to DN 15 (connection to the valve assembly). Your coupling: 22 x 15.
Scenario 2: Connecting a boiler to an existing system
You are replacing the boiler and the new one has DN 28 outputs (1" flange or compression fitting). Your existing system is DN 22. In order to directly connect the new pipe, you need to reduce at the output and return: 28 x 22. Exactly for this purpose, the Straight reduced coupling 28 x 22 is used, which provides a clean straight transition without the need for additional fittings.
Scenario 3: Reconstruction of an old steel installation
This is a trap we see very often. The customer has an old steel system with threaded pipe and thinks that a "standard reduced coupling" will solve it. Threaded pipe has a nominal designation DN 15 (½"), but its actual outer diameter is 21.3 mm – which is almost DN 22. A direct reduced coupling between copper and threaded pipe is therefore not a solution – you need a transition adapter with a thread. Direct reduced couplings are used exclusively for connecting pipe of the same material and system (copper to copper in the same standard range).
Scenario 4: Reducing the diameter in a long pipe run
In the hydraulic balancing of a larger family house or apartment building, it is sometimes designed that long pipe sections to remote circuits have one step smaller diameter, which helps to balance pressure losses. For example, the primary distribution is in DN 28, but the extension to a remote bathroom circuit is made in DN 22. A coupling 28 x 22 is installed at the point of reduction.
Scenario 5: Connecting a hot water tank to a heating circuit
An indirectly heated hot water tank has an exchanger that is supplied from the heating circuit. The tank circuit usually has DN 22, but the exchanger itself has DN 15 (1/2") connections. Here, two couplings 22 x 15 are used – one for the supply, one for the return of the exchanger.
Dimension table: overview of common combinations
For easier orientation, we provide an overview of the most common combinations of diameters and their typical use in heating practice:
| Coupling size | Larger diameter | Smaller diameter | Typical use |
|---|---|---|---|
| 15 × 12 | 15 mm | 12 mm | Capillaries, small circuits |
| 22 × 15 | 22 mm | 15 mm | Connecting radiators, floor heating circuits |
| 28 × 15 | 28 mm | 15 mm | Big step, side branches |
| 28 × 22 | 28 mm | 22 mm | Boiler/tank → distribution system |
| 35 × 22 | 35 mm | 22 mm | Larger boilers, industry |
| 35 × 28 | 35 mm | 28 mm | Primary lines, larger systems |
| 42 × 28 | 42 mm | 28 mm | Commercial boilers, CZT |
When a straight reducing coupling is the right solution and when it is not
A straight reducing coupling solves a simple task: connecting two pipes of different diameters in a straight line. However, there are situations where it is better to choose another type of pipe fitting. This topic is discussed in more detail in the article When to use a reducing coupling and when to choose another type of reducing fitting in this Knowledge Centre, but here is a brief summary:
- Reducing elbow – if you need to branch off and reduce at the same time, you should use a reducing T-piece, not a straight coupling
- Change of direction with reduction – for a bend with a different diameter, there are reducing elbows (bends) available
- Large diameter to small in one step – for example, 42 x 15 is not a commonly available straight coupling; it is also not hydraulically suitable. Usually, the reduction is done in two steps via an intermediate size
- Other materials – transitions from copper/steel or copper/plastic require special transition pieces, not standard compression or press-fit couplings
If you are unsure whether a straight reducing coupling is the right choice, I recommend reading the article How to choose the right straight reducing coupling for heating, where the entire decision-making process is systematically outlined.
Tolerances and dimensional accuracy: why quality matters
Straight reducing couplings of the Hepworth system (and other high-quality European manufacturers) are produced with very precise tolerances. According to the EN 1254-2 standard for compression fittings, the tolerance of the internal diameter of the socket ranges from +0.0 to +0.1 mm for diameters up to 28 mm. In practice, this means that if a pipe has an outer diameter of 22.0 mm, the socket of the coupling is calibrated for it.
Chinese or unknown copies may not meet these tolerances. At first glance, they may look identical, but after repeated temperature cycles (which happen every day during the heating season), poorly sealed joints gradually loosen. When moisture appears in a drywall surface after three years, it takes a long time to find the cause.
More information about material and pressure parameters can be found in the article Material and pressure parameters of straight reducing couplings for central heating.
How to check that you have selected the correct size before ordering
Before you click to place your order, I recommend going through this quick checklist. I have seen too many orders where work had to be stopped because the couplings arrived in the wrong size and a new delivery had to be waited for.
- ✅ Measure both ends with a sliding caliper – the larger and the smaller diameter
- ✅ Check that the marking corresponds to the outer diameter of the pipe, not the inner diameter or the nominal threaded size
- ✅ Confirm that both ends are made of the same material and system (e.g., both made of hard copper pipe according to EN 1057)
- ✅ For press-fit couplings, confirm that you have a crimping tool with jaws suitable for the system – not all crimpers are compatible with all systems
- ✅ Compare available variants (compression vs. press-fit) in relation to the installation location – in confined spaces, the outer dimensions of the coupling body can be decisive
- ✅ Order at least 10–15 % more pieces than you need – the same size is usually required in several places in the system
A detailed installation guide can be found in the article Installation of a straight reducing coupling step by step, where the preparatory work before the actual installation is also described.
External dimensions of the coupling itself: what to consider when installing in confined spaces
In addition to the pipe diameters for which the coupling is intended, the coupling itself also has its own external dimensions. This is important, for example, when running pipes in wall grooves, in surface-mounted installations, or in distribution boxes, where space is limited.
Compression couplings (sleeve + olive + body) are usually bulkier – for DN 22, the outer diameter of the coupling body can be 30–34 mm, which must be taken into account in the groove design. Press-fit couplings are more compact and have a smoother profile, but they require a crimping tool.
The overall length of a straight reducing coupling depends on the manufacturer and the size of the reduction. For a 22 x 15 coupling, the typical overall length is in the range of 45–60 mm. For a 28 x 22 coupling, it can reach 55–70 mm. Always check these values in the technical data sheet of the specific product – they are relevant, for example, when planning the length of cut pipe sections.
Most common mistakes when selecting dimensions and how to avoid them
Over the years of practice, we repeatedly encounter the same mistakes. Here they are summarized so you don't have to make them:
- Mixing up outer and inner diameter – the customer measures the inner diameter of the pipe (the opening), which is about 13 mm for DN 15 copper pipe, and orders a "13 mm coupling". Such a coupling does not exist. Always measure the outer diameter.
- Mixing up threaded and pipe diameter – a G ½" threaded fitting has an outer diameter of the thread of about 20.9 mm, which is easily confused with DN 22. These are, however, completely different types of fittings.
- Ordering "by eye" – an older pipe system looks massive, the customer estimates DN 28, but it actually has DN 22. Measuring with a caliper takes 30 seconds and saves hours of waiting for the correct parts.
- Ignoring the direction of reduction – a direct reduced coupling is symmetrical in terms of installation (can be installed in both directions), but hydraulically, the larger opening should face the side of higher pressure (usually from the boiler). In a properly dimensioned system, this usually comes naturally, but keep it in mind.
- Mixing systems from different manufacturers – compression fittings from different manufacturers may have the same nominal diameter, but different olive and nut geometries. Do not use a nut from one manufacturer on a body from another.
The topic of sealing and water leakage is also covered in a separate article Sealing of fittings: common problems and how to avoid them, where you will also find a diagnostic procedure for leaks.
Dimensional compatibility between systems from different manufacturers
Nominal diameters (15, 22, 28 mm) are defined by European standards, so piping from different manufacturers is dimensionally compatible. This means that a DN 22 pipe from one manufacturer can be easily inserted into a DN 22 coupling from another manufacturer – the outer diameter of the pipe is standardized and the same.
Where problems arise is in the sealing system and the method of fixing. Compression fittings from different manufacturers have different olives (sealing, front or back, ring or angled), and therefore the sealing components are not interchangeable. Press-fit fittings are even more system-closed – the crimping geometry from manufacturer A should not be used with the crimping tool from manufacturer B. More on comparing brands can be found in the article Fittings Hepworth vs. other brands: what to watch out for when choosing.
The conclusion is clear: the pipe dimension (DN) is standardized and compatible across manufacturers; the connection system (compression, press-fit, capillary) must, however, be consistent within one manufacturer and one joining technology.
Special cases: reductions in floor heating
Floor heating has its own specifics. A floor heating manifold is usually fed via a DN 22 copper pipe from the primary circuit, but the loops themselves are made of plastic pipe (PE-X or PE-RT) with an outer diameter of 16 mm or 20 mm. In this case, a direct copper-to-copper reduced coupling is not used at all – between the copper supply and the plastic loop is a manifold with its own connections.
Where a direct reduced coupling is used in floor heating is in connecting the supply and return pipes to the manifold. The manifold usually has DN 22 connections (G ¾" or directly copper 22 mm), but the supply to it may be in DN 28. Here again, a Direct reduced coupling 28 x 22 appears as a simple and clean transition.
Questions and answers (FAQ)
How do I find out whether I need a 22 x 15 or 28 x 22 coupling?
Measure the outer diameter of both pipes you want to connect. If one is 22 mm and the other is 15 mm, you need a 22 x 15 coupling. If one is 28 mm and the other is 22 mm, you need a 28 x 22 coupling. Simple rule: the first (larger) number = outer diameter of the larger pipe, the second (smaller) number = outer diameter of the smaller pipe. Measuring with a caliper takes half a minute and ensures correct selection.
Can I install a direct reduced coupling backwards (smaller diameter towards the larger source)?
Hydraulically, there is no difference – the medium flows in both directions according to the system's needs. Mechanically, the coupling is symmetrical in the sense that both ends accept the same diameter pipe. No rule "always larger diameter towards the boiler" is absolutely mandatory, but it is good practice, because when purchasing, it is immediately clear which end is larger and which is smaller.
Can I use two direct (non-reduced) couplings with an intermediate piece instead of one reduced coupling?
Technically yes, but it is unnecessarily complicated, more expensive (three parts instead of one), and each additional connection is a potential leak point. A direct reduced coupling solves the transition in one piece – it was designed exactly for that. Always use it when you need a direct transition between two different diameters.
What is the difference between a compression and a press-fit direct reduced coupling in terms of dimensions?
The nominal diameter (e.g., 22 x 15 or 28 x 22) is the same for both types – both are designed for the same pipe diameter. The difference is in the outer profile: compression couplings have sliding nuts and are bulkier (larger outer diameter of the body). Press-fit couplings have a more compact profile. In confined spaces, this property can be decisive. When ordering, therefore, check the outer dimensions of the coupling body in addition to the nominal diameter, if it is relevant for you.
Can I use a 22 x 15 coupling for another combination, for example, for 3/4" and 1/2" threaded piping?
No. Direct reduced couplings of the Hepworth system are designed for copper pipe with an outer diameter according to EN 1057. Threaded piping G ¾" and G ½" has different outer thread diameters (26.4 mm and 20.9 mm respectively) and a different joining method. For transition between threaded and copper piping, use transition fittings with an internal or external thread on one side and a compression or press-fit connection on the other.
How many direct reduced couplings are typically used during a heating system renovation in a family house?
It depends on the scope of the renovation and the number of circuits, but roughly: in a typical family house with 6–10 radiators and a central boiler, count on at least 12–20 pieces of 22 x 15 couplings (supply + return for each radiator or manifold), 2–4 pieces of 28 x 22 couplings (at the boiler and possibly the buffer tank), and a few pieces of other sizes at special points in the distribution. I recommend buying with at least a 15 % reserve, because bulk orders are usually more cost-effective and you will definitely use the surplus during future minor repairs.
Conclusion: dimension is the basis, quality is the guarantee
Selecting the correct size of a direct reduced coupling is not rocket science, but it does require careful measuring and a clear understanding of what the numbers in the designation mean. The outer diameter of the pipe is always the key data – and today, within European standards, it is well standardized for copper pipe. Just measure, compare with the table, and order.
Where customers make mistakes is mostly in mixing up outer and inner diameter, or in mixing copper and threaded dimensions. Both mistakes can be easily eliminated by a single measurement with a caliper before ordering. An investment of thirty seconds in measuring will save you hours of lost time and unnecessary costs for returning the wrong parts.
If you are dealing with a more complex distribution system where it is not clear exactly where and which reduction to use, I recommend also studying other topics in this Knowledge Center – especially How to correctly connect pipes of different diameters without water leakage and Maintenance and inspection of fittings in heating distribution systems. A well-planned distribution system with correctly selected reductions works safely, without leaks and without the need for interventions for many years.
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.