What do T-piece dimensions mean and how to read them correctly

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What do the dimensions of a T-piece mean and how to read them correctly

When you first look at a catalog of fittings for copper pipe, you may encounter notations like "22 x 22 x 15" or "28 x 28 x 28" – three numbers, three diameters. In practice, however, it turns out that most order errors occur precisely here, which can leave an installer on a construction site with the wrong piece in hand and wasting a working day on the way to the store. Over twenty years in the field, I have seen countless such situations. This article therefore examines the system of dimension labeling for T-pieces in every detail – from the basic logic of the three-digit notation, through the difference between outer and inner diameter, to concrete examples from the practice of single-family homes and apartment distribution systems.

Why are there always three numbers in the dimensions of a T-piece?

A T-piece – a fitting in the shape of the letter T – has three openings, i.e., three places where the pipe is connected. Each opening can have a different diameter, or they can all be the same. That is why the dimensions are always given as a triplet of numbers separated by a cross, for example 22 x 22 x 22 or 28 x 28 x 15. The order of the numbers is not random and follows a well-established logic that manufacturers and distributors in European practice consistently adhere to.

Read the triplet of numbers as follows:

First number – diameter of the main branch (inlet, "flow direction")
Second number – diameter at the other end of the main branch (outlet in the straight direction)
Third number – diameter of the branch (side branch, perpendicular to the main direction)

When all three numbers are the same, we speak of an equal T-piece. When the third number differs (usually smaller), it is a reduced T-piece. The difference between these two groups and when to use which is also discussed in the related article Equal vs. reduced T-piece: when to use which type.

In the diagram above, you can see the basic layout: sockets 1 and 2 form the straight flow direction, socket 3 is the side branch. Dimensions are always given in this order regardless of how the T-piece is physically oriented during installation.

Outer diameter vs. inner diameter – what do the numbers actually mean?

This is the point where the most confusion arises, especially if someone is moving from working with plastic pipe (PEX, PE-RT) to copper. For copper pipes and fittings according to the EN 1057 standard, all dimensions are given in outer diameter. The number 15 in the T-piece designation therefore does not mean that the opening is 15 mm – it means that you insert a pipe with an outer diameter of 15 mm into the fitting.

Why is this important? Because the wall of the copper pipe has its own thickness. A 15 mm pipe typically has a 1 mm wall, so the inner diameter is 13 mm. A 22 mm pipe has a wall of 1 mm (sometimes 1.5 mm), so the inner diameter is approximately 20 mm. A 28 mm pipe usually has a wall of 1.5 mm, so the inner diameter is approximately 25 mm. These values are used in flow and pressure loss calculations – which is the subject of the related article How to divide the heating pipe using a T-piece without losing pressure.

For correct installation, this means: never insert a pipe into a 22 mm fitting based on the inner diameter! Always work with the outer diameter.

Standard dimensions of T-pieces for home heating

In the practice of single-family homes and apartment heating distribution systems, three standard diameters are primarily used – 15, 22 and 28 mm. These are also the diameters covered by the Hepworth range available at atria.sk:

15 mm – the smallest standard diameter, typically for connecting radiators and individual heating units. The pressure and flow characteristics are sufficient to supply one unit, or a shorter circuit with one or two radiators.
22 mm – the medium diameter, floor distribution for a group of radiators on a floor, or an entire apartment. This is the most common dimension, which I see in 80% of orders in standard residential construction.
28 mm – a stronger backbone, used for main horizontal distribution, in larger houses or when the boiler supplies multiple circuits (heating + underfloor heating + hot water heating).

For each of these diameters, there is a parallel T-piece: T-piece 15 x 15 x 15 for radiators, T-piece 22 x 22 x 22 for medium distribution lines, and T-piece 28 x 28 x 28 for main risers and stronger circuits.

How to read a reduced T-piece – practical examples

A reduced T-piece is used when a smaller branch or sub-circuit is taken off from a larger distribution line. Reading the triplet of numbers is even more important here, as it depends on exactly where the reduction occurs.

Let’s look at the most common examples from practice:

22 x 22 x 15 – the main branch of 22 mm remains unchanged, the side branch is reduced to 15 mm. Typical use: main distribution line of 22 mm, branch to a radiator of 15 mm. This is by far the most commonly used reduced T-piece in residential installations.
28 x 28 x 22 – main line of 28 mm, branch to a sub-circuit of 22 mm. You can see this, for example, when connecting a manifold to underfloor heating.
28 x 22 x 22 – here the reduction is in the main direction. You enter with a 28 mm pipe, but the main line reduces to 22 mm. It is used less frequently, mostly when the pipe is intentionally narrowed after the T-piece due to pressure or hydraulic balance.
22 x 15 x 15 – input of 22 mm, the rest is 15 mm. This is a transition piece at the end of a distribution line.

Practical tip from installation: if you are unsure, draw a schematic on paper. Mark the flow direction and write the diameter of each connected pipe next to each opening. Then compare it with the order of numbers in the fitting designation. This usually takes two minutes and will save you a trip to the warehouse.

Physical dimensions of a T-piece – branch lengths and their practical reach

In addition to diameters, the term insertion depth (in English) appears when dealing with T-pieces. It refers to the length to which you insert the pipe into the fitting before soldering or pressing. For soldered copper fittings, these values are standardized and depend on the diameter:

Ø 15 mm – insertion depth approx. 12–14 mm
Ø 22 mm – insertion depth approx. 16–18 mm
Ø 28 mm – insertion depth approx. 18–22 mm

This value is important when measuring the overall length of the pipe in the route. If you forget to add the insertion depths, on a 10-meter distribution line with eight connections (each pipe inserted twice), you can easily be short by 20–30 cm of pipe. At first glance, a small detail, but in practice, it can be a source of frustration during final connections.

In addition to insertion depth, a T-piece also has an overall external length of the body. This determines how much space the fitting takes up in the pipe route. When planning pipe routing in limited space (e.g., behind a panel, in a suspended ceiling, or in a groove), this information should be verified in the manufacturer’s technical data sheet.

Standards and material standards behind the numbers

Copper fittings for potable water and heating installations are manufactured by manufacturers according to the standard EN 1254, specifically:

EN 1254-1: Capillary soldered fittings (most common type in heating)
EN 1254-2: Pressed fittings (press fitting)
EN 1254-4: Combined fittings (copper + thread)

Dimensions are always related to the EN 1057 standard for copper pipes. The Hepworth system, part of the Wavin group, complies with these standards, which means full compatibility with other European fittings of the same standard – you can combine fittings from different manufacturers as long as both meet EN 1254.

This is important to know also because the dimension notation is uniform thanks to the standard. If you see "22 x 22 x 15" from Hepworth, Geberit, or another European manufacturer, it always means the same: the main branch is 22 mm, the branch is 15 mm.

Practical step-by-step for reading dimensions directly when ordering

Based on experience, I recommend the following procedure, which works for installers as well as for technically capable homeowners who plan to install the system themselves:

Step 1 – Map the route and identify each branch. On a simple schematic (even a hand-drawn one is sufficient), mark each T-piece. Write the diameter of the connected pipe next to each opening.

Step 2 – Determine the main flow direction. The main branch is the one through which a larger volume flows. Its diameters are the first two numbers in the designation.

Step 3 – Identify the branch. The side branch – the third number. It is usually the same or smaller than the main branch.

Step 4 – Compare with available products. If you need a distribution line with a 22 mm main and 15 mm branches to radiators, you are looking for a T-piece 22 x 22 x 15. If all branches are the same, the ideal is T-piece 22 x 22 x 22.

Step 5 – Check the quantity. Each branch in the route = one T-piece. Do not forget the T-pieces at the collector, if one circuit supplies multiple floors.

In the schematic you see a typical example: the boiler supplies the main trunk Ø 28 mm, from which T-fittings 28 x 28 x 22 branch off to the sub-loops Ø 22 mm. From each sub-loop, individual radiators are then supplied via T-fittings 22 x 22 x 15. This is exactly how the dimensional system of the entire piping is read.

Where errors most often occur when reading dimensions

After years of practice, I have noticed several recurring scenarios in which plumbers and technically knowledgeable DIYers purchase the wrong fittings:

Mixing up the first and third number. The customer knows they have a 22 mm piping system and want a 15 mm branch, but orders a T-fitting 15 x 15 x 22 instead of 22 x 22 x 15. Although the dimensions appear to be "the same," the fitting is physically different – and you will not find a larger diameter in the branch for a smaller trunk in practice, because it makes no sense. If you come across such a designation, double-check what you actually need.

Mixing up the outer and inner diameter. The customer measures existing pipes with a caliper, but measures the inner diameter. Measured 20 mm is mistaken for a 20 mm fitting, although a 22 mm fitting is needed (because a 22 mm pipe has an inner diameter of ~20 mm). This error is also addressed in the related article How to choose the right T-fitting for copper pipe: diameters 15, 22 and 28 mm.

Wrong feeling of "similarity" of dimensions. Ø 22 mm and Ø 3/4″ (inch) are similar, but not identical. A 3/4″ pipe has an outer diameter of approximately 26.7 mm – that is a completely different world. Copper fittings EN 1254 are in millimeters, not in inches. Dimensional mix-ups between metric and imperial units are a source of serious problems when supplying from an old boiler with inch-based threads, where reductions are needed for the transition.

Ignoring the direction of reduction. Not every reduced T-fitting exists in both configurations. A T-fitting 22 x 22 x 15 (reduction in the branch) is a common product. A T-fitting 22 x 15 x 22 (reduction in the main direction) is less standard and not every supplier has it in stock. So before ordering, make sure to check not only the dimensions, but also the specific configuration.

I also recommend reading the article Common mistakes during T-fitting installation and how to avoid them, where not only ordering, but also installation errors are discussed, including incorrect orientation of the fitting before soldering.

T-fittings Hepworth – why exactly this system?

Hepworth is a British brand with a long history in the production of copper fittings, now part of the Wavin portfolio. Their capillary soldered fittings are made from cast copper (CW024A), which is an alloy with low arsenic and phosphorus content, optimized for soldering. The inner surface of the bores is precisely calibrated to tolerances according to EN 1254-1, which ensures a uniform capillary gap for the solder – typically 0.1 to 0.15 mm – essential for a strong and tight joint.

This means for the installer in practice that the fittings "fit" correctly and the soldering is predictable. Unlike cheaper alternatives, I have not encountered dimensional deviations with Hepworth that would require experience to compensate for. Even less experienced technicians achieve repeatable results with these fittings, which is important for domestic piping systems.

If you are interested in a comparison with other materials – for example PEX, multilayer or steel piping – I recommend reading the article T-fittings for copper pipes vs. other materials: what differs and what is better.

When dimensions are not enough – other parameters of the fitting

Dimensions are a necessary basis, but when selecting a T-fitting for a heating system, other parameters are also relevant:

Working pressure – Hepworth capillary fittings are dimensioned for a maximum working pressure of 10 bar at 110 °C. For domestic central heating, this is more than sufficient with a large margin (normal operating pressure 1.5–3 bar).
Operating temperature – copper fittings can withstand a continuous medium temperature of up to 110 °C, short-term higher. For low-temperature systems (floor heating, heat pumps), copper is technically "overdimensioned," but that is not a problem – rather it speaks to the durability of the joints.
Type of joint – capillary (soldering), press (crimping), threaded. The outer diameter dimension is the same for all types of pipe, but the actual joining system is different and they are not interchangeable.
Material – for copper pipes exclusively copper or brass fittings. Mixing with galvanized steel without dielectric insulation leads to galvanic corrosion. This topic is discussed in more detail in the article T-fittings for copper pipes vs. other materials: what differs and what is better.

Examples from specific projects

Project 1 – renovation of an apartment, 3+1, panel building. Existing piping from old pipes Ø 15 mm. The customer wanted to add one radiator in a new room. Solution: we cut the existing 15 mm line and installed a T-fitting 15 x 15 x 15, from the branch we led the new 15 mm pipe to the radiator. We verified the dimensions with a caliper measuring the outer diameter of the existing pipes (measured 15.05 mm = 15 mm pipe according to EN 1057).

Project 2 – new construction of a family house, two-story. The boiler supplies a hot water system with 12 radiators. Design: main trunk Ø 28 mm from the boiler to the manifold, from the manifold horizontal runs Ø 22 mm to each floor, from each horizontal run branches Ø 15 mm to individual radiators. In total, we used 2 pieces of T-fittings 28 x 28 x 28 at the manifold, 8 pieces of T-fittings 22 x 22 x 15 on the horizontal runs and 4 pieces of T-fittings 22 x 22 x 22 for extending the trunks. Each T-fitting was identified according to the above-described three-digit notation without mixing up.

Project 3 – industrial hall, heat connection. Here we encountered fittings in inches from an older piping system. New Hepworth fittings in metric dimensions had to pass through transition fittings. This situation is a typical example of why it is important to verify whether the existing piping is in metric or inch dimensions – the numbers in the fitting designation will not tell you that, you have to measure the actual outer diameter.

FAQ – Frequently asked questions about T-fitting dimensions

What does the number in the T-fitting name mean – is it the outer or inner diameter?

For copper fittings according to EN 1254 and EN 1057, it is always the outer diameter of the pipe that is inserted into the fitting. The number 22 in the designation "T-fitting 22 x 22 x 22" means that all three bores are intended for pipes with an outer diameter of 22 mm. The inner diameter of the bore itself is a few tenths of a millimeter larger, to create a capillary gap for the solder.

Can I rotate the T-fitting – does the orientation matter during installation?

For a parallel T-fitting (e.g. 22 x 22 x 22), the physical orientation does not matter – all bores are the same. For a reduced T-fitting, it does matter which bore is connected where: the larger diameter must correspond to the larger pipe and the smaller to the smaller one. Hydraulically, there is also a difference in whether the branch is in the direction of flow or against it – more about that in the article How to divide heating piping using a T-fitting without pressure loss.

How can I determine the diameter of my existing pipe if I don't know when it was installed?

The most reliable way is to use a caliper and measure the outer diameter. For EN 1057 copper pipes, the following values apply: 15.00 mm → Ø 15, 22.00 mm → Ø 22, 28.00 mm → Ø 28. If the measured value does not fall into these diameters (e.g. 26.7 mm), it may be an inch-based pipe (3/4″) – in this case, transition fittings are needed.

Is there also a T-fitting with a larger branch than the main branch?

No, in the standard range, because it would not make hydraulic sense – you would not be able to physically supply a larger branch than the main branch without reducing the flow in the main direction. If you need to supply a larger loop from a smaller distribution, the solution is to increase the diameter of the entire line before the branch point, not just the T-fitting itself. A similar situation occurred when adding floor heating to an existing radiator system – in such cases, it always ends with replacing the trunk.

Are Hepworth fittings compatible with other European manufacturers of the same size?

Yes, provided both fittings meet EN 1254-1 (capillary) or EN 1254-2 (press), the dimensions are standardized and the fittings are interchangeable. The difference may be in the capillary gap tolerances – Hepworth maintains tight tolerances, which can cause problems with soldering when using low-quality replacements (too large a gap = insufficient capillary action of the solder). Read more about this in the article Mounting a T-piece on a copper pipe: soldering step by step.

How can I quickly determine whether to order a parallel or reducing T-piece?

It is enough to answer one question: Does each branch of the T-piece have the same pipe diameter? If yes – a parallel T-piece, e.g. 15 x 15 x 15. If the diameters differ – a reducing T-piece, e.g. 22 x 22 x 15. Then simply write down the diameters in order: main branch inlet, main branch outlet, branch.

Conclusion: three numbers that decide the entire piping system

The dimensions of a T-piece are not just dry technical data – they are three numbers that in a compact form carry information about the entire layout logic at a given location. When you read them correctly, you are sure that the material fits, the joint will be tight and the hydraulics will work as designed. If you mix them up or misinterpret them, you can expect at best a trip to the warehouse, at worst a leak or pressure loss that takes hours to find.

The principle is simple: outer diameter, order of the outlets (inlet – outlet – branch), standardized compatibility. The rest is practice – and it comes with every new job. If you are unsure about the selection for your specific layout, take a look at the full range of Hepworth T-pieces and compare the available dimensions with your schematic.

Do you have a question on this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.