Difference between straight and reduced T-piece in copper piping

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Introduction: Why the type of T-piece matters more than it seems

When you work on a building site or the renovation of a central heating system, you will sooner or later come across a decision that seems trivial at first glance: should I use a straight T-piece or a reduced one? Both fittings look almost identical, both have the shape of the letter T, and both connect three pipe sections. And yet there is a fundamental difference between them—not only structural, but also hydraulic, economic, and operational.

In this article, we will look at the issue in detail from several perspectives. We will explain what a straight and a reduced T-piece actually are, where each of them belongs, what are the practical consequences of choosing the wrong one, and how to correctly read and select these fittings. At the same time, we will look at specific dimensions, real-life scenarios from practice, and the hydraulics that underlie the whole decision.

What is a straight T-piece and where can you find it

A straight T-piece (in English, "equal tee") is a fitting where all three outlets—the two straight ends and the side branch—have the same internal diameter. For example, a T-piece 22 × 22 × 22 means that the main line (both ends) and the branch are dimensioned for pipes DN 22 mm. This symmetry is elegant and simplifies the design of the piping layout in cases where you want to divide the flow into two equal branches with the same flow and the same dimensioning.

Straight T-pieces are typically used where:

both output circuits must have the same capacity (e.g., two identical series branches with the same number of radiators)
the hydraulic design calculates the same flow in both branches
the entire piping is run in one dimensioning without narrowing
it is about collectors or distributors, where each branch has the same load

In practice, you will encounter straight T-pieces mainly in the piping of technical water, in some industrial applications, and in collector distributors. In standard residential heating, they are relatively rare—and for good reason.

What is a reduced T-piece and how does it differ

A reduced T-piece (in English, "reducing tee") has at least one outlet of a different diameter than the others. Most often, the side branch is reduced—that is, the branch that branches off at a right angle to the main pipe line. This branch often leads directly to a radiator, a secondary circuit, or a smaller consumer, so it is logical and hydraulically correct for it to have a smaller diameter corresponding to the actual flow required.

Typical examples of reduced T-pieces in copper heating installations:

T-piece 22 × 22 × 15 – main line DN 22, branch DN 15 (a very common combination for feeding one radiator from a circuit)
T-piece 28 × 28 × 22 – main line DN 28, branch DN 22 (typical for a larger circuit with multiple radiators)
T-piece 22 × 15 × 22 – both arms of the main line DN 22, the central outlet (for joining into the pipe) DN 15
T-piece 28 × 22 × 28 – both ends DN 28, reduction in the central position to DN 22
T-piece 28 × 15 × 28 – main line DN 28, with a branch directly to DN 15 for a thin branch

Key detail: the diameters in the T-piece designation are always given in the order left end × right end (or central outlet) × branch. Different manufacturers may have slightly different conventions, so it is important to know how to correctly read the designation—about this we also wrote in the article Dimensions of reduced T-pieces: how to read the designation 22 × 22 × 15.

Hydraulic perspective: why reduction is important for flow

In order to understand why a reduced T-piece is not just a construction detail, but a real hydraulic necessity, we must stop at the principle of pipe sizing. The pipe diameter is selected according to the required flow of the medium and the allowed flow velocity. In hot water heating, velocities of 0.3 – 1.0 m/s are commonly used. If the pipe is too large for the actual flow, the velocity drops, the hydraulic resistance also drops, but other problems arise:

air venting decreases (air does not reach the air vents)
sedimentation of sludge is a risk in horizontal sections
heat losses increase unnecessarily in pipes of too large a diameter
unnecessary costs for material and insulation increase

On the other hand, if the pipe is too thin, the velocity exceeds 1.2 – 1.5 m/s, which causes noise, erosion of the inner wall, increased pressure losses, and in copper, additionally the risk of cavitation in bends. Correct dimensioning of each branch is therefore not a formality—it is the basis of a quiet and long-lasting system.

The reduced T-piece allows you to reduce the diameter of the branch directly at the junction to the correct size. An alternative – using a straight T-piece and then fitting a socket reducer – is technically possible, but it means one additional joint, more space in the installation trench or shaft, and potentially another leak point (with soldering, every joint is a risk). Professional practice clearly prefers the reduced T-piece wherever branch dimensioning requires a reduction.

Comparison of straight and reduced T-pieces: summary table

Property	Straight T-piece	Reduced T-piece
Bore diameters	All the same	At least one different
Number of joints at the junction	3 (+ possibly 1 additional reducer)	3 (everything in one fitting)
Hydraulic optimization	Only if all branches are the same	Each branch is optimal
Suitability for heating distribution	Limited, special cases	Standard, universal
Material costs	Lower fitting price, but + reducer	Everything in one piece
Space requirements	Higher (reducer increases length)	Compact
Typical application	Collectors, industrial, equal branches	Apartment and residential heating, radiators

Practical scenarios from real customer projects

Scenario 1: Apartment building, risers DN 28, radiators on DN 15

Imagine a typical apartment building from the 1970s undergoing renovation. The main riser is sized at DN 28 – it must supply several radiators on each floor. Each radiator has a DN 15 connection. What do you do at the junction? A straight T-piece 28 × 28 × 28 would give you a DN 28 branch, to which you would have to add a 28/15 reducer. That would be two joints, two soldering points, and unnecessary pipe length in the trench.

The correct solution is the T-piece 28 × 15 × 28 – the main line remains DN 28, the branch is directly DN 15. One fewer joint, a more compact installation, less work and less material. And importantly: the plumber can solder it all in one operation.

Scenario 2: Single-family house, loop DN 22, feeding a bathroom radiator via DN 15

In a single-family house, we run a heating loop in DN 22 around the entire house. At the bathroom, we branch off to a bathroom ladder radiator, which is sufficient with DN 15. In this case, a T-piece 22 × 22 × 15 comes into consideration. The main line continues further in DN 22, the side branch is immediately DN 15 – you go directly to the radiator valve without any intermediate fittings.

If you used a straight T-piece 22 × 22 × 22 and then a socket reducer 22/15 instead, the whole assembly would be 30–40 mm longer, with one additional joint. In a trench behind the plaster, every centimeter is important, and in addition, fewer joints = lower risk of problems during inspection.

Scenario 3: Underfloor heating, distribution unit

In underfloor heating, you usually have one supply from the boiler in DN 28, from which you feed a distributor. However, some systems have a transition unit, where the DN 28 loop supplies both the underfloor heating distributor (via DN 22) and another radiator in the ground floor directly. In this case, a T-piece 28 × 28 × 22 may be considered – the main line DN 28 continues to the next consumer, the branch DN 22 leads to the underfloor heating distributor.

This is a case where a straight T-piece 28 × 28 × 28 would technically also work, but then you would have to add a 28/22 reducer behind it, which would cost extra money and time. In addition, every unnecessary joint is a potential weak point after years of operation – oxidation of a soldered joint in a wet thermal insulation environment is not always visible from the outside.

Technical details: dimensions, tolerances and material

Copper T-pieces from Hepworth are manufactured according to the EN 1254-1 standard, which defines the dimensions of the socket, tolerances and material requirements. The material is copper class Cu-DHP (dephosphorised copper), suitable for soft and hard soldering. The inner diameter of the socket surface is calibrated to precise values that allow capillary soft soldering (up to 130 °C) or hard soldering (up to 250 °C and more) without leakage.

Important dimensions you work with in standard heating:

DN 15: pipe outer diameter 15 mm, inner approx. 13 mm, T-piece socket has an inner diameter tolerance of approx. 15.1–15.2 mm for capillary joint
DN 22: outer diameter 22 mm, inner approx. 20 mm, T-piece socket approx. 22.1–22.2 mm
DN 28: outer diameter 28 mm, inner approx. 26 mm, socket approx. 28.1–28.2 mm

The capillary clearance (difference between the inner diameter of the socket and the outer diameter of the pipe) is typically 0.05–0.2 mm. This fine clearance allows the molten solder to be drawn into the entire gap by capillary forces and create a watertight and solid joint. In the case of a reduced T-piece, the same requirements apply to each socket – only each socket has a different dimension, but the manufacturing process is identical.

The overall length of the fitting differs from the straight ones in the case of reduced T-pieces – in most cases, the central socket (where both dimensions are located) is slightly wider to ensure the body of the fitting has sufficient wall thickness when transitioning from a larger to a smaller diameter. For the plumber, this means that when planning the route in a groove or in an installation channel, you must take into account the specific geometry of the fitting – the manufacturer always provides a table of dimensions in the technical documentation.

How a reduced T-piece is made: a look into the production

It may surprise some to learn that a reduced T-piece is not made by welding two pipes of different diameters. It is manufactured either by cold forming from a single piece of copper pipe or by casting with machining of the sockets. In the case of cold forming, a side protrusion is pressed from the pipe, which is then machined to the required inner diameter – and it is precisely here that the narrowing of the branch occurs: the mandrel has a smaller diameter than the main line.

The result is a homogeneous, single-piece fitting without internal welds or joints. The copper wall has a uniform thickness all around, which ensures a long service life and resistance to the corrosive effects of the heat transfer medium. High-quality Hepworth fittings also undergo pressure testing during production – they are tested at a pressure significantly higher than the standard operating pressure of the heating system (usually 1.5–3 bar in apartment systems).

When a straight T-piece in heating really makes sense

It would be unfair to claim that a straight T-piece has no place in a heating system. There are situations where a professional might reach for it:

Collectors and distributors: If you are building your own copper collector for underfloor heating (which is nowadays more of an exception, as ready-made brass ones are available), all branches from the main line can have the same diameter.
Even load distribution: If both branches after the T-piece are truly identical (same number of radiators, same length, same hydraulic load), the same diameter makes sense.
Connection points at boilers: Some boiler connection points are designed so that all pipes have the same diameter and regulation is handled by valves and pumps.
Emergency solution in case of unavailability: If a reduced T-piece is not available at the moment and the installation cannot wait, a straight T-piece plus a reducer is an acceptable temporary solution – but always with the awareness that it is a compromise.

In practice, however, it holds true that in a standard apartment or family heating installation, a reduced T-piece makes up about 80–90 % of the used T-fittings. A straight T-piece is rather an exception.

Economic comparison: price of fittings vs. price of work

An argument that sometimes comes up on a construction site: “A straight T-piece is cheaper, I'll put a reducer and save money.” Let's look at it realistically. The price difference between a straight T-piece and a reduced T-piece of the same dimensions is typically in the range of a few percent – in absolute terms, we are talking about a difference of 20–80 cents per piece, depending on the dimension. A coupling reducer that you would need to buy additionally costs comparably.

What is crucial, however: with each additional soldered joint, the plumber spends extra time – on preparation, heating, soldering, and checking. With an hourly rate of a qualified plumber of 25–40 EUR/hour, one “saved” joint is actually more expensive if it takes an additional five minutes of work. In the installation of a family house with 15–20 radiators and 30–40 T-pieces, this is a real saving of several hours of work – in favor of reduced T-pieces.

Moreover: fewer joints = fewer potential leak points in the future. The cost of repairing a single leaking joint in plaster includes demolition and repair and is in the range of hundreds of EUR.

Most common mistakes when choosing a T-piece

From practice, I know that mistakes when choosing T-pieces repeat themselves. The most common ones are:

Mixing up the direction of the reduction: The plumber buys a T-piece 22 × 15 × 22 (where the reduced part is the “middle” position, i.e., the position where the fitting is connected to an existing pipe), but needs a T-piece 22 × 22 × 15 (where the reduced part is the branch). They look similar visually, but during installation, you will find a mismatch. More on reading markings in the article Dimensions of reduced T-pieces: how to read the marking 22 × 22 × 15.
Wrong orientation of the branch: A reduced T-piece is installed rotated and the branch points in the wrong direction – which, in rigid piping, means the need to replace the entire fitting.
Using a straight T-piece where a reduced one is needed: The most common case is when the project did not account for a reduction and the plumber reached for a standard piece that was available in stock. The result: the branch is oversized, the flow rate drops, and hydraulic regulation is complicated.
Mixing up outer and inner diameter: The diameter in the marking of the fitting always refers to the outer diameter of the pipe, not the inner one. Beginners sometimes look for a T-piece “for 15 mm inner diameter” – such a thing does not exist. Correctly: “DN 15” = outer diameter of the pipe 15 mm.

For more information about installation errors, read the article Common mistakes when installing copper T-fittings and how to avoid them.

Choosing the right reduced T-fitting: a brief procedure

If you are unsure which T-fitting belongs in your installation, follow these steps:

Determine the diameter of the main line – that is, the pipe from which you are branching off. For example, DN 22.
Determine the diameter of the branch – that is, the pipe leading to the radiator or another appliance. For example, DN 15.
Decide on the direction of the branch – is it perpendicular to the main line? If yes, it is a standard T-fitting. If it is in the same plane, it is a different type of fitting.
Check whether the main line continues with the same diameter – if yes, you are looking for a T-fitting X × X × Y, where X is the diameter of the main line and Y is the diameter of the branch.
Select the specific piece from the range – for example, from the category of reduced T-fittings Hepworth.

For a more detailed guide on selection, we also recommend the article How to choose the right reduced T-fitting for your heating system, where you will also find tips on hydraulic dimensioning of branches.

Most frequently asked questions (FAQ)

Can I use a straight T-fitting instead of a reduced one if I don't have the right one?

Technically yes – a straight T-fitting with a reducing coupling behind it will perform its function. However, it is not an ideal solution: you are adding one extra joint, increasing the overall length of the assembly, and increasing the risk of future leakage. If it is a temporary solution until the correct piece is available, it is acceptable. If it is a permanent installation in plaster, always reach for the correct reduced T-fitting.

Why is a reduced T-fitting more expensive than a straight one of the same dimensions?

The production of a reduced T-fitting is more complex – forming two different diameters in one fitting requires more precise stamping tools and more rigorous tolerance control. Despite this, the price difference is usually minimal and negligible in the overall cost of the installation compared to the time saved.

What is the difference between a T-fitting 22 × 22 × 15 and a T-fitting 22 × 15 × 22?

Both have the same three diameters, but the positions of the outlets differ. In the T-fitting 22 × 22 × 15, both ends of the main line are DN 22 and the reduced is the side perpendicular branch (DN 15). In the T-fitting 22 × 15 × 22, the middle position (the point of connection to an existing line) is DN 15 and both straight ends are DN 22 – this T-fitting is used differently, usually when you connect the fitting "laterally" to a thin pipe and continue with thicker pipe from both ends. In practice, the first variant (22 × 22 × 15) is much more common. If you are unsure, read the article Dimensions of reduced T-fittings: how to read the marking 22 × 22 × 15.

Is there anything special to do when soldering a reduced T-fitting compared to a straight one?

The soldering procedure is practically the same. The only difference is that with the smaller diameter outlet, you need to apply less molten solder and flux – the capillary gap is smaller and the smaller outlet heats up faster. Be careful not to overheat the smaller outlet while heating the larger one at the same time. An experienced installer will first heat the larger outlet, then smoothly move the torch to the smaller one. A detailed procedure is in the article Installation of a reduced T-fitting Hepworth: step-by-step soldering procedure.

Can I combine copper T-fittings from different manufacturers in one installation?

Yes, if all fittings meet the EN 1254-1 standard, they are compatible – the dimensions of the outlets are standardized and the pipes are also standardized (EN 1057). Hepworth fittings are fully compatible with standard copper pipes from any manufacturer of the corresponding dimensions. For certainty, however, we recommend not mixing fittings of different quality – cheap, no-name fittings may have worse tolerances, which complicates soldering and reduces the reliability of the joint.

Where can I find the full range of reduced T-fittings Hepworth?

The full current range including availability and prices can be found in the category of reduced T-fittings Hepworth on atria.sk. If you are looking for a specific dimension and cannot find it, consider that some dimensions are available only on order – in that case, do not hesitate to contact us.

Conclusion: a reduced T-fitting is the right choice for most situations

After going through hydraulics, construction, economics and practice, the conclusion is quite clear: for a standard installation of a hot water heating system in a family house or an apartment, a reduced T-fitting is the standard and preferred choice. It allows for proper dimensioning of each branch of the system, minimizes the number of joints, saves time and space during installation, and in the long run increases the reliability of the entire installation.

A straight T-fitting finds its place in specific cases, when all branches are truly equally dimensioned – which is rather an exception in heating practice. An investment in the right fitting from the beginning always pays off – whether in the form of saved work during installation, or in the form of trouble-free operation for decades without leaks or repairs.

If you are considering specific dimensions for your project, take a look at T-fitting 22 × 22 × 15 for the most common combination in family houses, or at T-fitting 28 × 28 × 22 for larger circuits. For further topics related to copper piping in heating, we also recommend reading Combining different diameters in a heating circuit using reduced fittings and How to correctly choose a reduction for a radiator branch from the main pipe.

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.