What diameter of straight connector do I need for my heating system
What diameter of straight coupling do I need for my heating system?
This is one of the most common questions we encounter when selling installation materials. A customer comes in with pieces of old pipe, or just with the dimensions from memory, and wants to know one thing: which coupling to buy. A seemingly simple question – in practice, however, it hides a whole range of factors that need to be considered. The pipe diameter is just the beginning. You need to distinguish between the outer and inner diameter, the type of material, pressure conditions, and the location within the system. This article will guide you through all these aspects from the basics to specific examples from real customer cases.
Why choosing the correct diameter is crucial
It may seem that a few millimeters here and there cannot make a big difference. In practice, this is not the case. A straight coupling of the wrong diameter can lead to a whole range of problems: from constant leakage at the joint, through increased pipe noise, to mechanical stress in the system, which only becomes apparent after a few months of operation. We have seen cases where a DIY enthusiast saved two euros on an incorrect coupling and then paid five times as much for repairs – a leak behind a kitchen unit, a ruined laminate floor, or moisture in the wall.
The correct coupling diameter means, above all, that the joint holds under all normal operating conditions – including thermal expansion, pressure surges, and vibrations from the circulation pump. This is not a marketing phrase, it is the physical reality of every heating system.
Outer diameter vs. inner diameter: which dimension is stated on the coupling?
This is the first and most common source of confusion. Straight couplings for copper pipe (as found in the Hepworth system) are labeled according to the outer diameter of the pipe. So when we say a 15 × 15 coupling, we mean it fits a copper pipe with an outer diameter of 15 mm. The inner diameter (bore) is a different matter and depends on the wall thickness of the pipe.
On the other hand, threaded couplings for steel pipes usually specify a nominal size (DN or inch thread), which does not correspond at all to the actual outer diameter. This is another source of confusion, but that is the topic of another article. In the context of direct crimp or push-fit couplings for copper and plastic pipes, a simple rule applies: the number on the coupling = the outer diameter of the pipe in millimeters.
How to measure the pipe diameter at home?
If you have a caliper (slide gauge) at your disposal, it is simple: place the caliper perpendicular to the pipe and read the outer diameter. If you don't have a caliper, you can use the sliding ruler method – wrap a thin thread exactly once around the pipe, cut it, measure the length, and divide by the number π (3.1416). You get the circumference from which you can calculate the diameter. Example: the thread is 47.1 mm long → 47.1 / 3.1416 ≈ 15 mm. So it is a DN15 pipe, outer diameter 15 mm.
In practice, the following three diameters are most common in domestic installations: 15 mm, 22 mm, and 28 mm for copper pipes, or 16 mm for plastic (PEX/MLCP) pipes. Less common, but still typical, are 10 mm (thin capillary lines for some boilers), 12 mm, and 35 mm (main horizontal distribution in older houses).
Overview of common diameters and where they are used
The following table summarizes the most common pipe diameters in domestic heating systems and their typical use. It is based on practice, where we have seen hundreds of installations from simple apartment circuits to large family homes with multiple boilers.
| Outer diameter | Typical use | Typical circuit |
|---|---|---|
| 10 mm | Capillary lines, some connecting hoses | Boiler → safety valve, vent circuit |
| 12 mm | Connection of individual heating units | Radiator, convector |
| 15 mm | Distribution to radiators, connecting lines | Standard apartment/house circuit |
| 16 mm | PEX/MLCP plastic pipe, underfloor heating | Underfloor loops, manifold |
| 22 mm | Main horizontal distribution, TÜV tank | Boiler → manifold, tank |
| 28 mm | Supply/return pipe of the boiler, larger tank | Boiler, heat pump, solar |
| 35 mm | Large connecting circuits, heat exchangers | Commercial buildings, older installations |
Diameter 15 mm: the most common in households
If you have a standard family house or an apartment with classic steel or aluminum radiators, there is a high probability that you have copper pipe with a diameter of 15 mm in your walls. This is the standard size for distributing heat to individual heating units. This diameter can handle the flow required for a standard radiator with a power of up to approximately 1.5–2 kW without significant pressure drop.
A straight coupling 15 × 15 is intended for this size. It is used when extending an existing pipe, when inserting a tee (if you need to add a branch), when repairing a cracked section, or when connecting two circuits. In practice, these are the most frequently purchased couplings of all diameters.
Typical situations where you need a 15 mm coupling
- Extending the pipe when moving a radiator (e.g., due to window reconstruction)
- Repairing a mechanically damaged section – for example, after drilling into the wall
- Connecting a new radiator to an existing system
- Replacing an old or corroded section of copper pipe
- Extending a branch when adding a room or a winter garden
Diameter 16 mm: beware of confusing it with 15 mm
This is one of the most common practical mistakes. A difference of just one millimeter – but the differences are significant. A 16 mm pipe is plastic (PEX, MLCP – layered aluminum-plastic, PEX-AL-PEX, etc.) and is commonly used in floor heating systems or in combined systems. This pipe cannot be connected using a coupling intended for 15 mm copper pipe – not even with mild force, as the sealing geometry and clamping mechanisms are different.
Therefore, if you have a floor heating system with plastic pipe, you need a straight coupling 16 × 16. This coupling is specifically designed for plastic pipe with an outer diameter of 16 mm, with a different clamping mechanism and sealing geometry compared to copper pipes.
In practice, it sometimes happens that a customer buys 15 mm couplings instead of 16 mm ones during a renovation rush, thinking that "one millimeter is not a problem." It is a problem. The seal does not fit properly, and either the connection cannot be closed, or it closes but leaks under pressure. If you are unsure, measure the pipe with a caliper.
Diameter 22 mm: main horizontal distribution
A 22 mm pipe appears as the main thermal artery – that is, the line from the boiler to the manifold, from the TÜV tank, or as a connecting loop between two consumer points. In a standard family house with five to eight radiators, this pipe is capable of transferring a power of 8–15 kW without flow issues.
Situations where you are looking for a straight coupling 22 × 22 are typically these: boiler replacement and the need to extend or modify the connection, adding a TÜV tank, installing a circulation pump on the main loop, or connecting two circuits (e.g., a solar circuit feeding a tank). In renovation practice, these are the connections where an incorrect coupling can cause the most damage, as a significantly larger volume of water flows through this pipe than through the branches to the radiators.
Diameter 28 mm: boiler and large heat source connections
A diameter of 28 mm is borderline in domestic installations – you will rarely encounter it in standard apartment practice. It is used for direct connection of condensing boilers, heat pumps, solar heat exchangers, or accumulator tanks. This pipe must handle high flows at relatively low speeds (to avoid noise and erosion).
A straight coupling 28 × 28 is purchased less frequently, but when you need it, you need the correct one. An incorrect choice at this diameter has immediate and significant consequences: under the working pressure of the boiler (typically 1.5–3 bar), an unsuitable or improperly installed coupling may leak immediately or after the first filling. At this diameter, we always recommend double-checking the correct insertion and securing before filling the system.
How to correctly determine the required diameter step by step
The following procedure is based on practice and will help you reliably determine which coupling diameter you need, even if you do not have the technical documentation for the installation at hand.
Step 1: Measure the outer diameter of the pipe
Use a caliper. Place it perpendicular to the pipe axis and read the value. If the pipe is insulated, expose a short section. Measure at a point where the pipe is not damaged or deformed. If you don't have a caliper, use the string method (described above). Always measure at least at two places and take the average.
Step 2: Determine the pipe material
Copper pipe has a characteristic golden-brown/orange-brown tint, or greenish patina if it is old. It is rigid and sounds metallic when tapped. Plastic pipe (PEX, MLCP) is white, gray, or has visible layers when viewed at the cross-section. It is more flexible. Steel (galvanized) pipe is gray and heavier. The type of material determines which type of fitting you will use (crimp for copper, compression or push-fit for plastic).
Step 3: Check the location in the system
Is it a connection to a radiator, or the main distribution line? Radiator branches are almost always 15 mm. The main line is 22 or 28 mm. For floor heating, it is 16 mm (plastic). This will help you verify whether the measured number matches your expectations – and if not, measure again.
Step 4: Check the type of existing fittings
If there are any fittings (elbows, tees) at the location, look for the number stamped or printed on them. Manufacturers usually mark the diameters directly on the fitting. This is the fastest way to verify without measuring.
Most common mistakes when choosing the fitting diameter
Over the years of practice, we have repeatedly seen these mistakes. We list them here so you can avoid them:
- Mixing up outer and inner diameter: The customer measures the inside of the pipe (e.g., 13 mm) and buys a 13 mm fitting – but it doesn't exist, or it is a different type. Always measure the outer diameter.
- Mixing up 15 mm and 16 mm: Visually almost indistinguishable, but functionally incompatible. In any case of doubt, always use a caliper.
- Using a threaded fitting where a crimp fitting is needed: A different topic, but when ordering from a catalog, people sometimes get confused in the order form. Pay attention to the type, not just the size.
- Ignoring pressure parameters: Even if the diameter matches, the fitting must be sized for the pressure in your system. Common domestic systems operate up to 3–4 bar, but pressure spikes after the boiler can be higher. Check the maximum pressure the fitting can withstand.
- Purchasing "off the shelf" without measuring: "I have a 15 mm at home and I need the same, so I buy a 15 mm" – works in 90% of cases, but the remaining 10% can lead to an expensive reconstruction.
- Using a fitting from a different system (e.g., for plastic pipe on a copper pipe): Materials may be dimensionally compatible, but the clamping and sealing mechanism is different. Never mix categories.
Pressure and temperature parameters: what to pay attention to
Selecting the correct diameter is a necessary condition – but not sufficient. Each fitting also has a maximum working pressure and maximum temperature. For domestic heating, the relevant values are: working pressure usually 6–10 bar (for most Hepworth crimp fittings), maximum temperature 110 °C for systems with a check valve, or 150 °C for pure copper crimping without plastic parts. Condensing boilers typically operate at 60–80 °C on the supply line, which is within the safe range for all standard direct fittings.
Problems arise in solar circuits – there, the temperature can rise to 150–180 °C during stagnation (when solar collectors overheat without transferring heat). In such cases, you need to use fittings certified for a higher temperature range and avoid using fittings with plastic parts in this circuit.
A more detailed discussion of this topic is covered in a separate article How to choose the right direct fitting for the pipe: diameter, material and pressure parameters, where you will find a complete overview of technical limitations for different types of fittings.
Hepworth fittings vs. generic ones: what is the difference in terms of diameters
Diameters are standardized according to European standards (EN 1254), so a 15 mm is 15 mm for every manufacturer. Where they differ is in the precision of production tolerances. Hepworth fittings have tight manufacturing tolerances, meaning that with a properly cut and de-burred pipe end, they fit precisely and the sealing is reliable. Cheaper generic fittings may have looser tolerances – the fitting may not resist when inserting (which seems more convenient), but this loose fit is a source of future leaks. More on this topic can be found in the article Hepworth direct fittings vs. other brands: what they differ in and what to choose.
Combining different diameters in one system
Real heating systems are not one-dimensional. In one house, you will find 28 mm from the boiler, 22 mm horizontal distribution, and 15 mm to the radiators. At the transition points between diameters, reducing fittings (e.g., 22 × 15) are used, not direct fittings of the same diameter. If you are dealing with such a situation, a direct fitting 15 × 15 or 22 × 22 will not help – you need a reducing fitting.
At the same time, copper and plastic pipes of different diameters cannot be directly connected with a standard direct fitting. Special transition fittings are used for this. This entire issue is thoroughly discussed in the article Can I connect pipes of different diameters with a direct fitting, where specific combinations and practical solutions are also presented.
Practical examples from real customer cases
Case 1: Repair after drilling into the wall
The customer drilled a hole for a cable and hit a copper pipe in the wall. Water started leaking into the structure. After uncovering, it turned out to be a radiator branch – diameter 15 mm, copper pipe. Solution: we cut out the damaged section (about 8 cm), used two direct fittings 15 × 15 and inserted a patch piece from a new copper pipe. The connection was completed in 45 minutes, including re-insulation and filling. This is a textbook case where a direct fitting 15 × 15 saved the situation.
Case 2: Adding a TÜV tank
The customer wanted to connect a 200-liter tank to an existing condensing boiler. The boiler's connection pipe had a diameter of 28 mm. The tank was also dimensioned for 28 mm. Since the tank was a bit further from the boiler than originally planned, the pipe had to be extended by 1.2 m. Solution: direct fitting 28 × 28 and extension. No problems, but the customer originally came with a request for a 22 mm fitting, thinking it was the "normal" size for boilers. Measuring showed 28 mm.
Case 3: Renovation of a bathroom with floor heating
During the bathroom renovation, the floor heating loop had to be moved. The pipe was PEX 16 mm. The customer bought fittings himself – and came with 15 mm fittings, because the store told him "for heating it is 15 mm". We had to return and exchange them for the correct 16 mm fittings. He lost a day of work. Always measure, always verify the material.
Case 4: Solar circuit and special requirements
The customer installed solar collectors. The circuit was dimensioned for 22 mm copper pipe. Problem: the temperature during stagnation could reach 160 °C. We used a crimp fitting with a high-temperature gasket – same diameter 22 mm, but a specially certified fitting. A standard 22 × 22 would not have been sufficient here.
FAQ – most frequently asked questions
How can I determine the pipe diameter if I don't have access to it (it is in the wall)?
If you cannot expose the pipe, look at existing fittings or valves nearby (they often have the size stamped on them), or check the building or boiler documentation. If that is not possible, estimate based on the purpose: a radiator branch is almost always 15 mm, the main line is 22 or 28 mm. In the worst case, order one size larger than you think – better to return for another size than to go twice.
Can I use a 15 mm fitting on a pipe with an outer diameter of 14.8 mm or 15.2 mm?
European standards for copper pipe (EN 1057) specify tolerances for the outer diameter of ±0.1 mm for standard building pipe. This means that a pipe labeled as 15 mm can have an actual outer diameter of 14.9–15.1 mm – and that is acceptable. The fitting is designed for this. If you have a pipe with a diameter of 14.5 mm or 15.5 mm, it is a different nominal size and a different type of fitting is needed.
What is the difference between a 15 × 15 coupling for copper and a 16 × 16 coupling for plastic, when the difference is only 1 mm?
The difference is not only in the diameter, but also in the construction. The coupling for copper pipe is designed for rigid, dimensionally accurate pipe with thin walls (approx. 1 mm). The coupling for plastic pipe takes into account a thicker wall (2–2.5 mm), higher material flexibility, and a different clamping mechanism. Therefore, mutual substitution does not work even with a small dimensional difference.
What diameter do I need when renovating an old installation with welded steel pipe?
The older welded system (steel pipe with an outer diameter of 21.3 mm for nominal bore DN15, or 26.9 mm for DN20) cannot be directly connected to a copper compression coupling without an adapter. You will need a transition fitting (copper-steel or threaded-copper). In practice, when renovating such systems, the entire section is usually replaced with copper pipe and the appropriate couplings, which is cheaper and more reliable than adapting.
Where on the coupling can I find the marked diameter if I forgot it when purchasing?
On almost all Hepworth fittings, the diameter is stamped or printed directly on the body of the coupling – usually in the middle part (between the two necks). Look for numbers such as "15", "22", "28", or "15×15". If the text is unreadable (for example, on old corroded fittings), measure the inner diameter of the coupling neck with a caliper and add the thickness of the gasket – or more simply, insert a piece of old pipe into the neck and check which dimension fits without play.
Is there any visual way to distinguish between 22 mm and 28 mm diameters without measuring?
Yes, in practice – 28 mm is clearly thicker at a glance. A 22 mm pipe fits into a closed hand so that the fingers almost touch, while a 28 mm pipe is noticeably looser. This is only an approximate test – we always recommend using a caliper. If you are buying online, it is better to double-check the measured diameter rather than rely on a visual estimate.
Conclusion: the correct diameter is the basis of a functional connection
Selecting the correct diameter of a straight coupling is not complicated if you proceed systematically: measure the outer diameter, identify the pipe material, and verify where the coupling is located in the system. For standard home installations, a 15 × 15 coupling covers most needs for radiator branches, a 22 × 22 coupling for main distribution lines, and a 28 × 28 coupling for direct boiler connections and larger heat sources. For plastic floor heating pipe, choose a 16 × 16 coupling.
If you want to go deeper into the topic – whether choosing between a compression and threaded coupling, renovating an old system, or solving a leaking connection – we recommend further articles in this Knowledge Center: Difference between compression and threaded straight couplings: when to use which, Tightness of straight couplings: why the connection leaks and how to fix it, and Straight couplings in heating renovation: most common mistakes in measurement and purchase. These articles will help you navigate the issue not only dimensionally, but also technologically and materially.
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.