How to choose the right reduced T-piece for your heating system

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Why the choice of a reduced T-piece really matters

Among the most underestimated decisions when designing or renovating a heating system is the selection of the fitting at the branch-off point. When a plumber or a homeowner hears the term "T-piece," most of them imagine just a piece of copper in the shape of the letter T that holds the pipe together. In practice, however, it is a much more complex component – and the wrong choice can cause problems ranging from weak radiator performance, through pipe noise, to a permanent imbalance of the entire circuit.

A reduced T-piece is a specific case: instead of three identical diameters, it has at least one different dimension – usually smaller in the branch. This seemingly small detail determines how much water (and therefore how much heat) flows into the side branch. If you overdimension the branch, the main pipe loses pressure and other radiators become cold. If you underdimension it, the connected radiator cannot sufficiently heat the room. In the following sections, we will go through everything you need to know – from basic geometry through hydraulics to specific dimensions and installation.

Anatomy of a reduced T-piece: what the three numbers in the designation mean

Each reduced T-piece from the Hepworth range is marked with a triplet of numbers, for example 22 × 22 × 15 or 28 × 22 × 28. The order of the numbers is not random – it follows a fixed logic that you need to know before you order or install anything.

The standard convention for copper fittings according to EN 1254-1 is as follows:

First number – inlet diameter (the inlet neck, from where the water comes from the boiler or the previous pipe section)
Second number – outlet diameter (the continuation of the straight pipe route behind the T-piece)
Third number – branch diameter (the side branch, usually towards a radiator or a secondary circuit)

This differentiation is key. Let's take a concrete example: T-piece 22 × 22 × 15 has a main flow pipe of 22 mm and a branch of 15 mm. In other words, water flows through a pipe with an outer diameter of 22 mm along the main route, while a 15 mm pipe branches off to the side branch – for example, to a smaller radiator or to a floor heating circuit. This is one of the most commonly used dimensions in family homes, where the main distribution runs in 22 mm and the supply to individual radiators is in 15 mm.

If you are interested in a more detailed explanation of the numerical designation system, I recommend the article Dimensions of reduced T-pieces: how to read the designation 22 × 22 × 15 in this Knowledge Center – there you will also find tables of all common combinations.

Hydraulics of the branch: why the branch diameter is crucial

Behind every radiator lies simple physics: the amount of heat a radiator delivers to a room depends on the water flow and the temperature difference between the supply and return. The flow, in turn, depends on the hydraulic resistance of the entire circuit – and one of its elements is the branch diameter in the T-piece.

In a standard two-pipe system with forced circulation, it holds that the smaller the branch diameter, the higher its hydraulic resistance, and the less water flows into a given radiator. This is intentional: we want each radiator to receive as much water as it needs – not more, not less. Therefore, the size of the branch is designed to naturally limit the flow without the need for excessive balancing with regulating valves.

A practical example from practice: in a four-radiator circuit, you have a main pipe of 22 mm and four radiators. If you made each branch also 22 mm (straight T-piece), each radiator would effectively "compete" with the others for the flow and balancing would be very sensitive. Instead, if you use 15 mm branches (which corresponds to T-piece 22 × 22 × 15), the natural resistance of each branch helps better distribute the flow and the system is more hydraulically stable.

This topic is covered in detail in the article When to use a reduced T-piece instead of a straight one in heating distribution in this Knowledge Center.

Overview of the most commonly used dimensions and where to use them

In the category of reduced Hepworth T-pieces, you will find several basic combinations of diameters. Here is a practical overview with typical use cases:

T-piece 22 × 22 × 15 – the most universal for family homes

T-piece 22 × 22 × 15 is probably the most commonly installed dimension in Slovak households. A 22 mm main distribution is standard for boilers with a power of up to 20–25 kW and circuits with 3–6 radiators. The 15 mm branch corresponds to the supply pipe to one radiator – whether panel, finned, or bathroom finned. This T-piece can be found in almost every renovated apartment or in the new construction of a family home.

T-piece 22 × 15 × 22 – reduction in the straight direction

T-piece 22 × 15 × 22 is an interesting case where the reduction is not in the branch, but in the straight continuation of the pipe. The inlet is 22 mm, the straight continuation behind the T-piece is 15 mm, and the branch is again 22 mm. This is used when we want a larger portion of the flow to go into the side branch (for example, to a larger radiator or to a group distributor), while the rest of the circuit continues through a narrowed 15 mm pipe to a smaller circuit. It is a more sophisticated way of hydraulically pre-distributing the flow, which requires at least a basic hydraulic calculation.

T-piece 28 × 28 × 22 – for larger systems with boilers 25–40 kW

T-piece 28 × 28 × 22 is used where the main distribution is run in 28 mm pipe. This is typical for houses with a larger number of radiators, for two-circuit systems (heating + hot water), or for boilers with higher rated power. The 22 mm branch then supplies either a group of two to three radiators in a smaller zone, or one larger fan coil. Very common is also its use in server rooms, wineries, and other spaces with higher cooling or temperature control requirements.

T-piece 28 × 22 × 28 – reduction in the straight direction in the 28 mm system

T-piece 28 × 22 × 28 serves a similar purpose as 22 × 15 × 22, but in a stronger dimension. The inlet and branch are in 28 mm, while the straight output is reduced to 22 mm. You can use it, for example, when a complete secondary loop in 28 mm (e.g., a branch for floor heating with its own pump) branches off from the main 28 mm pipe, while the continuing route of the main distribution narrows to 22 mm, as it supplies only a smaller number of remaining radiators.

T-piece 28 × 15 × 28 – a direct jump from 28 to 15 mm in the branch

T-piece 28 × 15 × 28 is a more specific case, where only a narrow 15 mm pipe branches off from a powerful 28 mm distribution – for example, to an expansion tank, an air vent, or a temperature sensor. It is less frequently used for direct radiator supply, as the difference in diameters is quite large here and the hydraulic properties require careful evaluation. In installation practice, you may encounter it in a technical room, where it is necessary to connect an auxiliary loop to the main distribution without significantly affecting the main flow.

How to calculate which branch diameter you need

Calculating the correct branch diameter does not have to be complicated if you know a few basic parameters. In practice, a simplified procedure is used:

Step 1: Determine the radiator power. Each modern radiator has its rated power in watts (W) listed in the catalog under standard temperature conditions (usually 75/65/20 °C or 70/50/20 °C). For example, a panel radiator 600 × 1400 mm has a typical power of 1,500 – 2,000 W.

Step 2: Calculate the required flow rate. The formula is simple: Q [l/h] = P [W] / (1,163 × ΔT [K]), where ΔT is the difference between the supply and return water temperatures. For example, for a radiator of 1,500 W and ΔT = 10 K: Q = 1,500 / (1,163 × 10) = 0.129 l/s = 464 l/h.

Step 3: Check whether the selected branch diameter can carry the flow rate at an acceptable water velocity. The recommended velocity in copper pipe fittings and piping is 0.3 – 1.0 m/s (ideally up to 0.7 m/s to avoid noise). For a 15 mm copper pipe (internal diameter approx. 13 mm), the maximum flow rate at 0.7 m/s is approximately 315 l/h. For a 22 mm pipe (internal diameter approx. 20 mm), it is approximately 760 l/h. This means that for a radiator with a flow rate of 464 l/h, a 15 mm branch is tight – a 22 mm branch is needed.

More on this topic can be found in the article How to correctly choose a branch reduction when feeding a radiator from the main pipe in this Knowledge Center.

Material aspect: why exactly copper Hepworth fittings

The Hepworth range is a long-term proven series of copper capillary fittings manufactured according to the British standard BS EN 1254-1. Copper fittings have several key advantages over galvanized steel and plastic in the context of heating systems:

Temperature: Copper withstands a continuous operating temperature of up to 110 °C, which covers both modern condensing boilers with higher supply temperatures and older low-temperature distribution systems equally.
Pressure: Standard copper Hepworth fittings can withstand an operating pressure of 16 bar at 110 °C – significantly beyond the scope of most domestic heating systems (normal operating pressure 1.5 – 3 bar).
Antibacterial properties: Copper naturally inhibits the growth of bacteria (including Legionella), which is relevant for systems combining heating and hot water preparation.
Solderability: Capillary Hepworth fittings are designed for soft soldering (Sn-Cu or Sn-Ag-Cu solder), which ensures hermetic and long-lasting joints without the need for threaded sealing.
Dimensional accuracy: Tolerances of diameters and capillary sleeve depth are very tight, which eliminates assembly errors and minimizes the risk of leaks.

A detailed comparison of copper, galvanized, and plastic fittings can be found in a separate article in the Heating HEPWORTH section.

Typical installation scenarios from practice

Scenario 1: Reconstruction of a radiator network in an older panel apartment

In practice, I have encountered dozens of apartment reconstructions where the original distribution system was made of steel pipes 3/4" and 1/2". The new owner wanted to completely replace the entire apartment with copper piping. Solution: the main vertical distribution between the apartments remained steel (the building management did not allow it to be changed), but the horizontal distribution in the apartment was completely replaced with copper 22 mm (main route) with 15 mm branches to individual radiators. A T-piece 22 × 22 × 15 was used at each branch, and the reduction from 1/2" steel to 15 mm copper was solved using a transition threaded fitting. Result: a quieter system, faster heating, no leaks after 8 years.

Scenario 2: New construction of a family house with a 30 kW boiler

A house with a floor area of 180 m², two storeys and 9 radiators. The designer planned the main horizontal distribution in 28 mm, risers between floors in 22 mm and branches to radiators in 15 mm. On the upper floor, at the point where the 28 mm main line branches off to a 22 mm riser, a T-piece 28 × 28 × 22 was used. On the lower floor, where the main 28 mm line branches off to a 22 mm branch for 4 radiators and the main line continues in 28 mm to the remaining 5 radiators, a T-piece 28 × 22 × 28 was used. This combination enabled a natural hydraulic flow distribution without excessive throttling by control valves.

Scenario 3: Underfloor heating in a bathroom connected to a radiator circuit

A bathroom in a new build had a combination of a panel radiator and an underfloor heating loop (18 mm pipe, its own pump, mixing valve). The main 22 mm distribution line ran through the bathroom. At the point of connection of the underfloor heating loop, a T-piece 22 × 22 × 15 was used, with the 15 mm branch leading to the mixing unit. This mixing unit had its own pump, so the hydraulic connection to the main circuit was minimal. The solution is elegant and generally common – the reduced branch limits the parasitic influence on the main circuit during operation of the underfloor heating pump.

Installation: key steps and common mistakes

The installation of a reduced T-piece Hepworth is capillary soldering – a technique in which the solder is drawn into the gap between the fitting and the pipe by capillary action. The correct procedure is as follows:

Cutting the pipe to size using a pipe cutter (not a saw – which leaves rough edges)
Deburring the inner and outer edge of the cut pipe
Mechanically cleaning the end of the pipe with sandpaper (to a shine) and similarly the inside of the fitting
Applying flux (soldering paste) to the end of the pipe and inside the fitting
Pushing the pipe into the fitting until it reaches the stop
Evenly heating the joint with a gas torch – first the fitting, then the pipe, not directly the solder
Placing the solder (Sn97Cu3 wire or with silver) against the edge of the fitting – capillary action draws it in automatically
Wiping off excess flux with a wet cloth after the joint has solidified

The most common mistake during installation of a reduced T-piece is incorrect orientation – swapping the inlet/outlet/branch ports. The result is either reversed flow or incorrectly directed reduction. Always check which port is which before soldering and fix the position of the fitting so that you do not rotate it during soldering.

The second common mistake: insufficient insertion of the pipe. If the pipe is not pushed all the way into the depth rib of the fitting, the capillary gap is uneven and the joint will not be airtight. Check this before each heating.

A detailed step-by-step procedure with photographs can be found in the article Installation of a reduced T-piece Hepworth: soldering step by step in this Knowledge Centre. I also recommend reading Common mistakes when installing copper T-pieces and how to avoid them.

Combination of different diameters in one circuit

In real installations, it is rare to work with a single pipe size. Usually, there is a hierarchy: boiler → main distribution (large diameter) → branches (medium diameter) → radiators (small diameter). At each transition between levels, a reduced T-piece or a direct reduction is used.

Example of a three-level hierarchy: 35 kW boiler → 28 mm main distribution → two 22 mm branches (each supplying one floor) → 15 mm offshoots to individual radiators. At the 28 mm → 22 mm transition, there will be a T-piece 28 × 28 × 22 (or 28 × 22 × 28 depending on the direction), and at the 22 mm → 15 mm transition, there will be a T-piece 22 × 22 × 15.

It is important to keep in mind during design that each narrowing means hydraulic resistance. If there are too many narrowings in succession without a significant change in flow (for example, a long series of radiators on one branch), the last radiator will have the smallest pressure drop and will be the coldest. The solution is either hydraulic balancing with valves or a different branching method (star distribution from a central manifold).

For more information on combining different diameters, read the article Combining different diameters in a heating circuit using reduced fittings.

Inspection and maintenance of connections

Copper brazed connections are essentially maintenance-free – provided they were properly made and the system is filled with suitable (neutral or slightly alkaline) water. Problems arise in the following cases:

Acidic water: A pH below 7.0 causes corrosion of copper pipe, typically visible as greenish stains around the connections. The solution is to adjust the pH of the filling medium.
Stray currents: Electrolytic corrosion occurs when copper is in unprotected contact with another metal (e.g., an aluminum radiator without a sacrificial anode). The result is pinhole leaks in the pipe or at the connections.
Mechanical stress: Fittings must not hang loosely without supporting the pipe. Thermal expansion of copper pipe (1.7 mm/m/10 °C temperature change) can create strong mechanical stress and crack the brazed joint if not properly supported.

I recommend visually inspecting all connections in the technical room once every 3–5 years. If you see white or green deposits at any connection, it is a warning sign. For more information on this topic, see the article Inspection and maintenance of copper pipe fittings in a central heating system.

Frequently asked questions (FAQ)

Can I use a T-piece 22 × 22 × 15 instead of a T-piece 22 × 22 × 22 (straight T-piece) when feeding a radiator?

Yes, and in most cases it is actually a better choice. A reduced 15 mm branch naturally limits the flow to the radiator, which makes hydraulic balancing of the entire circuit easier. A straight T-piece with a 22 mm branch should be used only if the radiator has a high output (typically over 2,000 W) and requires a high flow, or if you are feeding a group of radiators behind the branch, not just one.

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

The difference lies in where the reduction is placed. In a T-piece 22 × 22 × 15, the reduction is in the branch – the side branch is 15 mm and the straight run remains 22 mm. In a T-piece 22 × 15 × 22, the reduction is in the straight run – the straight run narrows to 15 mm and the larger flow is directed into the side branch of 22 mm. This is a crucial hydraulic difference, and the two fittings are not interchangeable.

Can I solder a reduced T-piece myself, or should I hire a professional?

Soldering copper pipe is a craft that you can master with practice, but it requires the right tools (gas torch, pipe cutter, deburring tool, sanding tool) and knowledge of the procedure. For installation in the primary heating circuit (where higher pressures and temperatures are present), we recommend leaving the work to a certified installer – also for the sake of boiler warranty. For secondary circuits (e.g., auxiliary piping to an expansion tank), an experienced DIY enthusiast can complete the job after studying the correct procedure.

What if the installed T-piece leaks after the first filling?

The most common causes are: insufficient insertion of the pipe (the joint is not fully around), insufficient heating during soldering, or surface contamination (grease, oxidation). Repair procedure: drain and empty the system, heat the joint until the solder melts, remove the pipe, clean both surfaces mechanically and chemically (with flux), and repeat the joint. Never repair a leaking joint without disassembling it – applying additional solder to an existing joint usually does not work.

Are reduced T-pieces from Hepworth suitable for systems with a condensing boiler?

Yes. Condensing boilers typically operate with a supply temperature of 50–75 °C and a return temperature of 30–60 °C, which is significantly below the nominal limit for copper fittings (110 °C). The only specific consideration is that condensing boilers prefer lower return temperatures – which is completely problem-free from the perspective of fittings. Copper and condensing boilers are a long-term proven combination.

Are there reduced T-piece sizes from Hepworth that are not available and need to be handled differently?

The Hepworth range covers the most common diameter combinations used in European heating systems. If you need an exotic combination (e.g., 35 × 28 × 15), it is usually solved with a series of standard fittings: a straight T-piece + direct reduction, or a T-piece + reduced coupling at a specific port. Combinations of multiple fittings are hydraulically equivalent and only add minimal resistance.

Conclusion: Choosing the right T-piece saves time and money

Selecting a reduced T-piece is not a routine decision, but a technical choice with a direct impact on the hydraulic behavior of the entire system. An incorrectly chosen branch size can lead to an unbalanced system, unnecessary noise, higher pump energy consumption, and ultimately uneven heating of rooms. On the other hand, a well-designed network of fittings – where each T-piece has the correct diameter ratio in the right place – operates quietly, efficiently, and without the need for unnecessary balancing.

Basic rules to remember: always read the labeling in the order inlet × outlet × branch, choose the branch diameter according to the required flow to the given section, consider the overall diameter hierarchy in the system, and consult a designer or experienced installer if in doubt. The investment in the right fitting is minimal compared to the costs of reopening walls or reworking an unbalanced system.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.