How to choose the right T-piece for copper pipe: diameters 15, 22 and 28 mm
How to choose the right T-piece for copper pipe: diameters 15, 22 and 28 mm
The T-piece is seemingly a simple fitting. Three outlets, one piece of copper, a few seconds of soldering – and you have a branch in the pipe. Despite this, I often encounter errors when selecting a T-piece on jobs, which then cause problems: wrong diameter, confusion between standard and reduced fittings, or simply buying a T-piece that does not belong in a particular place in the system. This article is therefore devoted to this most basic, yet often underestimated question: how to choose the right T-piece for a copper pipe in diameters 15, 22 and 28 mm.
We will not only look at dimensional specifications, but also at where each diameter is used in practice, what are the capacity limits of each size, and of course – we will refer to specific Hepworth products that cover these dimensions. If you are looking for information on what the numerical designations of T-pieces mean, we recommend reading our article What do the dimensions of a T-piece mean and how to read them correctly, where this issue is explained in depth.
Why is the selection of the T-piece diameter important at all?
When I design a heating distribution system or repair an older system, one of the first things I address is hydraulic balance. That is a fancy word for a simple thing: that each radiator gets enough heat and that the pump does not work unnecessarily hard. And it is precisely the T-piece where the medium flow is divided or joined – which means that its diameter significantly affects the hydraulic resistance at that location.
A T-piece that is too small in the main line causes throttling – the medium is forced through a smaller cross-section, the flow speed increases, the resistance increases, the pump works harder, and you may hear noise in the system. On the other hand, a T-piece that is too large in a branch, where you have a weak flow, can lead to poor heat distribution and to the fact that air is not properly vented in such a place. Choosing the correct diameter is therefore not only about physically "fitting" the fitting to the pipe – it is a hydraulic and operational-technical decision.
Diameter 15 mm: where you will find it and where it belongs
The 15 mm copper pipe is the absolute basis of Slovak and Czech heating distribution systems. You will find it everywhere where there is a branch to an individual radiator, a connection to a valve system, or a short section between a manifold and the first radiator in a small room. It is the thinnest commonly used diameter in central heating, and precisely for this reason it is the most widespread.
In practice, it looks like this: the main line of the heating circuit runs in 22 mm, but in every place where you want to make a branch towards a radiator, you need a T-piece. If that radiator is connected via a 15 mm pipe, you need either the same T-piece 15 × 15 × 15, or a reduced T-piece with larger outlets on the main line and a smaller one on the branch. I talk more about reduced fittings in the article Standard vs. reduced T-piece: when to use which type, but in general it applies: if your entire distribution is in 15 mm (e.g. a single-pipe system or a small apartment), the same T-piece 15 × 15 × 15 is what you are looking for.
Flow limits for 15 mm: in standard heating systems with a temperature drop of 10 °C, a flow of approximately 0.3–0.4 l/s can be passed through a 15 mm pipe, which corresponds to a power of about 12–17 kW. This is more than enough for a standard radiator (600–2 000 W), but for the entire household it is tight. Therefore, 15 mm is usually seen only on the last branches, not on the main lines.
Typical scenarios for 15 mm T-piece
- A branch to an individual radiator in an apartment or a family house
- Connection to a bathroom ladder radiator
- Connection to a thermostat or control valve at low flows
- Entire distribution in small new buildings with floor heating on a small area (although plastic piping is more commonly used today)
- Repair and extension of an existing distribution system, where the entire system is in 15 mm
Diameter 22 mm: the workhorse of Slovak heating
If 15 mm is the last branch, 22 mm is the backbone of most family house heating systems. This diameter can be found on the main distribution lines, on the line from the boiler to the manifold, between floors, and wherever it is necessary to transfer thermal power of several tens of kilowatts.
In practice: during the reconstruction of older boiler rooms, I often encounter distribution systems in 22 mm, where it is necessary to add another branch – for example, when adding a new room, when expanding the system with a bathroom radiator, or when changing the pipe route. In such cases, the T-piece 22 × 22 × 22 is the most commonly used fitting.
Hydraulically: through a 22 mm pipe, you can pass a flow in the range of 0.5–0.8 l/s during standard heating, which covers a thermal power of 20–35 kW at a drop of 10 °C. This is a whole standard family house with a boiler of about 20–25 kW. Therefore, 22 mm as a main line makes sense for houses with a floor area of about 100–200 m².
When to use the standard T-piece 22 × 22 × 22
The standard T-piece in 22 mm is the right choice when:
- You are adding a branch to a 22 mm main line and the branch will also be in 22 mm (e.g. towards a manifold or a larger circuit)
- You are connecting two branches of the same diameter into one outlet
- It is a location where the flow is divided into two comparable parts
- The system has no thermostatic control and there is no hydraulic imbalance that would be solved by a reduction
If you branch off from a 22 mm main pipe towards a single radiator supplied by a 15 mm pipe, a reduced T-piece 22 × 22 × 15 will likely serve better – the main pipe will remain a full 22 mm and the branch will be correctly dimensioned. However, this goes beyond the scope of this article; for details, see the topic Same vs. reduced T-piece: when to use which type.
Diameter 28 mm: powerful supply lines and larger buildings
The 28 mm copper pipe is less common in typical residential houses, but where it does appear, its role is clear: a main pipe with high thermal output. It typically refers to:
- Boiler rooms in buildings with boiler output exceeding 35 kW
- Primary circuit of a heat pump or boiler room in a two-generation house
- Larger industrial and semi-industrial buildings with central heating
- Connections to large-capacity TÚV (domestic hot water) storage tanks
- Horizontal supply lines in apartment buildings between apartment units
From a hydraulic perspective: through a 28 mm pipe, you can carry a flow of up to 1.2–1.5 l/s at a temperature drop of 10 °C, which corresponds to a thermal output of 50–60 kW. This covers a large family house, a smaller multifunctional building, or an older apartment block with a central heat source.
I encounter 28 mm pipes mainly during boiler room renovations, where an old cast iron boiler is replaced with a new condensing boiler or a heat pump. In such cases, it is necessary to make a branch somewhere in the primary circuit – for example, for a hydraulic balancer, for a TÚV circuit, or for a possible expansion vessel. And precisely here, the T-piece 28 × 28 × 28 comes in handy.
How to identify the existing pipe diameter before purchasing a T-piece
This is a very important question in practice, as many people are unsure what diameter their pipe is. The outer diameter of a copper pipe differs from the inner diameter, and if you confuse them, you might end up buying a fitting that does not fit.
For copper pipes, the European standard EN 1057 applies, according to which pipes are marked by their outer diameter. So when we talk about a "15 mm pipe", we mean a pipe with an outer diameter of 15 mm, while the inner diameter depends on the wall thickness (typically 0.7–1.0 mm), roughly 13–13.6 mm. Similarly, a "22 mm pipe" has an outer diameter of 22 mm and an inner diameter of about 20 mm, and a "28 mm pipe" has an outer diameter of 28 mm, with an inner diameter of about 26 mm.
Practical instructions for measuring: place a sliding caliper or micrometer on the outside of the pipe and measure the outer diameter. If you do not have a measuring tool, you can wrap a thread around the pipe and measure the circumference. The circumference of a 15 mm pipe is approximately 47 mm, of a 22 mm pipe approximately 69 mm, and of a 28 mm pipe approximately 88 mm. From the circumference, you can easily calculate the diameter (d = circumference / π).
Be careful not to confuse with imperial (inch) dimensions! Older steel pipes, threaded fittings, and some radiator connections are in imperial sizes (½", ¾", 1"). These pipes have different outer diameters than copper EN pipes, and Hepworth T-pieces for copper pipes cannot be easily slipped onto them. If you have a mixed system (copper and steel pipes) in your house, you will need transition fittings.
Material and structural properties of Hepworth T-pieces
The T-pieces available at atria.sk come from the Hepworth range – a renowned British manufacturer of copper installation technology, active in the market for decades. Their fittings are made from phosphorus-deoxidized copper (Cu-DHP, designation R 220 according to EN 1173), which is the standard for solderable fittings. This copper has a minimal oxygen content, which prevents porosity in the welds during soldering.
All three diameters – 15, 22, and 28 mm – are dimensioned for a minimum working pressure of 16 bar at temperatures up to 110 °C, which comfortably covers the requirements of central heating (typical working pressure 1.5–3 bar, maximum temperature 90 °C for boilers). For condensing boilers and heat pumps, these values are even more comfortable, as modern systems operate at even lower temperatures (40–60 °C for floor heating).
Soldering is done using hard or soft solder. Hepworth fittings are primarily intended for soft soldering using Sn97Cu3 or similar lead-free solders (since 2006, lead has been banned in installation technology by the European RoHS directive). For potable water, a special regime applies, but for heating circuits, lead-free solders are absolutely standard. For details on soldering techniques, see the article Mounting a T-piece on a copper pipe: soldering step by step.
What influences the choice of diameter in practice: calculations and rules
Choosing the diameter of a T-piece is not just a matter of "what diameter does my pipe have" – it is part of the hydraulic design of the system. For regular installers and DIY enthusiasts, the following practical rules apply, based on experience from dozens of jobs:
Rule 1: The T-piece must have the same diameter as the pipe into which you are installing it
This may sound trivial, but in practice, people sometimes buy a 22 mm T-piece when they have a 28 mm pipe at home – just because they did not check the size. Always measure the outer diameter of the existing pipe with a sliding caliper before purchasing.
Rule 2: The main run should not change diameter through a T-piece
If you have a main run in 22 mm and want to branch off to a radiator, the main run should continue in 22 mm. Reducing the diameter on the main run is a hydraulic mistake – it increases resistance and can restrict other branches. A reduced T-piece (e.g., 22 × 22 × 15) is the correct choice precisely because the main ports remain 22 mm and only the branch is reduced to 15 mm.
Rule 3: The diameter of the branch depends on the output of the branched circuit
Simplified: if the branch is to supply 1–2 radiators with a total output of up to 3–4 kW, a 15 mm branch is sufficient. If it is to supply an entire circuit (e.g., a floor of a house with 5–8 kW), use a 22 mm branch. If it is a primary circuit of a heat pump with an output of 10+ kW at low temperatures, consider 28 mm.
Rule 4: Think about the future
Very common in jobs: the customer wants to save money and installs 15 mm where 22 mm would be appropriate – "it's enough for now". In two years, they want to connect another radiator or replace the boiler with a larger one and find out that the entire distribution system needs to be reworked. Copper is a durable material (lifespan of 50+ years), so an investment in the correct diameter from the start always pays off.
Combining different diameters in one system
A real heating system in a family house usually includes all three diameters. A typical scheme looks like this: from the boiler, the primary circuit goes in 28 mm to the hydraulic balancer (hydraulic separator). From the hydraulic balancer, the secondary circuit in 22 mm comes out, forming the main house distribution. From this distribution, branches in 15 mm branch off to individual radiators.
At each branching point, a T-piece is used. When transitioning from 28 mm to 22 mm (e.g., at a manifold), a reduced T-piece or a combination of a T-piece and a reducer is used. When transitioning from 22 mm to 15 mm (at a radiator branch), a reduced T-piece 22 × 22 × 15 or 22 × 15 × 15 is also commonly used. And where the entire system is in the same diameter — for example, two 22 mm branches merge into one — the same T-piece is used, such as the T-piece 22 × 22 × 22.
If you are working with an existing distribution and need to add a new radiator or an entirely new branch, I recommend first drawing a scheme of the whole system with the diameter of each section, and then deciding exactly where to install the T-piece and what diameter you need. You will save a lot of work if you think it through before you start cutting the pipes.
Common mistakes when selecting a T-piece by diameter
Over the years of working with customers and during installations, I have seen several typical mistakes that are repeated when selecting T-pieces. Here are the most common ones:
- Measuring internal diameter instead of external: A customer measures the internal diameter of the pipe (e.g., 20 mm) and buys a 20 mm T-piece — which of course does not exist and does not fit. Always measure the external diameter.
- Mixing up with imperial dimensions: A customer has an old steel pipe 3/4" (external diameter 26.4 mm) and buys a 28 mm T-piece — it cannot be slipped over the pipe and does not fit.
- Purchasing a standard T-piece instead of a reduced one: A customer branches off from a 22 mm run towards a 15 mm radiator connection and buys a standard T-piece 22 × 22 × 22 — then has to add a reducer at the branch, which is an unnecessary fitting and an additional joint.
- Ignoring the flow direction: A T-piece is a symmetrical fitting, but in the system, it makes sense to orient it so that the flow in the main run is as smooth as possible. If you connect the T-piece "backwards" (the inlet goes through the perpendicular branch), the hydraulic resistance is higher.
- Using a too small T-piece on the main run: A customer has a 28 mm run, but at one point installs a 22 mm T-piece with a reducer — thus creating a bottleneck that restricts the flow through the entire system.
More about these mistakes and their solutions can be found in the article Common mistakes when installing T-pieces and how to avoid them.
Pressure resistance and safety: why diameter relates to safety
One more thing that is sometimes overlooked when selecting the diameter of a T-piece: wall thickness and pressure resistance. For copper pipes according to EN 1057, it is true that with increasing diameter, the wall thickness usually also changes. A standard wall thickness is 0.7 mm for diameters 15 and 22 mm and 0.9 mm for 28 mm (for the hard-drawn grade R250). For the soft grade R220, the walls are thicker.
Hepworth fittings are designed with sufficient reserve for both types of pipes. However, when soldering, it is true that the larger the diameter, the longer the heating time and the larger the area that must be properly tinned. A faulty soldering of a larger diameter (28 mm) is much more dangerous than with 15 mm, because a leak at a higher flow means a larger leak. Therefore, for 28 mm, I recommend entrusting the work to an experienced plumber if you do not have enough experience with soldering larger diameters.
Price perspective: is it worth saving on diameter?
The price of T-pieces increases with the diameter, which is logical — more material, higher production costs. A T-piece 15 × 15 × 15 is cheaper than a T-piece 22 × 22 × 22, and that in turn is cheaper than 28 × 28 × 28. Despite this, it is true that saving on diameter is not worth it. The price of the fitting is only a fraction of the total installation costs. If you choose a diameter that is too small, you will spend extra hours either on repairs or on solving hydraulic problems (unbalanced system, noisy piping, cold radiators). And that is much more expensive than the price difference between a 15 mm and a 22 mm T-piece.
On the other hand, an unnecessarily large T-piece also makes no sense. A 28 mm T-piece at a branch to one radiator is a waste, and at low flows in such a branch you may have problems with air venting. The correct diameter is always the one that corresponds to the hydraulic requirements of the given section — not larger, not smaller.
Overview table: T-piece diameter and its typical use
| T-piece diameter | Typical branch power | Typical use | Product |
|---|---|---|---|
| 15 × 15 × 15 | up to 15–17 kW | Branch to a radiator, small house distribution | T-piece 15 × 15 × 15 |
| 22 × 22 × 22 | up to 35 kW | Main distribution in a family house, branch to a circuit | T-piece 22 × 22 × 22 |
| 28 × 28 × 28 | up to 60 kW | Primary circuit, larger buildings, boiler rooms | T-piece 28 × 28 × 28 |
Most frequently asked questions (FAQ)
How do I find out whether I need a 15 or 22 mm T-piece if I don't know the diameter of my pipe?
The most reliable method is a sliding caliper — you place it on the outside of the pipe and measure the external diameter. If you don't have a caliper, you can wrap a thread around the pipe, measure the thread length, and divide by 3.14 — you get the external diameter. A 15 mm copper pipe has a circumference of about 47 mm, a 22 mm pipe about 69 mm, and a 28 mm pipe about 88 mm. If you are still unsure, bring a piece of the pipe (at least 10 cm) to the store and compare it physically with the fittings.
Can I put a 22 mm T-piece on a 15 mm pipe if I use sealing tape or hemp thread?
No, this is absolutely the wrong approach and dangerous. T-pieces for copper pipes are connected exclusively by soldering — the fitting must fit precisely on the external diameter of the pipe with a tolerance of about 0.1–0.2 mm. Sealant or thread cannot compensate for the gap in soft soldering, the joint would be leaky and mechanically unreliable. Always use a T-piece of the correct diameter.
Is it worth buying more T-pieces in stock if I am doing a larger renovation?
Certainly yes — especially T-pieces 15 × 15 × 15 and 22 × 22 × 22 are used in large quantities on jobs and their price is affordable. During a larger renovation, it is common for the plan to change a bit and you find out that you need one or two more T-pieces than you planned. Having a stock on hand will save you another trip to the store and unnecessary work delays.
Can I combine Hepworth T-pieces with fittings of another brand in one system?
Yes, provided both manufacturers produce fittings according to the EN 1254-1 standard (solderable copper fittings). The diameters are standardized, so fittings from different manufacturers are mutually compatible in terms of dimensions. It is important that the soldering is done correctly — the material of the fitting (Cu-DHP) and the work procedure are the same regardless of the brand.
What is the difference between a T-piece and a Y-piece (cross)? Does it matter which one I use?
A T-piece has a branch that is perpendicular (90°) to the main flow, while a Y-piece has a branch at a smaller angle (usually 45°). Hydraulically, a Y-piece is slightly more efficient when connecting two flows (less turbulent resistance), but in standard heating systems, the difference is minimal. T-pieces are far more common, cheaper, and readily available. For most heating distribution systems, T-pieces are the ideal choice.
What if I need a T-piece with a different diameter on each port – for example, 22 × 15 × 22 or 28 × 22 × 15?
Such combinations are solved using reduced T-pieces or by combining a standard T-piece with a reducing coupling. In the Hepworth range, you will find not only standard T-pieces but also various reduced variants. If you are looking for exact markings and selection procedures for reduced T-pieces, read our article Standard vs. Reduced T-piece: When to Use Each Type, where all combinations are discussed, including practical examples.
Conclusion: The correct diameter is the foundation of a functional distribution system
Selecting the right T-piece is not rocket science, but it does require at least basic knowledge of what is flowing in the system and how much energy it is transferring. Pipe diameters of 15, 22, and 28 mm cover almost the entire range of standard domestic and small commercial heating systems, with each having its place in the hydraulic scheme. The 15 mm is for the last branches and small circuits, the 22 mm is the heart of a family home distribution system, and the 28 mm ensures capacity where a large amount of heat is flowing.
Before each purchase, measure the outer diameter of the existing pipe, consider how much power the branch needs to transfer, and choose a T-piece so that the diameter matches the hydraulic requirement – not one size larger, not one size smaller. If you are unsure, you will find more information in other articles in our Knowledge Center, for example in the topic How to divide heating piping using a T-piece without pressure loss or Leakage in a T-piece in the heating system: causes and solutions. An overview of all available Hepworth T-pieces can be found in the T-piece category.
Do you have a question about this topic?
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