Same vs. Reduced T-Piece: When to Use Which Type
Same-size vs. reduced T-piece: when to use which type and why it matters
When you reach a pipe branching point, you face a decision that will affect the entire hydraulics of your branch: should I use a same-size T-piece or a reduced one? At first glance, this may seem trivial – after all, a T-piece is a T-piece, right? In practice, however, it turns out that this choice determines whether the branch will operate quietly and without problems for decades, or whether you will hear gurgling, lose pressure where you need it, and in a few years deal with leaks or an unbalanced circulation.
In this article, we will examine both types in depth – what they are, when each of them comes into play, what are the hydraulic consequences of the wrong choice, and how to decide in specific situations from everyday installation practice. We will also go through dimensioning, materials, and common mistakes that I repeatedly see on customer projects during renovations and new installations.
What is a same-size T-piece and what is a reduced T-piece
The basic conceptual difference is simple, but it is important to have it completely clear before we start comparing.
A same-size T-piece (also called symmetrical or equal-branch) has all three ports of the same diameter. This means that whether you look at the incoming flow or at the two outputs, they are all dimensionally identical. A typical designation looks like 15 × 15 × 15 or 22 × 22 × 22 – all three numbers are the same. In the Hepworth category, you can find for example T-piece 15 × 15 × 15, T-piece 22 × 22 × 22, or T-piece 28 × 28 × 28 – these are all same-size T-pieces.
A reduced T-piece has at least one port with a different diameter than the others. Most often, the branch (side) outlet is reduced, while the straight-through axis remains in the original diameter. For example, a T-piece 22 × 22 × 15 – the straight-through axis is 22 mm, but the branch is only 15 mm. It is also possible to reduce the straight-through direction (i.e., the continuation behind the T-piece is a smaller diameter than the inlet), which is less common, but occasionally necessary in over-dimensioned main branches.
If you are interested in a more detailed explanation of the three numbers in the designation, see the article What do the dimensions of a T-piece mean and how to read them correctly – there it is explained with examples, including less standard combinations.
Hydraulics in simplicity: what happens to the flow in a T-piece
In order to meaningfully compare both types, we need to understand what physically happens when a liquid (in heating typically water or a glycol mixture) enters a T-piece and divides into two branches.
The flow coming through the main axis has a certain speed and kinetic energy. At the branching point, the energy is divided between the straight branch and the branch. The branch always represents a higher hydraulic resistance than the straight branch, because the flow has to change direction. The sharper the branch (and a standard T-piece has a branch at a right angle, i.e., 90°), the greater the pressure difference that arises between the straight direction and the branch.
When a reduction is introduced – i.e., the branch is narrower than the main axis – the speed of the flow in the branch increases (flow = cross-section × speed, the flow does not change, the cross-section decreases). The increased speed means a higher dynamic pressure in the branch, but also higher pressure losses due to friction and turbulence. This is the key point to focus on when making a choice.
In practice, this means: if you need to bring a relatively small flow to the branch (e.g., one radiator or a small branch), a reduced T-piece allows you to maintain an appropriate water speed in the branch without having to feed a large pipe from a large pipe through a same-size T-piece and then immediately reduce it. If, on the other hand, you need the same or similar flow in the branch as in the straight-through axis, a same-size T-piece is the right choice.
When to use a same-size T-piece unambiguously
A same-size T-piece is the right choice when you divide a pipe into two branches with comparable flows, or when you need to maintain the same diameter in the branch for technological reasons.
Symmetrical pipe division
Classic example: a main supply of 22 mm comes to a point where the branch is divided into two equal loops of floor heating – each loop needs the same flow. Here you will use a T-piece 22 × 22 × 22 and both branches continue in the same diameter. Hydraulically, this is the cleanest solution, because both branches have the same conditions and balancing is simple.
Elbow for the same size pipe
If you are branching off from a 22 mm riser to a manifold that also requires the full 22 mm diameter (for example, in an open system or when connecting a larger radiator), you do not reduce the size – you use the same T-piece.
Technological reasons for maintaining the diameter
For some devices (boilers, storage tanks, manifolds), the flow is dimensioned for a specific pipe diameter and the manufacturer requires that the diameter not be reduced before or after the device. In such cases, using the same T-piece is not just an option, but a requirement.
Connecting measuring or drain valves
With small diameters, for example when installing a drain valve at the bottom of a branch or a thermometer on a 15 mm pipe, you use a 15 × 15 × 15 T-piece and connect the branch with a valve of the same diameter. Reducing in this case is not necessary, as the valve is the same size.
When to definitely use a reduced T-piece
A reduced T-piece comes into play when the flow in the branch differs significantly from the flow in the main line, or when the connected device requires it.
Feeding a single radiator from the main line
This is by far the most common case we see in practice. The main line runs in a larger diameter – for example, 22 mm – and you want to branch off to one panel radiator, which needs to be connected with a 15 mm pipe. You use a reduced T-piece 22 × 22 × 15 (if the main line continues in 22 mm) or 22 × 15 × 15 (if the 15 mm pipe continues after the T-piece). An alternative – the same T-piece 22 × 22 × 22 and a separate reduction 22/15 after it – is functionally acceptable, but it adds an unnecessary joint, which is every potential leak point.
Branch for a small heater or convector
Similarly, a branch for a convector or a bathroom fin radiator, which has a low output and does not require the full flow of the main line, uses a reduced T-piece. Here, the reduced T-piece also serves as a hydraulic barrier – it limits the amount of water drawn into the branch without the need for an additional regulating component (although a thermostatic valve is always required).
Transition from a dimensioned main line to a branch with lower load
In the hierarchy of the piping system – riser → horizontal branch → individual branches – the diameters typically decrease from the source toward the consumers. A reduced T-piece allows this transition to be made directly at the branch point, without a separate reducing coupling after the T-piece.
Space saving and minimizing joints
In tight spaces – behind drywall, in a groove, in an installation shaft – every additional joint is a nuisance. A reduced T-piece combines two functions (branching + reducing) into one piece, saving space and reducing the number of soldered joints during brazing.
Dimensioning and calculation: how to determine which type you need
In practice, the dimensioning of T-pieces is based on two factors: the flows in the branches and the water velocity in the pipe. The recommended water velocity in copper pipe for heating is 0.5 to 1.0 m/s for standard dimensions. Exceeding 1.5 m/s causes erosion of the pipe interior and noise, while too low a velocity (below 0.3 m/s) leads to sedimentation and air pockets.
Example calculation: You have a main line of 22 mm with a flow of 600 l/h. The branch needs to supply one radiator with a power of 1,200 W. With a temperature drop of 20 °C, the flow to the radiator is approximately 52 l/h. If you used a same T-piece 22 × 22 × 22 and a 22 mm pipe all the way to the radiator, the velocity in this pipe would be:
velocity = flow / cross-section = (52/3600) / (π × 0.011²) ≈ 0.038 m/s – this is catastrophically low, air will settle, the water will "stand still" and the radiator will heat poorly. The correct solution: a reduced T-piece and a 15 mm pipe → velocity ≈ 0.087 m/s, which is still low, but with 10 mm (copper 10/8 mm) it would be around 0.18 m/s – acceptable for connecting a single radiator.
This example clearly illustrates why choosing the diameter of the T-piece is not just an aesthetic matter. The diameter of the branch directly affects whether the radiator will be properly hydraulically supplied.
If you are interested in the topic of hydraulic balancing in more detail, please see the article How to divide the heating pipe using a T-piece without pressure loss – there are detailed explanations of local and linear pressure losses and practical balancing methods.
Material perspective: Hepworth copper T-pieces and their parameters
Hepworth is an established manufacturer of copper plumbing components, whose T-pieces are characterized by precise geometry of the collars and the quality of the internal surface processing. For brazing, the precision of the collar diameter is critical – tolerances are typically in the H8/h8 range, which ensures the correct clearance for capillary (hard) brazing.
Available same T-pieces in the Hepworth category cover the basic dimensions of a typical family house: 15 × 15 × 15 T-piece for small branches and valves, 22 × 22 × 22 T-piece for main lines and 28 × 28 × 28 T-piece for risers and larger horizontal lines. Reduced combinations – for example 22 × 22 × 15 or 28 × 22 × 22 – are part of the same product line, but you need to look for them in the category Hepworth T-pieces.
From the perspective of thermal resistance and pressure parameters, the copper T-pieces from Hepworth are designed for working pressures typical of standard heating systems (up to 10 bar at standard temperatures up to 110 °C), which with a large margin covers the requirements of domestic boiler circuits, where the working pressure typically does not exceed 2–2.5 bar and the temperature 90 °C.
Learn more about the differences between copper T-pieces and alternative materials in the article Copper Pipe T-Pieces vs. Other Materials: What Differs and What Is Better.
Installation considerations: Same vs. reduced during soldering
From the perspective of soldering installation, there is no fundamental difference in the procedure between a same-size and a reduced T-piece. The same rules apply: the pipe must be aligned and cleaned, the socket of the T-piece as well, apply flux, heat properly (evenly, not directly on the joint), and the solder will flow into the capillary all around. Different heating parameters apply for each socket diameter.
Where a reduced T-piece becomes more complicated: when heating the entire T-piece, you must consider that different-sized sockets require different temperatures. The larger socket (22 mm) requires longer heating than the smaller one (15 mm). If the procedure is incorrect, it is possible that the smaller socket overheats and the solder runs out, while the larger one is still not sufficiently hot. The correct procedure: heat the larger sockets first, then the smaller ones, and solder from the heavier to the lighter joint. When soldering multiple sockets at once – if you are soldering the entire T-piece at once with three pipes – use a larger torch and be careful that the heat does not escape into already completed joints.
A detailed installation guide can be found in the article Installing a T-Piece on a Copper Pipe: Soldering Step by Step.
Typical practical scenarios: How to decide in specific situations
Allow me to analyze several scenarios that repeat in contracts and where I see a decision for a same-size or reduced T-piece with specific consequences.
Scenario 1: Reconstruction of an apartment building, shared riser
A 28 mm riser runs through four floors. On each floor, it branches off to an apartment circuit. The flow in the apartment circuit is significantly lower than the total flow in the riser. Here, a reduced T-piece 28 × 28 × 22 or even 28 × 28 × 15 (depending on the apartment's output) is clearly the right choice. The riser remains 28 mm in full height, and each floor receives a diameter corresponding to its load.
Scenario 2: Single-family house, division into two parts of the building
The boiler supplies two wings of the house – the old and the new. Both wings have comparable output and flow. Here, a same-size T-piece 22 × 22 × 22 is the right choice – both branches run in the same diameter, each has its own circulation pump or balancing valve, and the system is hydraulically symmetrical.
Scenario 3: Bathroom stair radiator on 15 mm piping, branch from 22 mm main line
This is a textbook situation for a reduced T-piece 22 × 22 × 15. If you use a same-size T-piece 22 × 22 × 22 and then reduce it afterward, you will lose space, add a joint, and additionally risk hydraulic imbalance, because the 22 mm "stub" before the reduction functions as an unintended pressure accumulator. The reduced T-piece here also plays the role of a fine hydraulic barrier.
Scenario 4: Measuring device on the pipe
You want to install a thermometer on the water side after the circulation pump, on a 22 mm pipe. The thermometer has a 1/4" or 1/2" threaded connection. Here, you will use a same-size T-piece 22 × 22 × 22 and thread a threaded reducer onto the branch for the thermometer. Reason: the thermometer is a blind branch (no flow passes through it), so its "diameter" is not dimensionally relevant for the flow – it only matters that you can reach it.
Scenario 5: Two-pipe system with individual connections for each radiator
A classic modern two-pipe star system: from the manifold, a separate 15 mm (or 12 mm) pipe goes to each radiator. At the manifold – if it is a soft manifold collector – T-pieces are not needed, as the manifold replaces them. But if you are working without a manifold and feeding each radiator via branches from the main 22 mm branch, each branch is a reduced T-piece 22 × 22 × 15. The last branch of the branch is usually a reduced T-piece, where the flow path changes to 15 mm: 22 × 15 × 15.
Most common mistakes in selection and their consequences
In contracts, I repeatedly encounter the same decisions that turn out to be problematic in practice. Not because they are functionally non-functional, but because they unnecessarily complicate installation, increase costs, or cause hydraulic problems.
Mistake 1: Same-size T-piece everywhere, reduction separately. Some plumbers have a stock of same-size T-pieces and always add the reduction separately. Result: unnecessary joints, higher chance of leakage, more space in the installation shaft. A reduced T-piece is technically cleaner and more professional a solution wherever possible.
Mistake 2: Reduction in the flow path instead of in the branch. If you reduce the direct flow path and keep the full diameter in the branch, you reverse the hydraulic logic of the system – the branch will dominate and the direct branch will be throttled. This can cause hydraulic overloading of the branch and undercooling of other consumers on the direct branch.
Error 3: Wrong order of numbers in the designation. The designation of a T-piece is always: inlet (run-through axis) × outlet (run-through axis) × branch. If you mix this up, you will buy the wrong piece. Read more about reading designations in the article What do the dimensions of a T-piece mean and how to read them correctly.
Error 4: Using a reduced T-piece with a too small branch on the main branch. If the branch is too small (e.g., 15 mm on a 28 mm run-through axis), turbulence at the branch can be significant and cause noise at higher flow rates. For larger diameter differences, it is sometimes more advantageous to use the same T-piece and attach an external reducing elbow behind it, which reduces the diameter more smoothly.
To read about other common installation errors, see the article Common errors in T-piece installation and how to avoid them.
Summary of the decision-making process: a simple selection procedure
If you are unsure which type to choose, use this simple decision-making process:
- Step 1: What is the diameter of the run-through axis (pipe that comes in and continues)? This is the first and second number in the T-piece designation.
- Step 2: What diameter does the branch need? If it matches the run-through axis → same T-piece. If it is smaller → reduced T-piece.
- Step 3: Does the diameter also change in the run-through axis behind the T-piece (the continuing pipe is smaller than the incoming one)? Yes → reduced T-piece with a different combination of numbers (e.g., 22 × 15 × 15).
- Step 4: Check the result: does the flow and water speed in the branch correspond to the recommended values (0.3–1.0 m/s)? If not, change the branch diameter.
If you have doubts about the correct sizing of the circuit, you can also read the article How to choose the right T-piece for copper pipe: diameters 15, 22 and 28 mm, where there are specific tables of recommended diameters for various outputs and flows.
Frequently asked questions (FAQ)
Can I replace a same T-piece with a reduced one and vice versa if I only have one available?
Yes in an emergency, but with consequences. If you use a same T-piece instead of a reduced one, you will have to add a separate reduction – which is technically functional but adds a joint. If you use a reduced T-piece instead of a same one (i.e., in a situation where both branches should have the same diameter), the smaller branch will limit the flow and the other branch will not be hydraulically supplied correctly. This is a more serious problem. In most cases: using a same instead of a reduced is acceptable with a separate reduction, the opposite is not.
Is a reduced T-piece mechanically stronger or weaker than a same one?
The mechanical strength of a T-piece depends on the material and wall thickness, not on the symmetry of the branches. Both types – same and reduced – are mechanically equivalent when made by the same manufacturer and material. A reduced T-piece has a thinner wall at the smaller branch, but this corresponds to the lower pressure in the smaller pipe, so safety parameters are maintained. Both variants have the same certifications and pressure classes.
Why are there fewer reduced T-pieces in the catalog than same ones?
Same T-pieces have only three possible diameters (15-15-15, 22-22-22, 28-28-28 for standard dimensions), while reduced T-pieces have many more combinations – each combination of three different diameters is a separate product (22-22-15, 28-22-15, 28-28-22, 28-22-22, etc.). Manufacturers and distributors therefore usually stock only the most common combinations. Less common combinations are ordered on request or solved by combining a same T-piece with a reducing coupling.
Can I install a reduced T-piece rotated – i.e., with the smaller diameter as the inlet?
Physically yes, but hydraulically it is undesirable. If you bring water into the smaller branch and then divide it into two larger outlets, the speed at the inlet will be high and in the body of the T-piece there will be a sudden slowdown and pressure rise. This causes turbulent losses and in the extreme case noise. Correct orientation: the larger diameter is always on the inlet (or run-through axis), the smaller diameter is the branch.
Is there any situation where a reduced T-piece is counterproductive even where you would expect it?
Yes – in low-temperature systems with very low flows (e.g., condensing boilers with large floor heating areas and small temperature differences of 30/40 °C), the water speed is generally low. If you reduce the branch even further, the speed will drop below 0.2 m/s and air will constantly settle in the branch. In such cases, it may be advantageous to keep a larger diameter in the branch (same T-piece) and regulate the flow using thermostatic or balancing valves.
How can I tell that I used the wrong type of T-piece after starting the system?
Signs of a wrong choice are: a radiator or branch supplied from the branch is always cold or significantly underheated compared to others (branch diameter too small), or conversely – one circuit takes too much water and the others suffer (branch diameter too large, insufficient hydraulic resistance). Another sign is noise at the T-piece – hissing or cracking during flow, which indicates too high speed or turbulent losses at the reduction. The solution is always to replace the T-piece with the correctly dimensioned type.
Conclusion: The right T-piece saves time, money and problems
Deciding between a same and a reduced T-piece is not just a technical formality. It is one of the basic dimensioning decisions that will affect the hydraulic behavior of the entire distribution. A same T-piece belongs where both branches are equal – symmetrical distributions, connections to equally sized devices, maintaining the diameter. A reduced T-piece is used when the flow in the branch is significantly lower than in the run-through axis, or when you want to save a joint by combining two functions into one piece.
The main rule from practice, on which you can rely: if you are thinking about whether to add a reduction after the T-piece, better buy a reduced T-piece directly. Every additional joint is a risk – and in heating, where joints work under cyclic pressure and temperature changes for decades, this is doubly true.
If you are unsure about a specific type for your project, take a look at the available products directly in the category Hepworth T-pieces – and if you have doubts about hydraulics or dimensioning, read other articles in the Knowledge Center, for example Leakage of a T-piece in the heating system: causes and solutions or Maintenance and inspection of T-piece connections in central heating systems, where you will learn what makes a well-chosen and properly installed T-piece a durable and trouble-free part of your heating system in the long run.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your home? Write to us – we're happy to help.