How to properly connect pipes of different diameters without water leakage
How to properly connect pipes of different diameters without water leakage
Anyone who has ever dealt with the renovation or expansion of a heating distribution system will sooner or later encounter the same problem: connecting two pipes that simply do not have the same diameter. This may involve connecting a new radiator to an older system, extending a branch from a riser of a different size, or integrating floor heating into an existing system. In each of these cases, it is crucial not only to choose the right type of fitting, but above all to install it correctly – so that the connection lasts for years of operation without a single drop of leakage.
In this article, we will look at the entire issue in depth: from the physical principles of the joint's tightness, through specific dimensions and the selection of the appropriate coupling, to the installation procedure and the most common mistakes we see in practice on customer jobs. We will focus primarily on the Hepworth crimping system, which forms the basis of most modern heating distribution systems today, but the principles apply more broadly.
Why water leaks occur precisely at transitions between different diameters
Before we get into the solutions, it is important to understand why reduced joints are potential problem areas. Several factors act together physically.
Thermal expansion is the first and most important factor in heating distribution systems. A copper pipe of 5 meters in length will extend by approximately 5.6 mm when heated from 20 °C to 80 °C. This means that each joint in the system is working – alternating between compression and relaxation. If the connection is geometrically inaccurate, this movement will gradually deepen the sealing wear over time.
Pressure in the system – a standard closed heating system operates at a pressure of 1.5 to 2.5 bar. During hydraulic pressure testing of the system (leak test), the pressure is usually increased to 1.5 times the operating pressure, i.e. up to 4 bar. A reduced coupling must be able to withstand these loads without deforming the seat geometry.
Different wall thickness of the pipe at different diameters causes the mechanical stiffness of both ends of the joint to be different. It is then easier for the operator to prefer the "stronger" end during installation and not fully crimp the weaker one into the stop.
In the diagram you can see what happens inside a typical straight reduced coupling. Two things are key: the geometry of the reduction cone, which smoothly transitions from a larger to a smaller diameter and also serves as a stop when inserting the pipe, and the position of the O-rings, which must always be under pressure when the pipe is correctly inserted – only then can they ensure tightness.
Types of pipe connections of different diameters – an overview of available solutions
Before you reach for a specific fitting, it is good to know what options you have. A straight reduced coupling is not always the best solution – it depends on the installation context.
Straight reduced coupling (crimped or with O-ring)
The cleanest and most commonly used solution for systems made of copper, crimped systems (Hepworth, Viega, Sanha), or PEX/composite pipes. It connects two pipes of different diameters directly, without the need for transition pieces or threads. The advantage is compactness, low hydraulic loss and reliability when installed correctly.
Combination of two reducing bushings with threads
An older solution that is still found in renovated houses from the 80s and 90s. A threaded bushing is attached to each side, with a short nipple in between. Disadvantage: more joints = more potential leakage points, and in addition, threaded connections with hemp sealing or Teflon require long-term inspection.
Reduced reducer with internal thread
Used mainly where one side transitions to threaded fittings (valves, ball valves). In such a case, it is not a direct reduced coupling in the true sense of the word – it is a transition piece.
Welded or soldered joint
Traditional heating with hard-soldered copper still exists. A properly made soldered reduction joint is extremely reliable, but it requires an experienced plumber, suitable conditions (oxygen supply, safety), and after installation it is not removable without damage. In today's practice, crimped joints have largely replaced it.
For a more detailed comparison of individual solutions and when to use which, I recommend the article When to use a reduced coupling and when to use another type of reducing fitting, where you will also find a clear decision-making scheme.
Most commonly used dimensions and their practical meaning in heating distribution systems
Heating systems in single-family homes and apartments typically work with these average combinations of diameters. Let's go through them from the perspective of real practice.
Transition 28 × 22 mm
This is a classic combination for the main distribution loop. Boilers or heat pumps usually come with a G1" or G1¼" connection, which corresponds to outer pipe diameters of about 28–35 mm. The main distribution route is usually run in DN28 (outer diameter 28 mm, wall 1 mm for copper), and the transition to DN22 is made for branching off to individual radiator circuits. Straight reduced coupling 28 × 22 is therefore one of the most commonly used dimensions on every larger project. In practice, you will find it on every manifold or at the point where a section circuit branches off from the main riser.
Transition 22 × 15 mm
This is another very common combination – a transition from the distribution branch (DN22) to the connection of a specific radiator or group of radiators (DN15). A diameter of 15 mm is standard for radiator connections in new builds and renovations. Straight reduced coupling 22 × 15 therefore appears literally on every project where a new radiator circuit is being installed. A single family house with five radiators can easily consume five to ten of these couplings.
Other common dimensions
Depending on the system, you may also encounter transitions 35 × 28 mm (for boilers with larger heat exchangers), 22 × 18 mm (in older systems with pipes according to the German standard), or 15 × 12 mm (for underfloor heating with low-profile pipes). Dimensions and their correspondence with fittings are discussed in more detail in the article Dimensions of straight reduced couplings: which size do you need.
Material properties and their impact on joint tightness
One of the things customers often underestimate is the material of the fitting and its compatibility with the pipe. Not every coupling is suitable for every system.
Brass crimp fittings (e.g. Hepworth)
Brass (CuZn) is a long-proven material in heating. It has good resistance to temperatures up to 110 °C (which covers condensing boilers with higher peak temperatures), good mechanical properties, and does not corrode in a closed heating circuit with treated water. Hepworth fittings are designed for crimping with special tools, where the crimping profile ensures an accurate and repeatable joint. The O-rings are EPDM, which ensures resistance to temperatures and chemical additives in heating water (corrosion inhibitors, glycol up to a certain concentration).
Stainless steel crimp fittings
These are mainly used in systems where there is a higher risk of sedimentation (lake house) or where the water is more aggressive. They are more expensive, but they do not react to electrochemical corrosion with copper pipe as problematically as some cheaper alloys.
Plastic reducing couplings (PPR, PEX, PE-RT)
For underfloor heating systems or when using plastic piping, plastic fittings are used – either crimped with a metal core or welded (polyfusion). Different installation rules apply to these materials, but the principle of correct insertion of the pipe to the stop remains the same.
A detailed comparison of materials and pressure parameters can be found in the article Material and pressure parameters of straight reduced couplings for central heating.
Step by step: correct installation of a straight reduced coupling
Almost every leak I have encountered on a complaint case had the same origin: either the pipe was not inserted to the stop, or the crimping tool profile was incorrect, or the pipe surface was not sufficiently prepared before insertion. Here is a detailed procedure that eliminates these fixable errors.
1. Pipe preparation
The pipe must be cut to the required length with a straight-cutting pipe cutter – not with a mechanical saw, which leaves burrs and an uneven cut. The end of the pipe is cleaned by reaming or deburring. Any sharp edge or burr can damage the O-ring during insertion, which leads to a leak. If you are working with copper pipe, it must have an outer diameter within the tolerance of ±0.1 mm according to EN 1057 – do not buy questionable pipe where the diameter varies along the length.
2. Measuring the insertion depth
Each crimp fitting has a specified insertion depth – for DN15 it is typically 20–22 mm, for DN22 about 25–27 mm, for DN28 about 28–30 mm (depends on the manufacturer, always check the installation documentation). Mark the depth on the pipe with a marker or special pen. This step is often skipped in a rush – and it is the most common cause of later leaks.
3. Inserting the pipe into the fitting
Insert the pipe straight (not at an angle), evenly, without wobbling. Most Hepworth fittings have an indicator window or recess where you can see if the pipe is inserted to the correct depth. If this check is missing, check whether the marked line on the pipe matches the edge of the fitting. The pipe must sit firmly – it must not be loose or tilted.
4. Crimping
Perform crimping using approved crimping tools (not pliers or other tools). The jaw profile must exactly match the profile of the fitting (for Hepworth this is usually an M-profile or V-profile – see the catalog). One crimping stroke compresses the fitting profile into a precise grip around the pipe and at the same time compresses the O-ring into a defined position. After crimping, visually check whether the profile is symmetrical and whether the deformation is outside the joint area.
5. Inspection before commissioning
Before embedding or covering the piping, always pressure test with cold water at 1.5 times the operating pressure for at least 30 minutes. Inspect each joint by touch (using dry hands, not gloves) and visually. Perform thermal loading (the first heating of the system) carefully – slowly increase the temperature and re-inspect the joints after 20 minutes of operation.
A detailed illustrated installation guide can also be found in the article Installation of a direct reduced coupling step by step, where these steps are elaborated for various types of crimping tools.
Hydraulic consequences of diameter reduction – what to know when dimensioning
A reduced coupling is not a neutral element from a hydraulic perspective. Any change in pipe diameter means a change in flow velocity and pressure loss. This is important when dimensioning the entire piping system.
For orientation: if you accelerate water from DN22 (cross-section approx. 380 mm²) to DN15 (cross-section approx. 177 mm²), the flow velocity almost doubles. According to the Bernoulli equation, this also means an increase in local dynamic pressure loss. For standard flow rates in residential heating (up to 0.3 m³/h per circuit), this loss is negligible and will not affect the system's function. Problems arise when you design a long piping system with multiple reductions in sequence without compensating with balancing valves.
The golden rule for practice: reduce the diameter where the branch loses capacity (feeding fewer radiators), not earlier. If you reduce too early, you increase the pressure loss across the entire branch and the end radiators will be underheated.
The graph clearly shows the exponential nature of the dependence – the transition from DN22 to DN15 brings about a 2.3-fold increase in pressure loss per meter. Therefore, it is important that the DN22×15 coupling is placed as close as possible to the radiator (not at the beginning of a 6-meter run), and that the overall hydraulics are properly balanced.
Most common mistakes when connecting pipes of different diameters
Over the years on site, I have seen the same mistakes repeated again and again. I list them here so you can consciously avoid them.
Mistake 1: New product, but old O-ring
Fittings are sometimes stored for a long time or purchased from unreliable sources. EPDM O-rings degrade under UV radiation and ozone – if the fitting has been lying in the sun in a car or in an unheated warehouse for a year, the ring may be brittle and microscopically damaged during crimping. Solution: purchase fittings from verified suppliers and check whether the packaging is intact.
Mistake 2: Unbeveled pipe end
This is probably the most common cause of leakage after installation. A sharp edge can cause micro-cuts in the O-ring, which will fully manifest only after the first temperature cycle. In practice, this means: the boiler room is not in operation in summer, heating is turned on in September, and the next day the customer calls to say it is leaking. Solution: after each cut, deburr the pipe using a double-action deburring tool both from the outside and inside (although the inside is less critical for sealing).
Mistake 3: Wrong profile of crimping jaws
Hepworth uses a specific profile for the crimped joint. If you use jaws intended for another series (e.g., Viega Profipress), the crimping geometry will be different and the O-ring will not be compressed to the correct value. This is not always visible at first glance – the joint may look fine, but it will start to leak under pressure and heat. Solution: always use tools certified for the given system.
Mistake 4: Mechanical stress on the joint immediately after crimping
If you bend, pull or otherwise mechanically stress the crimped joint immediately after installation (for example, in a tight free space you lean against the pipe), you may disrupt the position of the O-ring before the system stabilizes. Solution: leave the joint without mechanical stress for at least 5 minutes, better longer, and then pressure test the system.
Mistake 5: Skipping depth control for reduced couplings
A direct reduced coupling has a different internal diameter on each side. This often leads to the technician pushing the pipe only from one side to the stop and leaving the other side in a half-position when routine work is being done. Solution: always mark both pipes before installation and check both sides independently.
Further practical tips on the topic of leakage and how to prevent it can be found in the article Joint tightness: common problems and how to avoid them.
Special situations in practice
Connecting pipes of different materials (copper to plastic)
In practice, you quite often encounter the need to connect a copper pipe to a plastic one, for example during a renovation where the existing piping is in copper and the new branch is made of PEX. A direct reduced coupling alone is not sufficient for this – you need a transition fitting (copper–PEX adapter) and then a possible diameter reduction after it. An alternative is to use a reduced coupling with a combined connection (one side is copper crimping, the other side is PEX). These combined fittings exist and are a solution "in one place".
Connecting in hard-to-reach places
What to do if you need to crimp in a place where the crimping tool physically won't fit? In such cases (e.g., in a wall cavity or a narrow installation shaft), use crimping tools with extended jaws or special right-angle adapters for the machine crimping tool. Never replace crimping with "clamping with pliers" – such a connection is not reliable.
Reconstruction of an existing leaking joint
If you have an existing joint that is leaking water and it is a crimped fitting, you cannot simply patch it or pull it tight – you must replace it. Crimped fittings are one-time use only: after crimping, they are cut off with special pliers (removal pliers) or cut off, and replaced with a new fitting. That is why it is important to have a few spare pieces on hand for each job.
Choosing the right coupling according to the specific situation
To make quick decisions, here are some brief practical rules:
- Branching a new circuit from an existing DN28 riser to a smaller diameter: use a straight reduced coupling 28 × 22 directly at the output from the riser or distributor.
- Connecting a radiator to a DN22 distribution branch: use a straight reduced coupling 22 × 15 at the point where the branch leads to the radiator supply pipe.
- Connecting pipes made of different materials: use a specialized transition fitting, not a standard straight reduced coupling.
- Reduction is part of the fitting (e.g., a radiator valve with a reduced inlet): a separate coupling is not needed – the fitting itself handles the diameter reduction.
- You need to reduce by more than one step (e.g., 28 to 15): it is recommended to use two couplings in sequence (28×22 and 22×15) instead of one extreme reduction – this reduces mechanical stress on the joint and improves hydraulic transition.
For more information on strategic fitting selection, see the articles How to choose the right straight reduced coupling for heating and Hepworth couplings vs. other brands: what to watch out for when choosing.
Long-term reliability and maintenance of couplings in heating systems
A properly crimped fitting should last the entire lifetime of the heating system – at least 25–30 years under normal operation. Despite this, there are factors that affect long-term reliability.
The quality of heating water is key. Hard water (with high calcium and magnesium content) gradually forms deposits on the internal surfaces of fittings and pipes, which can lead to local underpressure flow and vibrations in extreme cases. Using treated water (pH 7.5–9, total hardness max. 3–4 °dH, corrosion inhibitor according to the boiler manufacturer's recommendation) eliminates these problems.
Regular system pressure checks allow you to detect any leak before it causes damage. Monitor the pressure gauge on the expansion tank – a drop in pressure by more than 0.2 bar per month when the system is off is a signal to check the joints.
Annual visual inspection of all accessible joints (in the boiler room, in the pipes running behind plaster in inspection panels) should be standard. Look for damp spots, salt deposits or rust – these are indicators of micro-leaks.
A systematic approach to maintenance is described in the article Maintenance and inspection of couplings in heating systems.
Frequently asked questions (FAQ)
Can I connect 22 mm and 15 mm pipes using two different threaded fittings instead of a straight reduced coupling?
Technically yes, but it is not recommended for closed heating systems. Threaded connections must be sealed with hemp or Teflon tape, which can gradually lose tightness during repeated temperature cycles. Moreover, each threaded connection is a potential leak point and requires regular inspection. A straight reduced coupling of the crimped system is more reliable, compact and less maintenance-intensive in the long run.
What to do if I don't have a crimping tool and need to perform a quick repair?
For emergency repairs, there are self-sealing fittings (e.g., Speedfit or similar systems), where the pipe is simply inserted and the seal is ensured mechanically without crimping. These solutions are intended for temporary use or for low-pressure applications, not as a long-term solution in central heating under full pressure. If you don't have access to a crimping tool, rent one or call a certified heating technician – it is cheaper than repairing the damage after a water leak.
What is the maximum temperature and pressure at which I can use a straight reduced Hepworth coupling?
Typical brass crimped Hepworth fittings are certified for temperatures up to 110 °C and pressure up to 16 bar (PN16). In standard central heating, you operate at 75–80 °C and 2–3 bar, which is well within the safe zone. For solar thermal systems (where temperatures may briefly exceed 120 °C), special fittings with high-temperature O-rings must be used – standard EPDM seals may not be suitable for these applications.
Do I need to use sealing compound or tape when installing a straight reduced coupling?
No. Crimped couplings have integrated EPDM O-rings that ensure airtightness without any additional sealing. Adding Teflon tape, hemp or sealing compound to a crimped joint is incorrect and can actually cause problems – foreign material may prevent proper seating of the O-ring or cause its chemical degradation.
How do I know that the crimping was done correctly?
After crimping, check: 1) the crimp profile is symmetrical around the entire circumference of the fitting, not deformed on one side, 2) the pipe cannot be easily pulled back (with proper crimping, this is not possible without special tools), 3) the inspection window (if the fitting has one) shows the correct insertion depth, 4) a pressure test with cold water shows no leaks or moisture.
Is there a difference in installation between vertical and horizontal piping?
From the perspective of crimped coupling installation, there is no difference – the procedure is the same. It is important, however, to ensure the correct slope of the pipe in horizontal runs (minimum 3–5 mm/m towards the drain valve), so the system can be drained. A reduced coupling placed at the lowest point in the run may be in a difficult position when filling the system – air may get trapped, which is not a problem with the coupling itself, but a design issue with the piping layout.
Conclusion: a reliable connection is not a coincidence
Connecting pipes of different diameters without water leakage is not black magic – it is a combination of correct fitting selection, careful preparation by the plumber and strict adherence to the installation procedure. The straight reduced coupling of the crimped Hepworth system is part of the standard in modern heating precisely because it offers an extremely reliable connection with minimal maintenance requirements when installed correctly.
Key principles in summary: always insert the pipe to the stop and check the insertion depth, use the correct crimping tool for the system, deburr each end of the pipe before installation, and pressure test the system before covering the piping. These four steps guarantee that you won't have to return to the joint in years to come.
The full range of straight reduced couplings including other dimensions can be found in the straight reduced couplings category on atria.sk.
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.