Dimensions of Reduced T-Pieces: How to Read the Marking 22 x 22 x 15
Dimensions of reduced T-fittings: how to read the marking 22 x 22 x 15
When you first look at a catalog of copper fittings, three-digit markings such as 22 x 22 x 15 or 28 x 22 x 28 may seem a bit mysterious at first. In practice, however, they hide a simple and logical system that gives you immediate information about where the fitting fits, where it branches off, and what diameter each of its three bores has. This article will explain this system in depth – from the basic anatomy of a T-fitting through concrete examples from real installations to the most common mistakes that even experienced fitters and lay customers make when ordering.
Anatomy of a T-fitting: three bores, three dimensions
A reduced T-fitting is a fitting in the shape of the letter T. It has two coaxial bores – forming the "straight line" of the letter T – and one perpendicular bore that branches off from that straight line. In professional jargon, these three openings are called differently: the run, the straight bores (or "legs"), and the branch. It is precisely the order in which these three dimensions are written in the marking that forms the basis of the entire system of reading.
There is a fundamental difference between a straight and a reduced T-fitting: in a straight T-fitting, all three bores have the same diameter, for example 22 x 22 x 22 – such a marking is usually not even listed in the catalog, because it is redundant. A reduced T-fitting has at least one bore of a different diameter than the others. It is precisely this reduction in one of the bores that is the reason why the marking contains three different or partially different numbers.
In the diagram above, you can see the basic layout of the three bores of the T-fitting. Bore 1 and Bore 2 form the run – they are aligned with each other. Bore 3 is the perpendicular branch. The marking of the fitting always describes these three dimensions in the exact order: run bore 1 × run bore 2 × branch.
Exact interpretation of the three-digit marking
Now let's look at some concrete examples. The format of the marking is always the same: three numbers separated by crosses or spaces, where each number is the outer diameter of the pipe in millimeters, to which the respective bore fits.
Example 1: T-fitting 22 x 22 x 15
This is probably the most commonly used reduced fitting in private homes. T-fitting 22 x 22 x 15 means the following:
- First number – 22: diameter of the first run bore. Fits a copper pipe ø22 mm.
- Second number – 22: diameter of the second run bore. Also ø22 mm – the run is therefore the same on both sides.
- Third number – 15: diameter of the branch. Fits a copper pipe ø15 mm.
This fitting is typically used on the main distribution pipe DN22, from which a supply or return pipe DN15 to a radiator branches off. The run "loses nothing" – it remains ø22 mm – and from it, a smaller branch is taken to the heating body. This is precisely the most classic scenario: the main horizontal pipe runs through the entire room, and from each 22 x 22 x 15 fitting, a shorter connection leads to the radiator.
Example 2: T-fitting 28 x 28 x 22
T-fitting 28 x 28 x 22 works on the same principle, just in different dimensions:
- 28 – 28: the run remains ø28 mm on both sides.
- 22: the branch is ø22 mm.
This is a typical fitting for stronger main lines, for example in older houses with cast iron radiators, where the original larger pipes are being renovated and smaller branches are connected to them. Or in systems where the boiler supplies several circulation loops and the supply from the boiler itself is ø28 mm, but one of the distribution pipes connects a ø22 mm branch for one of the loops.
Example 3: T-fitting 22 x 15 x 22 – watch the order!
This is where the biggest source of confusion lies. T-fitting 22 x 15 x 22 looks very similar at first glance to 22 x 22 x 15, but it is a completely different fitting:
- 22: first run bore ø22 mm.
- 15: second run bore ø15 mm – the run is REDUCED here!
- 22: branch ø22 mm.
In other words: in the run direction, the pipe reduces the diameter from 22 to 15 mm, while the branch leads to a pipe of ø22 mm. Such a fitting is used in completely different places – for example, where you want the main pipe to continue with a smaller diameter after the branch (because the next section serves fewer radiators and does not need full flow), but from the branch location, you need to supply something of a larger diameter.
The confusion lies at the heart of the most common ordering mistake. At first glance, the numbers appear the same – 22, 22 and 15 – but their order significantly changes the function of the fitting. That is why it is important never to rely on the assumption that "they have the same numbers" and always carefully read the full designation, including the order.
Why is the numbering in this order?
The standard notation is based on European standards for copper plumbing, specifically the EN 1254 standard, which defines fittings for soldering copper. The order is always: straight run (larger side) → straight run (second side, may be reduced) → branch. In practice, you can remember this easily: read the T-piece from left to right along the straight line, and then add the branch upwards (or downwards, depending on the installation position).
For Hepworth fittings, which are the subject of this category, this standard applies without exception. All dimensional designations are given in the outer diameter (OD – outer diameter) of the copper pipe in millimeters. It is important to know this because in other systems (for example, in plastic or steel installations), the inner diameter or threaded designation may be used – here it is always the outer diameter.
Overview of the most commonly used reduced T-pieces and their typical use
In practice, during renovations and new builds of single-family homes, we mainly work with pipe diameters of 15, 22 and 28 mm. Let's look at the full range of available reduced T-pieces and where each of them finds its application.
22 x 22 x 15 – king of radiator circuits
As I mentioned, the T-piece 22 x 22 x 15 is by far the most commonly used. In practice, you see it everywhere where the main distribution pipe (ø22 mm) supplies individual radiators connected via a branch of ø15 mm. For example, in a typical two-story single-family house, the main circuit in ø22 mm copper is run from the manifold to individual rooms on the ground floor, and from each T-piece 22 x 22 x 15, a short branch of ø15 mm leads to a convector or panel radiator.
28 x 28 x 22 – strong main lines
The T-piece 28 x 28 x 22 is used where the primary circuit – for example, from the boiler to a hydraulic diverter or distributor – runs in ø28 mm and from this primary circuit, secondary circuits in ø22 mm branch off. A typical scenario: a boiler with a power of 20–30 kW, from which ø28 mm runs, and on the first floor, the main pipe branches off to a circuit for a bathroom radiator with a finned heater (ø22 mm) and a circuit for underfloor heating (ø22 mm). At each branching point, this T-piece is used.
22 x 15 x 22 – special reduction in the straight run
As we have seen, the T-piece 22 x 15 x 22 reduces the straight run. In practice, I have encountered it, for example, during renovations, where the original pipe of ø22 mm led to a dead-end branch, which was reused for a larger distribution, but beyond that point, the pipe continued to only one further appliance, for which ø15 mm is sufficient. This fitting thus combines the function of a straight run reduction and a T-piece in one part – saving space and one joint.
28 x 22 x 28 and 28 x 15 x 28 – reduction in the middle of the straight run
For completeness: the T-piece 28 x 22 x 28 has the straight run reduced in the central section to ø22 mm, while both end ports are ø28 mm, and the branch is also ø28 mm. Similarly, the T-piece 28 x 15 x 28 has the central straight port of ø15 mm. These combinations occur less frequently – usually in non-standard layouts, where it is necessary to maintain a larger diameter on both sides of the straight run, but in the middle (for example, due to space constraints or to achieve a certain hydraulic resistance), the central port is reduced.
Outer diameter vs. nominal diameter: what the designation depends on
One question that customers often ask is: why is a copper pipe called a "fifteen" or a "twenty-two"? It refers to the outer diameter (OD) in millimeters, which is the European standard for copper plumbing. The inner diameter depends on the wall thickness of the pipe, which varies according to the class (for example, for heating, a pipe with a wall thickness of 1 mm is commonly used for ø15, 1 mm for ø22 or 1.5 mm for ø28).
For you as a customer or installer, it is important to know only one thing: the number in the T-piece designation always corresponds to the outer diameter of the pipe you are inserting into the respective port. If you have a pipe marked as "Cu 22×1" (copper, outer diameter 22 mm, wall thickness 1 mm), you must look for a T-piece with the number 22 on the corresponding port.
Hepworth fittings are manufactured with precise tolerances for soldering (capillary or hard soldering). The port of the fitting has an inner diameter slightly larger than the outer diameter of the pipe – it is precisely into this gap that the solder flows when heated. This gap is standard 0.05–0.2 mm, which allows the capillary effect and ensures a strong, tight joint.
Practical scenarios from customer practice
Scenario 1: Heating renovation in a single-family house, 8 radiators
The customer is renovating the heating system in a house built in 1985. The boiler is new, a condensing one, and the supply and return lines run in ø22 mm from the boiler to a horizontal distribution. On each floor, there is a horizontal "floor line" in ø22 mm, and from it, branch connections to radiators in ø15 mm. The customer needs 8 branches – 4 on the supply, 4 on the return. The obvious choice: 8 pieces of T-piece 22 x 22 x 15. The main line remains full-profile ø22 mm, and each branch to the radiator is ø15 mm. Simple, clear, hydraulically correct.
Scenario 2: Installation of floor heating with a 25 kW boiler
The boiler is connected via a ø28 mm pipe, and the floor heating distributor is connected via a ø22 mm branch. At the same time, a branch for the bathroom radiators (ø22 mm) is taken off from the same main pipe in ø28 mm. Here, 28 x 28 x 22 T-fittings are used – the main flow direction in ø28 mm remains unchanged, and the branches in ø22 mm lead to the distributors. If we need the pipe to continue in only ø22 mm after the last branch (e.g., to the second circuit boiler), we use an oval reducer or a T-fitting with a reduced main flow direction at the last position.
Scenario 3: Repair after a failure – replacing fittings
From experience, I know that in 20 % of cases where a customer complains that a "T-fitting does not fit", it is a mix-up between 22 x 22 x 15 and 22 x 15 x 22. The correct piece was missing on site, the installer took "whatever was at hand" and installed a fitting with a reduced main flow direction instead of a reduced branch. Result: the pipe after the fitting is suddenly ø15 mm instead of ø22 mm, the hydraulic balance of the circuit is disturbed, and the radiator at the end of the circuit heats poorly. The solution is simple – replace the fitting with the correct one, but this means cutting out the joint and re-soldering. Prevention: always physically read the marking on each fitting before installation, not just from the box.
Hydraulic consequences of correct (and incorrect) selection
Selecting the correct T-fitting is not only about mechanical compatibility – it has direct consequences for the hydraulic balance of the entire circuit. A copper pipe with a diameter of ø15 mm has a cross-sectional area of approximately 154 mm², whereas ø22 mm has 314 mm² and ø28 mm has as much as 491 mm². These are significant differences. If you mistakenly install a T-fitting with a branch of ø22 mm instead of ø15 mm to a radiator where a ø15 mm was planned in the design, you end up in a situation where the radiator valve and thermostatic head operate under different pressure conditions than they were set for.
And vice versa: if you install a T-fitting with a branch of ø15 mm at a location where a ø22 mm branch should be (e.g., to a floor heating distributor), you restrict the flow and the distributor will not be able to supply all the floor heating circuits with sufficient flow. Result: some rooms are cold, others are overheated, the circulation pump works with too high pressure differentials and wears out faster.
If you are interested in a deeper understanding of hydraulic design and how to choose the correct dimension of a T-fitting from the perspective of hydraulic balance, also see other articles in this Knowledge Centre, specifically How to correctly choose a branch reducer when connecting a radiator to the main pipe and Combining different diameters in a heating circuit using reduced fittings.
Orientation of the T-fitting during installation: does the direction of the branch matter?
Another question that often arises in practice is: does it matter how you physically rotate the T-fitting – with the branch pointing up, down, left, or right? From a technical perspective for capillary soldering, it does not – the joint is equally strong in any position, because the capillary effect works horizontally and even downward. The only case where it matters is when venting the system: if the branch is oriented upwards and the radiator is at the end of it, air will more easily reach the vent valve during system filling. However, this is not a question of the T-fitting itself, but of the overall installation layout.
For more information on the correct installation procedure, including the orientation of the main and branch directions of the fitting, see the article Installation of a reduced T-fitting Hepworth: soldering step by step.
How to avoid confusing fittings when ordering: a practical guide
Based on experience from many projects, I have developed a simple procedure to avoid confusion when ordering:
- Draw a sketch on paper. A simple sketch is enough: draw the pipe routes, mark the diameters, and for each branch, write down the required size for each port. From this sketch, you can directly derive the correct marking of the T-fittings.
- Write down the marking in the order: straight-run-straight-run-branch. Always: first both straight ports, then the branch. Do not invent your own system.
- Physically check the delivered fittings. Before you start soldering, take each fitting in your hand and read the marking directly on it. Do not rely only on the box label.
- If in doubt, use a caliper. Measure the copper pipe that you will insert into the port. The outer diameter must match the number in the marking.
- Check the technical data sheet. Hepworth fittings have technical data sheets with dimensional sketches – you can find them on the product pages at atria.sk. In case of uncertainty, this is the fastest way to verify.
Related tips and typical installation errors are thoroughly discussed in the article Common mistakes when installing copper T-fittings and how to avoid them.
Material and quality: why Hepworth
All T-fittings in this category are made from dezincified copper (deoxidized copper CW024A according to EN 1412), which is suitable for capillary soldering. Copper is ideal for heating systems for several reasons: it has excellent resistance to corrosion at operating temperatures up to 110 °C (which also covers condensing boilers with heating circuit temperatures), it has high mechanical strength, it is resistant to biofilms and microbiological corrosion and – which is important for renovations – it is recyclable, so the material retains residual value at the end of the system's life.
Hepworth fittings comply with the European standard EN 1254-1 (capillary fittings for copper piping) and undergo final dimensional tolerance control. The fittings are designed for a maximum working pressure of 16 bar at temperatures up to 110 °C in dry steam, which is more than sufficient for standard heating systems in apartment and residential construction (normal working pressure is 1.5–3 bar, maximum safe pressure is 3–6 bar).
Most frequently asked questions (FAQ)
What does it mean if two numbers in the T-fitting designation are the same and one is smaller – is it a reducing or straight T-fitting?
If two numbers are the same and the third is smaller, it is a reducing T-fitting – one of the branches (either one of the straight runs or the branch) has a smaller diameter. A straight T-fitting would have all three values the same, for example 22 x 22 x 22, but such fittings are not commonly listed in the catalog, as they are usually marked only by the diameter without repeating it three times.
Can I use a T-fitting 22 x 22 x 15 for a pipe of ø15 in the straight run, if I insert the thinner pipe into the larger branch?
No, under no circumstances. The opening of the ø22 mm branch and the ø15 mm pipe would leave a 3.5 mm gap – the capillary effect would not work and the joint would be mechanically unstable, leaky and dangerous. Fittings and pipes must always match in diameter. If you need to change the diameter in the straight run, use either a reducing bushing or another type of T-fitting with the corresponding dimensions.
How can I tell which branch is the "side branch" and which are the "straight runs" when I have the T-fitting in my hand?
Simply: the straight branches are aligned with each other – if you were to thread a straight rod or look through them, it would pass straight through. The side branch is the one that is perpendicular to this direction. With branches of the same size, you can tell by looking. If the side branch has a smaller diameter, you can identify it by the size of the opening.
What is the difference between 28 x 22 x 28 and 28 x 28 x 22?
In the 28 x 22 x 28 fitting, the central straight branch is reduced to ø22 mm – meaning the pipe in the straight direction passes through a narrowing of ø22 mm. The side branch is ø28 mm. In the 28 x 28 x 22 fitting, the straight direction is fully maintained at ø28 mm and only the side branch is reduced (ø22 mm). These are functionally different fittings for different situations in the distribution network.
Is it important to follow the flow direction in a reducing T-fitting?
For capillary copper T-fittings from Hepworth, the flow direction is not critical in terms of the function of the fitting – they are not one-way components like check valves. Hydraulically, it is always more advantageous if the flow goes from the larger branch to the smaller one in the side branch (not the other way around through an expansion). In practice, however, at normal flow speeds in residential heating (up to 1.5 m/s), this does not make a measurable difference.
Where can I find a table of all available combinations of reducing T-fittings from Hepworth?
A complete overview is available directly in the category Reducing T-fittings on atria.sk, where all available combinations of diameters are clearly listed. If you are looking for a combination that you do not see there, you can solve the dimension by combining a straight T-fitting with a separate reducing bushing or coupling – more on this is covered in the article Combining different diameters in the heating circuit using reducing fittings.
Conclusion: the numbering system is your guide in the catalog
The three-digit numbering system for reducing T-fittings may seem a bit unintuitive at first glance, but once you understand the basic rule – the first two numbers are the straight direction, the third number is the side branch – it becomes a reliable guide for every order and every installation. The most important lesson from practice is this: do not neglect the order of the numbers, because 22 x 22 x 15 and 22 x 15 x 22 are completely different fittings in terms of installation and hydraulics, even though they contain the same digits.
Before each job, make a simple sketch of the circuit with marked pipe sizes, directly derive the designations of the required T-fittings from it, and physically verify them when receiving the materials. This three-minute check can save you hours of work related to cutting out incorrectly installed joints and re-soldering them. And if you are unsure, check out other articles in this Knowledge Center – from selecting the right T-fitting through the soldering process to checking completed joints – everything you need is all in one place.
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.