Can I connect pipes of different diameters with a straight coupling
Can I connect pipes of different diameters with a straight coupling? Complete technical guide
This question arises regularly – whether during the renovation of an older heating system, when finishing pipe runs, or when a customer brings in a boiler with a different connection diameter than the existing pipe. The answer is not entirely simple, and it certainly is not enough to say just "yes" or "no." It depends on exactly what you mean by the term "straight coupling" and what your specific situation is. In this article, we will go into depth – from basic terminology through physical limitations to specific practical solutions.
What is a straight coupling and what is it actually used for
A straight coupling is a fitting designed to connect two pipe sections in one line – without changing direction, without branches. It is available in pressed, threaded, bolted, or push-fit versions. The basic function is to ensure a hermetic, pressure-resistant connection between the ends of two pipes.
The key parameter of any straight coupling is the diameter of both ends. Here is the first important difference that most people do not fully consider:
- Equal diameter straight coupling – both ends have the same diameter. For example, straight coupling 15 x 15 or straight coupling 22 x 22. It is used exclusively to extend a pipe of the same diameter or to repair a broken section.
- Reducing straight coupling (reduction) – the ends have different diameters. For example, 22 x 15 or 28 x 22. This is precisely the fitting that answers your question.
This seemingly small terminological nuance has significant practical consequences: if you are looking for a way to connect pipes of two different diameters, you need a reducing coupling, not a "standard" straight coupling. A straight coupling with identical ends will not solve your problem.
Why you cannot force pipes of different diameters into a standard straight coupling
This question may seem trivial, but in practice, we often encounter creative attempts to solve it that do not work in the long run. Let us explain why.
Every straight coupling – whether pressed, threaded, or push-fit – has precisely dimensioned ends. In pressed couplings, the O-ring and sleeve are calibrated for a specific outer diameter of the pipe (e.g., 15 mm, 22 mm, 28 mm). Tolerances are in the range of tenths of a millimeter. If you were to force a 15 mm pipe into a coupling designed for a 22 mm pipe, a gap would form, which cannot be sealed airtight or watertight without special gaskets – and even if it could, the joint would fail at the first pressure surge.
In the case of threaded couplings, the situation is even clearer: the thread has a defined geometry (G ¼, G ½, G ¾, etc.), and a pipe with a different thread simply cannot be screwed in.
The conclusion is therefore clear: a straight coupling with equal ends cannot be used to connect pipes of different diameters. For such a connection, you always need a reduction or a combination of fittings.
When you really need to connect two pipes of different diameters – real scenarios from practice
Over the years of practice, we have seen dozens of situations where this need arises. Here are the most common ones:
1. Connecting a new boiler to existing piping
An older gas boiler had a connection of 1 inch (DN 25), while a new condensing boiler has a connection of ¾ inch (DN 20). The piping in the house is old steel pipe with a diameter of 25 mm. Solution: a reducing fitting at the boiler inlet/outlet, the rest of the piping remains unchanged.
2. Extending piping from copper to plastic (or vice versa)
A basic copper installation of 22 mm is connected to floor heating with 16 mm piping. Here, a simple diameter reduction is not enough – you also need a transition fitting between materials (copper → multicor, e.g., via a threaded reduction or adapter).
3. Reconstruction of part of the piping
During a bathroom renovation, part of the piping is changed from DN 15 to DN 22 due to a higher flow rate for a new bathroom radiator. A reduction is needed at both ends of the changed section.
4. Repairing a failure on a combined piping system
In an old house, irregular diameters may occur – pipes from different eras, different manufacturers. A situation where you need to connect 18 mm with 15 mm or 20 mm with 22 mm is not uncommon.
5. Connecting valves and equipment
A ball valve, check valve, or expansion tank may have a different connection diameter than the main piping. A reducing coupling or a reducing nipple solves the transition here.
What types of fittings are available for connecting pipes of different diameters
Now that we know that a standard straight coupling does not solve this issue, let's look at what can be used in practice.
Reducing straight coupling (reduction)
This is the cleanest solution. A reduction looks like a straight coupling, but one end has a larger and the other a smaller diameter. It is produced for all common combinations: 22 × 15, 28 × 22, 28 × 15, 35 × 28 and more. Available in crimped and threaded versions. The installation is identical to a standard coupling – one step, one component, no unnecessary joints added.
Threaded reduction (reducing nipple / reducing bushing)
If you are working with threaded fittings, a reducing nipple (external thread on both ends) or a reducing bushing (internal thread on both ends) will solve the transition between two threaded pipe ends. For example, from G ¾ to G ½.
Combination of two fittings and a short pipe section
In some cases – for example, when the required reduction is not in stock or does not exist in a given combination – you can use two fittings with a short pipe section in between. For example: supply pipe 28 mm → reduction 28×22 → short section 22 mm → reduction 22×15 → pipe 15 mm. This solution is functional, but unnecessarily adds joints (each joint is a potential weak point).
Transition adapters (copper → plastic, steel → copper, etc.)
If you are also changing the pipe material, a special adapter is required that combines different types of ends on each side (e.g. crimping collars on one side, threading on the other). These components also exist with a reducing function.
Straight couplings of the same diameter – where to find them and what they cover
To give you a real overview of what you can find in the category of Hepworth straight couplings for the same diameters:
- Straight coupling 15 x 15 – the most commonly used size for domestic hot and cold water distribution, radiator connections and secondary branches. A diameter of 15 mm is the standard in Slovakia for most apartment and house installations.
- Straight coupling 16 x 16 – used mainly for multi-layer (multicor/pex-al-pex) pipes in floor heating, where a diameter of 16 mm has become an industrial standard. Note: 15 mm copper and 16 mm multicor are not interchangeable without a reduction!
- Straight coupling 22 x 22 – main horizontal lines in family houses, boiler connections with higher output, inlet pipes to distributors and storage tanks. 22 mm is a typical diameter for primary circuits.
- Straight coupling 28 x 28 – risers, main branches in larger installations, solar circuits, connections to hot water storage tanks with higher flow. 28 mm is used everywhere where 22 mm is not sufficient to cover the required flow.
Physical and hydraulic consequences of changing the diameter in a pipe
When connecting pipes of different diameters, it is not just a question of a mechanical connection – changing the diameter has real consequences for the flow of the medium in the system. These consequences must be understood before you decide where and how to use a reduction.
Flow velocity and pressure loss
The flow rate does not change – what enters the larger pipe must also exit the smaller one. When the diameter decreases, the flow velocity must necessarily increase. A simple relationship applies: Q = v × A (flow rate = velocity × cross-section). When transitioning from DN 22 to DN 15, the cross-section decreases from 380 mm² to 177 mm² – less than half. The flow velocity thus more than doubles, which brings increased pressure loss.
In practice, this means: if you reduce the diameter too early or over a long section, the circulation pump must overcome greater resistance, which can lead to insufficient flow in distant circuits, system imbalance or noisy flow (cavitation, hissing in the pipe).
Where reducing the diameter is fine and where it is problematic
Reducing the diameter is completely fine where the required flow is also reduced – for example, when branching off from a main riser to a single radiator. On the contrary, reducing the diameter on the main line without also reducing the flow load is problematic. A typical mistake: a customer shortens the main 22 mm line and connects three more radiators after the narrowing – the system will stop flowing evenly.
Hydraulic shock and transition through a reduction
Every sudden narrowing generates a local pressure loss (local resistance). Reductions with a gradual conical transition have lower local resistance than abrupt transitions. Therefore, crimped reductions with a smooth transition are always a better choice in primary circuits than threaded adapters with a sharp transition.
How to correctly choose a reducing coupling – step-by-step procedure
If you have decided to use a reduction, here is a proven procedure that eliminates the most common mistakes:
- Measure the outer diameter of the existing pipe – not the internal diameter (bore), but the outer one. Use a caliper. Note: "15 mm pipe" has an outer diameter of 15 mm, while a "16 mm multicor pipe" has an outer diameter of 16 mm – these are not the same dimensions!
- Determine the outer diameter of the second pipe you want to connect to.
- Determine the material of both pipes – copper, steel, multi-layer, PEX, etc. The type of coupling (crimping, threaded, push-fit) depends on the material.
- Determine the operating pressure and temperature of the system – for standard heating up to 6 bar and 95 °C, different certifications apply than for boilers with higher pressure. More about choosing the right coupling can be found in the article How to choose the right straight coupling for pipe: diameter, material and pressure parameters.
- Choose the type of reduction – crimped (for permanent installations, requires crimping pliers), threaded (for serviceable locations, at fittings) or push-fit (quick installation, lower pressure). A more detailed comparison can be found in the article Difference between crimped and threaded straight couplings: when to use which.
- Verify the correct orientation – the larger end belongs to the larger pipe. This may sound trivial, but such mistakes can happen even during quick repairs.
Most common mistakes when connecting pipes of different diameters
After years of experience, we know that the same mistakes keep happening. Here are the ones you should be careful about:
Mixing up outer and inner diameter
The customer says: "I have a 15 mm pipe." In reality, they don't know whether they mean the outer or inner diameter. A 15 mm copper pipe has an outer diameter of 15 mm and an inner diameter of 13 mm. A 16 mm multicor pipe has an outer diameter of 16 mm. These two pipes are not the same and will not fit into the same fitting. Always measure the outer diameter.
Plastic 16 mm and copper 15 mm – a tricky pair
This is by far the most common source of confusion. A 1 mm difference may seem negligible, but from the perspective of fittings, it's two completely different worlds. If you need to connect a 15 mm copper installation to a 16 mm multicor pipe, you need a transition fitting or adapter, not a regular reducer in the sense of "a bit larger to a bit smaller".
Ignoring the flow direction in asymmetric reducers
Most reducers are symmetrical – they work the same in both directions. Some special reducers (e.g., with an integrated check valve or bleed hole) are directionally oriented. Always check the technical datasheet.
Using a reducer in the wrong place in the system
A reducer placed too close to the pump in the main heating circuit can cause cavitation and noise. A reducer at the inlet to a manifold without pressure balancing can unbalance the entire system. Always consult the placement of the reducer with a designer or experienced technician.
Mixing DN and outer diameter for steel pipes
A steel threaded pipe DN 15 has an outer diameter of 21.3 mm – not 15 mm! The DN (Diameter Nominal) marking for steel pipes corresponds to the bore, not the outer diameter. This is a crucial trap when combining old steel piping with copper fittings.
Material compatibility when connecting different diameters
A reducing coupling must be compatible with the material of both pipes. This may seem obvious, but in practice, situations arise where you need to connect different materials and different diameters at the same time. Such a solution requires either a special combo adapter or a short transition piece with two fittings.
Example: copper pipe 22 mm → 22×15 reducer (copper-copper, crimped) → short piece of copper 15 mm → crimped transition copper 15 → threaded G½ → valve with G½ thread. Each of these steps is one connection – the fewer connections, the better.
For a deeper look at this issue, we recommend the article Straight couplings Hepworth vs. other brands: what they differ in and what to choose, where compatibility between different systems and manufacturers is also discussed.
Practical tables – common diameter combinations and recommended solutions
| Pipe 1 | Pipe 2 | Recommended solution | Note |
|---|---|---|---|
| Copper DN 22 | Copper DN 15 | Crimped reducer 22 × 15 | Direct step, one connection |
| Copper DN 28 | Copper DN 22 | Crimped reducer 28 × 22 | Standard step for risers |
| Copper DN 15 | Multicor DN 16 | Transition adapter copper 15 → multicor 16 | Different material and diameter! |
| Steel DN 25 (ext. Ø 33.7) | Copper DN 22 | Threaded adapter G1 → crimped copper 22 | Beware of DN ≠ outer diameter |
| Copper DN 22 | Copper DN 22 | Straight coupling 22 × 22 | Extension, not a reducer |
| Copper DN 28 | Copper DN 15 | Reducer 28×22 + piece 22 mm + reducer 22×15 | Two-step reduction, 28×15 reducer rare |
When is a reducing coupling unsuitable and what to use instead
There are situations where you might consider a reduction, but in reality, another solution is more appropriate:
Connecting diameters with too large a difference
Reducing from DN 35 to DN 15 in one step creates extreme hydraulic conditions – a huge increase in flow velocity, noise, vibrations, and erosion of the fitting. In such a case, it is better to perform the reduction in steps: 35→28→22→15, or alternatively, realize this transition via a distributor or collector.
Transition to a significantly different pressure system
If you are connecting a supply with a working pressure of 3 bar to a section where pressure reaches 8–10 bar (e.g., an inlet from the public water supply), a simple reduction is not sufficient. You need a pressure regulator or a reducing valve.
Series connection of multiple reductions in a short section
Two reductions immediately one after the other without an intermediate section can cause turbulent flow and increased pressure loss. Leave at least 5× D (five times the diameter of the larger pipe) of straight pipe between two reductions.
Sealing of the joint at different diameters – what to pay attention to
Every reducing coupling is just as prone to leaks as a standard straight coupling – that is, practically not at all if installed correctly. But problems appear quickly with incorrect procedures. A detailed guide on solving leaks can be found in the article Sealing of a straight coupling: why the joint leaks and how to fix it.
Key rules for a tight reducing joint:
- The pipe must be pushed fully into the socket until it reaches the stop (with push-fit systems, this is checked via an indicator window).
- With crimped reductions, verify that the crimping was done in the correct position and with the correct jaw profile (B, TH or V profile according to the manufacturer).
- The pipe ends must be cut straight and deburred – a slanted cut or burr can damage the O-ring.
- Threaded reductions must be sealed with suitable sealing material (hemp + paste, or PTFE tape with sealing paste). Never use only PTFE tape on its own – it does not have sufficient long-term pressure resistance.
FAQ – Frequently asked questions about connecting pipes of different diameters
Can I fit a straight coupling 22×22 to a 22 mm pipe on one side and a 15 mm pipe on the other side?
No. A 22×22 coupling has both sockets dimensioned precisely for the outer diameter of 22 mm. A 15 mm pipe would float loosely in the socket, the O-ring would not grip it, and the joint would be leaky from the very beginning. For such a connection, you must use a reducing fitting 22×15.
Are Hepworth reducing couplings compatible with couplings from other manufacturers?
Crimping geometry and diameters are partially standardized (EN 1254, ISO 8434), but the crimping profile (B, TH, V) may differ between manufacturers. A Hepworth crimped fitting should be crimped with jaws designed for the specific profile. Physical compatibility with pipes from other manufacturers (same outer diameter) usually works, but always verify this with the fitting manufacturer. More in the article Hepworth straight couplings vs. other brands: what they differ in and what to choose.
Are there push-fit (non-crimping) reducing couplings?
Yes, push-fit reductions do exist and are suitable for pressures up to 10 bar and temperatures up to 95 °C (depending on the manufacturer and specific product). They are particularly practical for repairs and renovations where a crimping tool is not available. Disadvantage: in some countries and for certain certifications (e.g., gas), push-fit couplings are not permitted.
What is the difference between a "reduction" and a "transition"?
In everyday language, these terms overlap, but technically: a reduction refers to a fitting where both ends are of the same type (e.g., both crimped), but of different diameters. A transition (or transition fitting) refers to a fitting where the ends are of different types (e.g., one crimped, the other threaded) – they may or may not also have different diameters. In Hepworth catalogs, you will find both clearly separated.
How can I determine the diameter of my existing pipe if I don't have a caliper?
Emergency method: wrap a piece of pipe with a thread and measure the circumference. Diameter = circumference / π (3.14). For common diameters, the following applies: circumference of approx. 47 mm → DN 15, circumference of approx. 69 mm → DN 22, circumference of approx. 88 mm → DN 28. This method is not accurate to 0.1 mm, but it is usually sufficient for selecting a fitting. Ideally, however, always have a caliper on hand.
Can a reducing coupling reduce the performance of a heating system?
Yes, if it is used in the wrong place or in too large a range. Every pipe narrowing increases hydraulic resistance. If you reduce the diameter of the main heating circuit and do not adjust the circulation pump settings accordingly, the system may have insufficient flow and some radiators may not heat properly. For any change in diameters on the primary circuit, we recommend a hydraulic recalculation or consultation with a heating designer.
Conclusion: Straight coupling for different diameters – a simple rule
The answer to the title question is now clear: a standard straight coupling of the same diameter is not sufficient for connecting pipes of different diameters and should not be used for that purpose at all. For every combination of different diameters, there is a correct fitting – a reducing coupling, a transition fitting, or a combination of multiple elements – and that is the correct and safe solution for the given situation.
Straight couplings of the same diameter – such as straight coupling 15 × 15, straight coupling 22 × 22, straight coupling 28 × 28, or straight coupling 16 × 16 – are indispensable for extending, repairing, and completing pipe runs within one diameter. But only there.
If you are unsure which fittings your specific project requires, we recommend reading the articles What diameter of straight coupling do I need for my heating system and Straight couplings during heating renovation: most common mistakes in ordering and purchasing before purchasing – both of these texts directly follow up on the topic we have discussed here and will help you avoid unnecessary mistakes and expenses.
Do you have a question about this topic?
Not sure how to proceed or dealing with a specific situation in your home? Write to us – we are happy to help.