How to choose the correct Hepworth corner elbow according to pipe diameter
Why the pipe diameter is the most important parameter when choosing an elbow
When ordering Hepworth elbows, I often encounter one recurring mistake in practice – the customer chooses the fitting based on the bend angle or based on "how it looks in the photo", and the diameter is only considered as an afterthought. The result is always the same: the elbow either does not fit on the pipe, or it fits too loosely and the connection will not be tight. An elbow is a passive element of the piping system, which must accurately match the outer diameter of the pipe it is being soldered or crimped onto. It is not a component where a tolerance of a few dozen micrometers does not matter – quite the opposite, a soldered copper joint only works properly if the gap between the inner diameter of the fitting and the outer diameter of the pipe is within the range of capillary action of the solder, typically 0.05 to 0.2 mm around the entire circumference.
Hepworth produces its elbows in precise nominal diameters that correspond to the European standard for copper pipes used in heating and water supply systems. That is why you encounter designations such as 15x15, 22x22, or 28x28 in this category – the first number and the second number indicate the diameter at both ends of the elbow, because it is a symmetrical fitting with two identical sockets.
Hepworth elbow sizing system – what do the numbers 15, 22 and 28 mean
The numerical designation of Hepworth elbows always indicates the outer diameter of the copper pipe in millimeters, onto which the fitting is mounted. It does not refer to the inner diameter of the pipe or to the nominal bore DN, but to the actual outer dimension measured with a caliper on a straight part of the pipe. This system is common throughout Europe for copper piping and Hepworth strictly follows it, which facilitates compatibility with other manufacturers of copper fittings.
Below is a simple cross-sectional diagram of a pipe, showing exactly where the diameter is measured and why it is important to distinguish between the outer and inner diameter:
It is precisely this outer diameter D that must exactly match the inner diameter of the elbow socket. If you are unsure what diameter of pipe you have installed at home or on the construction site, I recommend reading the separate article "Dimensions of Hepworth elbows 15, 22 and 28 – what diameter do I need", where this topic is discussed in detail, including a conversion table to inches.
Why Hepworth uses exactly these three diameters
The dimensions 15, 22 and 28 mm are not random – they correspond to the most common diameters of copper pipe used in residential and family heating systems in Europe. 15 mm is used for end branches to radiators and smaller appliances, 22 mm for main risers and distribution branches in family homes and smaller buildings, and 28 mm for main lines with higher flow, for example in boiler circuits, collectors or in larger apartment buildings. In the atria.sk range you will find exactly these three basic sizes, which cover the vast majority of standard orders.
How to correctly measure the diameter of an existing pipe
If you are renovating an older system or adding piping and you do not have documentation available, the most reliable method is physical measurement. I recommend the following procedure, which I use regularly during site inspections:
- Measure the pipe with a caliper on a straight, undamaged section, at least 10 cm away from any joint, bend or fitting – the material is often slightly deformed near joints.
- Measure in two perpendicular planes, since older pipes may have a slight oval shape – round the diameter to the nearest whole millimeter upwards.
- If you do not have a caliper, place a standard measuring tape around the pipe and divide the measured circumference by the number 3.1416 (pi) – you will get a diameter accurate enough to determine the type of elbow.
- If you are unsure between two sizes (for example 15 vs. 18 mm), always try to dry-fit the elbow on the pipe before buying a larger quantity – the fitting should slide on with minimal resistance, not wobble loosely or require hammering.
Overview of Hepworth elbows by diameter and recommended use
To quickly orient yourself on which diameter corresponds to which type of system, I have prepared a clear table based on common practice in the installation of heating systems in family homes and apartments:
| Elbow diameter | Typical use | Typical flow rate | Product |
|---|---|---|---|
| 15 x 15 mm | Connections to radiators, end branches, bathroom circuits | up to approx. 0.3 l/s | 90° elbow 15 x 15 |
| 22 x 22 mm | Main distribution branches in family homes, risers | 0.3 – 0.8 l/s | 90° elbow 22 x 22 |
| 28 x 28 mm | Boiler circuits, collectors, main lines in larger buildings | 0.8 – 1.5 l/s | 90° elbow 28 x 28 |
This table is only indicative – the actual flow rate also depends on the speed of the medium flow and the hydraulic calculation of the entire system. If you want to examine the topic of dimensions in more detail, including a comparison of the advantages and disadvantages of each diameter in specific situations, I recommend the article "Comparison of 15x15 vs 22x22 vs 28x28 elbows – where to use which one."
Practical scenarios – when to reach for which diameter
Scenario 1: Apartment renovation with radiator connections
In a typical apartment with a panel or brick core, I often encounter the situation where the main line to the riser is run in 22 mm pipe, but the last section to the radiator is reduced to 15 mm. In such a case, you will need a combination of 90° elbow 22 x 22 on the main branch and 90° elbow 15 x 15 on the end connection to the radiator body. The transition between diameters is handled via a reducer, not via an elbow – this is a common mistake when the installer tries to "solve everything with one piece" and the elbow becomes deformed or leaks.
Scenario 2: New family home construction with a boiler room
In larger family homes with boiler output exceeding 20 kW, the main line from the boiler to the manifold is often run in 28 mm pipe to reduce pressure losses and noise from the flow. Here, the 90° elbow 28 x 28 is used, especially in the mechanical room, where the piping is often run in right angles along the wall or around the boiler.
Scenario 3: Minor repair or extension of an existing piping system
When repairing older piping systems where the piping is already installed, it is crucial to measure the diameter on-site, as older buildings may have atypical dimensions (for example, inch-based piping with diameters close to 15 or 22 mm, but not identical). In such cases, I always recommend first verifying compatibility on a short piece of pipe before ordering a larger quantity of fittings.
What happens when the elbow diameter does not match – practical consequences
An incorrectly selected diameter of a right-angle elbow will manifest in one of two ways in practice, and both are problematic in terms of the long-term functionality of the system.
If the internal diameter of the elbow is larger than the external diameter of the pipe, the joint will have too large a gap for the solder to capillary rise evenly around the entire circumference. The result is a joint that appears well-soldered at first glance, but in reality contains microscopic voids that will become apparent after several weeks or months as slow leaks or dripping at the joint – typically during temperature changes and thermal expansion of the piping.
If, on the other hand, the internal diameter of the elbow is smaller, the fitting cannot be mounted on the pipe without using force, which often leads to microscopic deformations of the copper, damage to the surface layer, or even cracking of the fitting during installation. In the case of crimped joints, this also causes the crimping jaws to not create even pressure around the entire circumference, and the joint will not meet the declared pressure resistance.
More detailed causes of leaks, including those not directly related to diameter (such as insufficient surface cleaning or incorrect soldering temperature), are discussed in the article "Common problems and leaks in right-angle elbows – causes and solutions."
Reductions and combination of different diameters in one piping system
In most real heating systems, the pipe diameter gradually decreases from the boiler toward individual radiators – this is a common and correct hydraulic practice. The main branch from the boiler may be 28 mm, the distribution branch on the floor 22 mm, and the radiator connection 15 mm. It is important to note that the elbow itself is never used to change the diameter – this is the role of reductions (reduced couplings), which are installed either before or after the elbow, depending on where the direction change is needed and where the diameter change is required in the space.
A common mistake I encounter during inspections of completed projects is the attempt to "save" one fitting by slightly expanding or narrowing the elbow by hammering. This significantly reduces the mechanical strength of the joint, and in systems with operating pressure above 2.5 bar (common in high-rise buildings or with an expansion tank under higher pressure), there is a real risk of failure precisely at this location.
Difference between selecting diameter and selecting the bend angle
Diameter and bend angle are two completely independent parameters of a right-angle elbow, and it is advisable to verify both simultaneously when ordering. While the diameter determines which pipe the fitting fits onto, the angle determines the change in direction of the route. A standard Hepworth right-angle elbow is manufactured at a 90° angle, which covers the vast majority of typical jobs when running piping along walls and room corners. If you need a different angle, for example when bypassing an obstacle or following a sloped ceiling, I recommend reading the article "Difference between a 90° elbow and other pipe bend angles," where this topic is discussed separately.
Step-by-step process for selecting the correct right-angle elbow
To simplify the process, I have summarized the entire selection process into a simple step-by-step procedure that I recommend following with every order, whether it is a single piece for a repair or a larger order for an entire house:
The first step is always to accurately measure the diameter of the pipe to be connected. The second step is to determine what angle the route requires – in the vast majority of cases, this will be a standard 90° elbow. The third step is to decide how the joint will be created – by soldering or crimping, which may slightly affect the specific type of fitting selected. Finally, the fourth step is to place an order for the specific size from the range – 15x15, 22x22, or 28x28.
A detailed step-by-step guide for the actual installation after selecting the correct elbow can be found in the article "Installation of a Hepworth right-angle elbow step by step," and if you are dealing with the joint technology, I recommend "How to correctly solder or crimp an elbow onto copper piping."
Calculating the required number of elbows for larger orders
For larger renovations or new constructions, it is advisable to calculate in advance how many elbows of a specific diameter you will need to avoid repeated orders and delays in work. In practice, I recommend going through the piping layout room by room and noting every point where the route changes direction by 90° (or another angle), along with the pipe diameter in that section. In a typical family home with 6–8 radiators, the actual consumption is usually as follows:
- 15x15 mm – approximately 10 to 20 pieces for end connections to radiators and corners in bathrooms
- 22x22 mm – approximately 8 to 15 pieces for main distribution branches between floors
- 28x28 mm – approximately 2 to 6 pieces in the mechanical room for the boiler circuit
These numbers are only approximate and can vary significantly depending on the house layout and the way the piping is routed (underfloor vs. on the surface). A precise calculation process with a concrete example can be found in the article "How to calculate the required number of elbows for a heating system." I always recommend ordering 5–10% extra elbows as a reserve for possible mistakes during installation or additional route adjustments – especially with soldered joints, it occasionally happens that a fitting has to be cut off and replaced after an unsuccessful attempt to make the joint.
Practical experience – most common mistakes when selecting diameter
Over the years of working with copper piping and Hepworth fittings, I repeatedly encounter similar scenarios that have led to additional complications on site:
One of the most common situations occurs during the renovation of older apartments, where the original piping was steel with inch-based dimensions, and during the transition to copper, the internal diameter of the old piping is confused with the external diameter of the new one. This leads to ordering an elbow one size larger than actually needed.
The second common mistake occurs with combined systems where part of the distribution consists of copper pipe and part of plastic or multi-layer pipe with a copper transition – in this case, the diameter of the copper part must be measured separately, independently of the nominal diameter of the plastic pipe, because transitions are manufactured for the exact outer diameter of copper, not for the DN of the plastic pipe.
The third situation, which often appears in larger projects carried out by multiple installation teams, is inconsistent pipe diameters within one route – when one team uses 22 mm pipe and another connects with 15 mm without verifying the project. The solution here is to perform a thorough diameter check with each material handover on site and clearly mark the planned diameters in the project documentation before installation begins.
Inspection and maintenance after installation
After selecting the correct diameter and successfully installing the corner elbow, it is advisable to include these connections in the regular system inspection, especially in the first weeks after commissioning, when the pipe gradually "settles" during thermal cycles. I recommend checking the connections during the first full heating of the system to operating temperature and again after approximately two weeks of normal operation. A detailed inspection and maintenance procedure can be found in the article Maintenance and Inspection of Connections on Heating System Corner Elbows.
Summary – how to remember the correct diameter selection
Selecting a Hepworth corner elbow according to the pipe diameter can be simplified to one rule: always measure the outer diameter of the existing or planned pipe directly on site, never rely solely on estimation or on information from the project without verification on site. The three basic dimensions available – Corner Elbow 15 x 15, Corner Elbow 22 x 22 and Corner Elbow 28 x 28 – will cover almost all common orders in heating systems for family homes and apartments. When in doubt, it is always better to invest a few minutes in precise measurement than to deal with leaks and repairs after months of operation.
Frequently asked questions about selecting the diameter of a corner elbow
How can I tell if I have 15 mm or 22 mm pipe if I don't have a caliper?
The easiest way without a measuring tool is to compare with common objects – 15 mm pipe is approximately the thickness of a thicker pencil, 22 mm corresponds approximately to the diameter of a two-euro coin, and 28 mm is slightly larger than the diameter of a two-euro coin. This is only an approximate estimate – when ordering a larger quantity of material, I always recommend precise measurement using a measuring tape or caliper.
Can I use a 22x22 elbow on a pipe with a 20 mm diameter?
No, the tolerance between the inner diameter of the elbow and the outer diameter of the pipe is too small to safely combine two different nominal diameters. A 2 mm difference is too large for capillary soldering and the connection would not be tight or mechanically strong. In such a case, it is necessary to use a reducer or another type of fitting specifically designed for that diameter.
Can one elbow be used for two different diameters on both ends?
Corner elbows in this category are symmetrical, meaning both sockets have the same diameter (e.g., 15x15). If you need a transition between two different diameters in one corner of the route, the solution is to combine an elbow with a reducer, not to look for an asymmetric fitting, which is not commonly produced in this range.
Is there a difference between a soldering elbow and a compression elbow for the same diameter?
Yes, even if the nominal diameter is the same, the socket construction slightly differs according to the intended connection method – compression fittings have internal sealing O-rings and a different socket depth, while soldering fittings are smooth and without seals. It is therefore important to select a product corresponding to the technology you plan to use for installation.
What should I do if I have a combination of old and new pipe with different tolerances in one route?
In this case, I recommend always physically verifying the fit of the elbow on the old section of pipe before final installation, as older copper pipe may have slightly different production tolerances after decades compared to current standards. If the fit is not smooth, the solution is either to slightly sand the surface of the old pipe with sandpaper or to use a transition fitting.
Is it worth ordering elbows all at once for the whole house, or gradually according to stages?
For larger projects, it is more advantageous to order the material all at once according to a pre-calculated number of pieces for each diameter – you will save on shipping and avoid delays caused by waiting for additional orders. I recommend, however, always calculating a small 5-10 % reserve for each diameter, as described in the article on calculating the required number of elbows.
Conclusion
Selecting the correct Hepworth corner elbow according to the pipe diameter is not a complex technical discipline, but it requires diligence and precise measurement directly at the site. Three basic diameters – 15, 22 and 28 mm – will cover the majority of common heating system distributions in family homes and apartments, and if you follow the basic rule of verifying the outer diameter of the pipe before ordering, you will avoid the most common problems I encounter in practice – leaky connections, unnecessary returns and lost time on site. If you have any doubts about a specific dimension or type of connection, do not hesitate to look into the related articles in the Knowledge Centre, which cover installation, soldering and maintenance in greater depth.
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