How to properly connect pipes of different diameters using reducing couplings
How to correctly connect pipes of different diameters using reduced fittings
Anyone who has ever started installing wall heating sooner or later encounters a situation where they need to connect two pipes of different diameters. This may involve a transition from a main distribution pipe of 22 mm diameter to a branch for individual heating elements of 10 mm diameter, a reduction in the supply to convector units, or simply a situation where the designer has planned a distribution with multiple pipe dimensions. This is common in practice – and for these situations, there are reduced fittings and reduced T-pieces, which, if correctly selected and installed, can connect the system into a trouble-free, tight and hydraulically balanced system.
This article will explain in detail the difference between straight and reduced fittings, how to consider the correct diameter for a specific branch of the distribution, what are the typical mistakes when connecting different dimensions and how to complete the whole process even without years of experience as an installer. We will also include concrete examples from practice, diagrams and recommendations for the most common configurations.
Why do we connect pipes of different diameters at all?
At first glance, an ideal heating distribution might seem to be made of one pipe diameter from the boiler to the last radiator. In theory, maybe. In practice, it doesn't work that way, and there are several good reasons for this.
The first reason is hydraulic. The longer the distribution and the more it branches, the greater the resistance to water flow. If you choose a pipe that is too small for the main circuit, you risk insufficient flow and uneven heating. On the other hand, a pipe that is too large in each small branch is expensive and unnecessarily takes up space in the wall or floor. Therefore, in practice, individual sections of the distribution are dimensioned according to the actual flow requirements – the main branch (supply and return pipe) has a larger diameter, and the side branches to individual units have a smaller diameter.
The second reason is structural. Wall heating places special requirements on the distribution – the pipes are embedded in the wall or in the plaster, the space requirements are limited and the installation must be carried out before the final construction work. In these conditions, it is unnecessary to install thick pipes where thinner ones would suffice.
The third reason is simply practice: many wall heating projects work with storage tanks, distributors and collectors, where several smaller branches converge into one larger circuit. In these cases, reductions and reduced T-pieces are essential.
Basic terms: straight fitting, reduced fitting, reduced T-piece
Before we get into specific recommendations, it is good to be clear about the terminology. Fittings for Hepworth systems (as well as other plastic and metal fittings for heating) are divided according to whether all the openings are of the same diameter or of different diameters.
Straight fitting (e.g. Straight fitting 10 x 10) is used to directly extend a pipe of the same diameter – typically where the pipe is not long enough, or where it is necessary to connect two pipe sections during a repair. It has no reducing effect.
Reduced fitting has two openings of different diameters. It allows a transition from a larger diameter to a smaller one (or vice versa). In wall heating, we most often encounter reductions of 22/15, 22/10 or 15/10 mm.
Reduced T-piece combines both functions at once: it allows a branch from the main pipe to a branch of a different diameter. Real-life examples include, for example, T-piece 22 x 10 x 22 (main pipe Ø22 mm, branch Ø10 mm and continuation Ø22 mm), or T-piece 22 x 10 x 10 (inlet Ø22 mm, branch and continuation Ø10 mm – used at the end of the main distribution, where it branches into two smaller branches). Another common case is T-piece 15 x 10 x 15, where the main pipe of 15 mm branches into a branch of 10 mm.
The correct naming of the fitting is always read in the order: inlet × branch × outlet. This rule is important when ordering – mixing up the order can lead to ordering a fitting that cannot be installed in the correct way.
When to use a reduced fitting and when to use a reduced T-piece?
This is a practical question that even experienced installers ask when designing a distribution on site. The answer depends on exactly what you want to achieve.
Reduced fitting is used when you want to simply change the diameter of the pipe without a branch. Typical example: the main distribution comes to a distribution unit in a larger diameter and you continue from there in a smaller diameter. Or: you are repairing a section of pipe where there was an original reduction and you need to connect a new pipe of a different diameter.
Reduced T-piece is used whenever you need to make two branches from one pipe – with at least one of them having a different diameter than the input pipe. In practice, this is the most common scenario when supplying individual heating units (convector units, panels, radiators) from the main circuit of wall heating.
Practical example from a job: during the renovation of a family house in Trnava, we solved wall heating in three rooms. The main circuit was dimensioned for Ø22 mm (supply and return pipe). From each room, one heating panel was connected via a branch of Ø10 mm. At each connection, we used T-piece 22 x 10 x 22 – inlet 22 mm from the boiler, branch 10 mm to the panel, outlet 22 mm continues to the next room. At the end of the distribution of the last room, a T-piece 22 x 10 x 10 was used, where the main pipe of 22 mm branched into two branches of Ø10 mm (supply and return to the last panel).
Hydraulic consequences of incorrect pipe diameter selection – what happens if you choose wrong?
One of the most common problems we see in practice is the underestimation of hydraulics – that is, what happens to the water flow when the pipe is narrowed or widened at the wrong place. These are phenomena that only become apparent after the system is put into operation, and then the correction is time-consuming and expensive (especially with floor heating, where the joints are embedded).
Too small diameter on the main loop: If the main pipe is dimensioned too narrow (for example Ø10 mm instead of Ø22 mm for a long distribution with multiple outlets), pressure losses are high, the pump is operating at the edge of its capabilities, and distant heating units receive insufficient flow. Result: some rooms are cold, others are too hot.
Too large diameter on the branch to the radiator: The opposite situation occurs when the branch to an individual radiator is unnecessarily oversized. This can cause hydraulic imbalance – too much water flows to one radiator at the expense of others. The solution is proper dimensioning or the use of regulating valves.
Incorrect orientation of the reducer: A reduced coupling or T-piece mounted in reverse (larger diameter where it should be smaller) causes unnecessary turbulent flow and increased system noise. In some cases, air locks can also occur.
Practical tip from experience: always draw – even on paper – a scheme of the entire distribution with all diameters and connection points before installation. This will help you avoid most errors.
How to choose the right reducer fitting – step by step
Selecting the right fitting is not complicated if you know what to look for. Here is a procedure that works for every project:
1. Determine the diameters of the pipes you want to connect
Measure the outer diameter of both pipes. Note – Hepworth fittings are marked by the outer diameter of the pipe (OD), not the inner one. The most common values for floor heating are 10 mm, 15 mm, and 22 mm. If you have a pipe from another system (for example Cu or steel), check whether the dimensions are compatible.
2. Determine whether it is a direct connection or a branch
If you are connecting two pipes in sequence (extension), you are looking for a reducer coupling. If you need to create a branch from one pipe in another direction, you are looking for a reduced T-piece.
3. Record the dimensions in the correct order
For T-pieces: inlet × branch × outlet. For example, a T-piece 22 x 10 x 22 means: inlet pipe 22 mm, branch 10 mm (for example to a radiator), outlet 22 mm (continuation of the main distribution). This order is standard and sellers identify the fitting according to it.
4. Consider the material and temperature parameters
Hepworth fittings are intended for systems with a temperature up to 95 °C and pressure up to 10 bar – these values cover the vast majority of domestic heating systems. If you have a specific system (for example industrial heating or a solar loop with higher temperature), verify the parameters.
5. Check compatibility with the existing system
Not every fitting can be directly connected to every pipe. The Hepworth system works on the principle of crimping or swaging, where the fitting is firmly swaged onto the pipe using special tools. It is not compatible with standard threaded or welded connections without an adapter.
Typical configurations in practice – concrete examples
Let’s look at a few real situations that repeat in the design and installation of floor heating:
Configuration 1: One main loop with multiple branches to radiators
This is by far the most common case. The main distribution (supply + return pipe) runs from the boiler room or technical room in a large diameter (22 mm) through the whole house. At each location where there is a heating radiator, a smaller branch (10 mm or 15 mm) is taken off from the main pipe to the radiator. The pipe returns at the end of the loop.
For each branch you will need one reduced T-piece on the supply pipe and one on the return pipe. If the loop is 22 mm and the branches are 10 mm, you will use T-piece 22 x 10 x 22 for each intermediate branch, and T-piece 22 x 10 x 10 for the last node, where the main pipe ends and branches off.
Configuration 2: Two-pipe system with 15 mm branches
In some projects, a medium diameter of 15 mm is used as the main distribution (for smaller houses or shorter routes). The branches to the radiators are then Ø10 mm. Here, T-piece 15 x 10 x 15 is used. This solution is more compact and suitable, for example, for apartments or family homes with floor heating in two to three rooms.
Configuration 3: Direct branch without further continuation
Sometimes you need to create a branch from a larger pipe and end the pipe there – that is, it does not continue in the same diameter. Example: the main distribution 22 mm comes to the last radiator and there it is divided into two branches 10 mm (supply and return). For this, you use T-piece 22 x 10 x 10, where the inlet is 22 mm, one branch is 10 mm and the other arm is also 10 mm.
Configuration 4: Series of identical radiators connected in sequence
If you have, for example, five convector radiators in a row in a corridor, you can connect them to a common 10 mm circuit, with each radiator being a branch. In this case, 10 x 10 x 10 T-pieces are suitable – the same diameter on all three ports, since the entire circuit is Ø10 mm. Reductions are not necessary here, as long as the circuit sizing is consistent.
Installation of reduced fittings and T-pieces – what you need to know before starting
Proper installation is just as important as choosing the right fittings. Even the highest quality fittings will leak if installed incorrectly. For the Hepworth system, the following rules apply:
Pipe preparation: The pipe must be cut perpendicular to its axis – an angled cut will result in incomplete insertion into the fitting and subsequent leakage. Use the correct pipe cutter (pipe cutter), not a saw. After cutting, deburr the pipe edges – plastic or metal debris inside the pipe can cause problems during operation.
Insertion depth check: Each fitting has a specified minimum insertion depth. Before installation, mark the pipe with a marker or tape at a distance equal to this depth. After crimping, check whether the mark has approached the fitting's port. An insufficiently inserted pipe will not be properly crimped, and the joint will be leaky.
Crimping with the correct tool: Hepworth fittings are crimped using crimping pliers with a V, M or U profile, depending on the specific system. An incorrect profile or low-quality pliers will cause deformation of the fitting and improper crimping. Once crimped, the joint cannot be opened and reused – in case of an error, the fitting must be cut off and a new one used.
Fitting orientation: A reduced T-piece must be mounted in the correct direction – the inlet side (larger diameter) must face away from the heat source, the outlet (continuation of the main circuit) must go further into the system, and the branch must lead to the radiator. Swapping the inlet and outlet sides on a symmetrical T-piece (e.g. 22 x 10 x 22) will not destroy the function, but with an asymmetrical T-piece (e.g. 22 x 10 x 10), you must be careful.
A detailed step-by-step installation procedure is available in a separate article Installing Hepworth fittings step by step, where the entire process is documented, including common mistakes.
Inspection after installation and pressure test
After completing the installation of all joints – including reduced ones – it is essential to perform a pressure test on the system before backfilling the trenches or applying plaster. This is a rule that must not be skipped, as repairing a leaky joint after it is covered is always expensive and complicated.
The pressure test is carried out as follows: the system is filled with water, purged of air, and then the pressure is increased above the operating pressure (usually 1.5 times the maximum operating pressure, but at least 6 bar). The pressure is left to act for at least 30 minutes and the manometer reading is monitored. If the pressure drops by no more than 0.1 bar per hour, the system is tight.
In practice, we recommend performing the pressure test in stages – first after the rough installation, while the joints are still accessible. If a leak appears, it is easier to fix. A detailed procedure for solving leaks is covered in the article Causes and solutions for leaks at Hepworth fitting joints.
Hydraulic balancing of the system with different diameters
When connecting pipes of different diameters, you inevitably change the hydraulic conditions in the system. Each change in diameter means a change in flow and pressure conditions. This aspect is especially important in larger installations with multiple circuits.
In wall heating systems with reduced T-pieces, a problem of uneven heating may occur: branches closer to the boiler receive more flow than distant branches. The solution is hydraulic balancing using control valves (balancing valves or thermostatic valves with adjustable flow setting). These valves are installed on each branch and allow the correct flow to be set regardless of the distance from the boiler.
Another option is the so-called Tichelman connection (reverse loop), where the supply and return pipes cross – the first in the supply order is the last in the return pipe and vice versa. In this way, pressure losses in individual branches compensate for each other and the system is naturally hydraulically balanced. More on designing pipe layouts can be found in the article How to design a pipe layout for wall heating using T-pieces.
Common mistakes and how to avoid them
To conclude the technical part, we summarize the most common mistakes we see in wall heating and reduced joint installations:
- Incorrect fitting diameter selection: The customer orders a 15 x 10 x 15 fitting, but needs a 22 x 10 x 22. Always double-check the pipe diameter before ordering.
- Incorrect orientation of the T-piece: An asymmetrical T-piece (e.g. 22 x 10 x 10) installed backwards – the 10 mm inlet pipe is connected to the 22 mm port. Result: leakage or incorrect function.
- Insufficient pipe insertion: The pipe is not inserted to the required depth before crimping. The joint looks fine, but leaks during the pressure test (or later during operation).
- Crimping with the wrong plier profile: Using pliers with the wrong profile for the specific fitting system. Each system (Hepworth, Viega, Geberit, etc.) has its own pliers.
- Skipping the pressure test before covering: This is the most expensive mistake. Repairing an embedded or plastered installation can cost several times more than the original installation.
- Mixing fittings from different systems: Hepworth fittings are not compatible with fittings from other manufacturers (even if they have the same nominal diameter). Always use components from one system only.
More about errors when working with T-pieces and fittings can be found in the article Common mistakes when installing T-pieces and fittings for wall heating.
Table: Overview of common configurations and recommended fittings
| Situation | Main loop | Branch to the body | Recommended fitting |
|---|---|---|---|
| Side branch from the main loop | Ø22 mm | Ø10 mm | T-piece 22 × 10 × 22 |
| End of the loop with a branch | Ø22 mm | Ø10 mm + Ø10 mm | T-piece 22 × 10 × 10 |
| Middle loop with a branch | Ø15 mm | Ø10 mm | T-piece 15 × 10 × 15 |
| Branch in a small loop | Ø10 mm | Ø10 mm | T-piece 10 × 10 × 10 |
| Extension of pipe of the same diameter | Ø10 mm | — | Straight coupling 10 × 10 |
Most frequently asked questions (FAQ)
Can I use a reduced T-piece in reverse – that is, connect a larger pipe to the side with the smaller neck?
No. Each neck of the fitting is dimensioned precisely for a specific pipe diameter, and incorrect connection may result in leakage or even an inability to complete the joint at all. The fitting must always be connected in accordance with its marking (inlet × branch × outlet). If you need a reversed reduction direction, order a fitting with the opposite diameter sequence.
Is there a difference between using two separate fittings (reduction + T-piece) instead of one reduced T-piece?
Technically, both solutions work, but using one reduced T-piece is always better. Fewer joints mean a lower risk of leaks, a shorter installation route, lower hydraulic resistance, and material and labor savings. Using two fittings instead of one is a solution for situations where the correct combined fitting is not available.
How is the marking of a fitting read, for example, T-piece 22 x 10 x 10?
The marking is always read in the order: inlet × branch × outlet. For the T-piece 22 x 10 x 10, this means: the inlet pipe has a diameter of 22 mm, a branch with a diameter of 10 mm branches off from this T-piece, and the continuation (outlet) also has a diameter of 10 mm. Thus, from one larger pipe, two smaller branches are created. This is the standard marking used by all manufacturers and suppliers in the industry.
Do I need to use special tools for wall heating with plastic Hepworth fittings, or are standard plumbing pliers sufficient?
Yes, you must use special pressing pliers (press tools) with a profile matching the Hepworth system. Standard plumbing pliers, hose clamps, or other improvised solutions are not sufficient – the joint would not be reliable and would leak during pressure testing (or in operation). You can rent this tool, or have the pressing done by a certified plumber.
Can I combine Hepworth fittings with another system (e.g., Viega or Geberit)?
No. Although the nominal diameters (10, 15, 22 mm) may be the same, the geometry of the necks, the profile of the pressing joint, and the sealing are different for each system. Mixing systems leads to leaks and loss of warranty. If you need to connect different systems (e.g., during a renovation), use transition adapters designed by the manufacturer.
Is pressure testing mandatory, or can I skip it if I'm sure I've done everything correctly?
Pressure testing is a mandatory part of the handover documentation for any heating installation and is also required by most insurers in case of water damage. Moreover – and this is even more important from a practical standpoint – even experienced plumbers make mistakes. Pressure testing is the only reliable way to detect a leak before the installation is covered. Skipping it is risking your own wallet.
Conclusion: The right fitting in the right place saves time and money
Connecting pipes of different diameters is not a technically complex task if you know what you are doing and why. The key is the correct identification of the location and direction of the reduction, the selection of the appropriate fitting (reduced coupling or reduced T-piece), and precise installation according to the manufacturer's instructions. With the Hepworth system for wall heating, you have a complete range of fittings for all common diameter combinations – from simple straight couplings through reduced T-pieces to special configurations.
From long-term customer experience, we know that investing in the right materials and thorough installation always pays off – in contrast, saving on the wrong fitting or skipping pressure testing can cost several times more in the event of a failure after the installation is covered. For more information on selecting fittings, sizing distribution systems, and overall design of a wall heating system, we recommend further articles in this Knowledge Center – for example, How to choose the right Hepworth fittings for wall heating, What pipe diameters do I need for wall heating, or Hepworth couplings and T-pieces: differences between straight and reduced fittings.
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.