How to choose the right Hepworth vents for wall heating

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Introduction: Why the correct selection of fittings for floor heating matters

Underfloor heating belongs to the category of comfortable, energy-efficient heating solutions that have been gaining increasing popularity in recent years – not only in new buildings, but also in the renovation of older houses or apartments. The foundation of every well-functioning underfloor heating system is a high-quality pipe distribution system, and thus the correctly selected fittings that connect, branch or change the direction of individual pipe sections. Fittings are the most common places where leaks, malfunctions or hydraulic imbalances occur – not because the fittings are bad, but because they were poorly selected or improperly installed.

The Hepworth system has long been a proven and professionally recognized solution for underfloor heating pipe distribution. This British brand has decades of development behind it, and its fittings are characterized by high manufacturing precision, reliable tightness and good compatibility with various types of pipe. In the Slovak market, they are distributed among others also by the e-shop atria.sk, where you will find the full range for underfloor heating.

In this article, we will in detail examine how to choose Hepworth fittings, what all influences the selection of a specific piece, and what mistakes are most commonly made. We will start from the basics and go up to more advanced details that even experienced plumbers will appreciate.

What are Hepworth fittings and what types do we know

Fittings are connecting materials that allow the construction of branched pipe networks from individual straight pipe sections. In the case of underfloor heating, these are usually pipes of smaller diameters – most commonly 10 mm, 15 mm and 22 mm – where the emphasis is on minimal installation depth, pressure resistance and long service life without the need for repairs.

Hepworth produces fittings in several basic typologies:

Straight connectors – connect two same or different pipe diameters in one axis (straight or reduced)
T-pieces – allow branching, i.e. connecting a side branch to the main distribution; they can be all the same (all three nozzles of the same diameter) or reduced (one or more nozzles of a different diameter)
Elbows (bends) – for changing the direction of the pipe (90°, 45°)
Closing and control fittings – ball valves, drain valves and others.

For the purposes of this article, we will focus mainly on straight connectors and T-pieces, because it is precisely these fittings that form the basis of every underfloor heating distribution system and their selection is the most complex and most common source of problems.

Pipe diameters and the Hepworth fitting marking system

Before you start ordering fittings, you must know exactly what pipe diameters you are using in the underfloor heating system. Hepworth fittings are marked by the outer diameter of the pipe in millimeters. For example, a connector marked as "10 x 10" is intended for a pipe with an outer diameter of 10 mm at both ends.

In practice, we encounter the following diameters in underfloor heating systems:

10 mm – the most commonly used diameter for individual underfloor heating circuits; low hydraulic resistance, small installation depth, suitable for short circuits of up to about 15–20 meters
15 mm – used for longer circuits or higher performance, or as a transitional diameter between 10 mm and larger distribution pipes
22 mm – primarily for main distribution pipes (collectors, risers), from which individual circuits are fed via T-pieces

If you are interested in a more detailed look at which diameters to choose for your specific project, we recommend reading the article What pipe diameters do I need for underfloor heating, where you will find hydraulic calculations and performance tables for each dimension.

Why it is important not to confuse outer and inner diameter

One of the most common mistakes made by beginning installers is confusing the outer and inner diameter. Hepworth fittings are always marked by the outer diameter – and it is precisely according to this that you must order them. If you have a pipe in your hand and are unsure of its dimensions, measure the outer diameter with a caliper. For example, common plastic pipes such as PE-X or PE-RT for underfloor heating have an outer diameter of 10 mm and an inner diameter of about 6.3–7.0 mm depending on the wall thickness and manufacturer.

Straight connector: when and where to use it

A straight connector is the simplest type of fitting – it connects two pipes of the same diameter in a straight line. In practice, it is used mainly in the following situations:

extending the pipe on a longer section, when one reel is not enough
repairing a damaged section of pipe (cutting out and replacing with a new piece)
connecting to an existing distribution of the same diameter
connecting two different materials (e.g. PE-X and Cu) via a transition connector

A typical example from practice is a situation where a customer, during a bathroom renovation, buys a 10 mm PE-X reel with a length of 25 meters, but finds out that he is still missing 4 meters to complete the circuit. Instead of buying another reel, it is sufficient to use a Straight connector 10 x 10 and connect the rest from another reel. Of course, each connection is a potential leak point, so it is important to minimize the number of connections in the floor – ideally the entire circuit is made from one piece without connections.

Reduced straight connectors

In addition to equal connectors, there are also reduced ones, where both nozzles are of different diameters – for example 22 x 15 or 15 x 10. These are used when transitioning from a larger to a smaller diameter, for example when connecting a 10 mm circuit to a 15 mm collector hose. More on this topic can be found in the article How to correctly connect pipes of different diameters using reduced connectors.

T-piece: the heart of every branched distribution

A T-piece is a fitting that allows you to branch off a pipe – from one incoming pipe you create two branches. This is a key component when building a branched wall heating system, where individual circuits branch off from the main distribution to different rooms or wall zones.

In the Hepworth range, we find T-pieces in various combinations of diameters. For wall heating, the most relevant are:

T-piece 10 x 10 x 10 – all three ports have the same diameter of 10 mm; used when you want to divide one 10 mm circuit into two smaller 10 mm circuits, or when connecting a side branch of the same diameter to a running distribution
T-piece 15 x 10 x 15 – the running distribution has a diameter of 15 mm, the side branch 10 mm; a typical solution for connecting one wall heating circuit (10 mm) to a main distribution of 15 mm
T-piece 22 x 10 x 22 – the running distribution has a diameter of 22 mm, the side branch 10 mm; used in larger installations with a main distribution of 22 mm, from which 10 mm circuits are fed
T-piece 22 x 10 x 10 – the inlet port is 22 mm, both outlet ports are 10 mm; a specific application at the end of a 22 mm distribution and connecting two 10 mm circuits (e.g. at a collector)

How to read the T-piece designation

The designation of Hepworth T-pieces always has the form "A x B x C", where the letters represent the port diameters. The convention is as follows:

A – first (through) port = inlet of the main distribution
B – side branch (perpendicular port)
C – second through port = outlet of the main distribution

If A and C are the same, it is the same running distribution. If B is smaller than A and C, it is a reduced T-piece – the main distribution has a larger diameter, the branch a smaller one. This is also the most common situation in wall heating, where smaller circuits are fed from a larger distribution pipe.

How to choose the right T-piece for a specific situation

The choice of the correct T-piece depends on several factors. In practice, we most often encounter three basic scenarios:

Scenario 1: Simple distribution with one diameter (10 mm)

Small projects – for example, one room or one apartment – where the main distribution is the same diameter as the circuits (10 mm), and you want to make a branch at each radiator or wall panel. The solution is simple: T-piece 10 x 10 x 10 for each branch. This is more complex hydraulically (pressure losses are higher), so it is recommended to use a flow restrictor or control valve on each branch to ensure even heat distribution.

Example: a customer has a small studio and wants to heat two walls in the living room and one in the kitchen. The total length of the circuits is about 30 meters. The distribution from the manifold runs in a 10 mm hose, and a T-piece 10 x 10 x 10 is used at each branch. A simple, economical solution for small projects.

Scenario 2: Larger project with a main distribution of 15 mm and circuits of 10 mm

This is probably the most common scenario for wall heating in family homes or larger apartments. The main supply and return pipe has a diameter of 15 mm, from which individual circuits of 10 mm are fed. At each connection of a circuit to the main distribution, a T-piece 15 x 10 x 15 is used. The flow in the main distribution is reduced at each T-piece by the flow of the branches, so it is important to hydraulically balance the system – or use a collector (manifold), to which all circuits are connected equally.

Practical example: a family home of 120 m², where three rooms have wall heating. Each room has one 10 mm circuit of 18–22 meters in length. The main distribution of 15 mm runs from the boiler room to each room with T-pieces 15 x 10 x 15 at each branch. The solution works reliably with proper flow balancing using individual control valves.

Scenario 3: Large project with a main distribution of 22 mm

In larger projects (office spaces, larger family homes, apartments), the main distribution is 22 mm and feeds a larger number of 10 mm circuits. T-pieces 22 x 10 x 22 are used here, which allow the full diameter of the main distribution to be maintained while feeding individual circuits. A special case is the T-piece 22 x 10 x 10, which is used at the end of a 22 mm main distribution, where it branches into two 10 mm branches – for example, at a terminal collector or at a circuit distributor.

Material properties of Hepworth fittings and their compatibility

Hepworth fittings for wall heating are made of plastic – specifically polybutylene or polypropylene materials, depending on the specific product range. They have metal (brass or stainless steel) insert parts that ensure strength and sealing of the connection. The system is designed primarily for use with PEX and PERT pipes.

Basic technical parameters to keep in mind when selecting:

Maximum working pressure: usually 6 bar at normal temperatures (up to 70°C); this is completely sufficient for wall heating, as the operating pressure of such systems is around 1.5–3 bar
Maximum working temperature: 70°C continuously, up to 95°C short-term; wall heating typically operates at 35–50°C, so there is no problem here
Compatibility: Hepworth fittings are compatible primarily with pipes of the same manufacturer or with certified alternatives; always check whether your pipe has the same outer diameter and whether its diameter tolerance is in line with the requirements of the fitting

Important practical note: never mix fittings from different systems without verifying compatibility. For example, Hepworth fittings are not mechanically compatible with fittings from other manufacturers, even if they have the same diameter marking – the insertion mechanism (clip or screw system) may be different, and the connection may leak or loosen.

Number of fittings: how to create a material list

One of the most common questions we receive from customers is: "How many T-fittings and couplings do I need?" The answer depends on the project, but we can provide a few practical recommendations.

Procedure for designing the piping layout and counting fittings

Draw a floor plan and mark where the heated walls will be
Design the main piping layout (supply and return pipe) from the heat source (boiler, heat pump) to each room
For each location where a heating circuit branches off, mark a T-fitting
For each location where two pipe sections are joined (extension), mark a straight coupling
Count the total number of each type of fitting

As a practical guideline: in a typical single-family house with 4–5 rooms and wall heating, plan for 8–16 T-fittings (supply and return branch for each room) and 4–10 straight couplings. A more precise calculation depends on the layout and design of the piping. A detailed guide on designing the piping layout can also be found in the article How to design a piping layout for wall heating using T-fittings.

Extra quantity and what to consider when ordering

Always order 10–15% more fittings than you calculate. The reason is simple: during installation, a fitting may be damaged (cracked during assembly), there may be an error in the design, a need for an additional branch that you did not plan, or simply a loss during drilling or cutting. It is much cheaper to have a few extra fittings in stock than to stop work and wait for an additional order.

Common mistakes when selecting fittings – experience from dozens of projects

Over the years of working with fittings for wall heating, we repeatedly encounter several typical mistakes that customers make when selecting them. We describe them so you can avoid them.

Mistake 1: Confusing the dimensions of the T-fitting

The customer orders a T-fitting 22 x 10 x 22 and the installer mounts it in the wrong direction – the branch (10 mm) goes into the main line and the 22 mm neck goes to the circuit. Result: a 22 mm circuit at a location where there should be 10 mm, and vice versa. This happens when the installer does not create a clear plan before ordering and buys "on the fly". Solution: always draw a schematic, label the necks, and confirm it when ordering.

Mistake 2: Mixing standard and reduced T-fittings

At a location where a T-fitting 15 x 10 x 15 was supposed to be installed, someone installed a T-fitting 10 x 10 x 10 – because it was "available" and they thought it would work. Result: the main line is narrowed to 10 mm at this location, which causes a significant pressure drop and reduced flow for all circuits downstream. This problem often becomes apparent only after the system is started, when the last rooms in the layout are not sufficiently heated.

Mistake 3: Ignoring the flow direction in T-fittings

A T-fitting has a hydraulically advantageous and disadvantageous flow direction. Ideally, the main flow should pass through the straight ports (A and C) and the branch should only take a portion of the flow through the side port (B). If you reverse the T-fitting so that the main flow goes through the side port, pressure losses are significantly higher. In practice, this is less critical in small systems, but in larger layouts, it can cause hydraulic imbalance.

Mistake 4: Too many joints in the wall

Every joint is a potential leak point. We have seen cases where a T-fitting and a straight coupling were installed in the wall every meter – because the customer had little pipe and many fittings. The correct approach is to minimize the number of joints in the wall to an absolute minimum and ideally place them at accessible locations (behind a cover, near a maintenance opening). More on this topic can also be found in the article Common mistakes when installing T-fittings and couplings for wall heating.

Checklist before ordering fittings

Before placing the final order, go through this checklist:

✅ Do I know the outer diameter of all pipes in my system (10 mm, 15 mm, 22 mm)?
✅ Do I have a schematic of the piping layout with all branches and flow directions marked?
✅ For each branch, have I selected the correct T-fitting (standard or reduced)?
✅ For each extension of the same pipe, have I selected the correct straight coupling?
✅ Have I verified the compatibility of the fittings with the type of pipe I am using?
✅ Have I ordered 10–15% more pieces than my minimum requirement?
✅ Am I planning the installation according to the manufacturer's instructions and do I have the right tools available?

Compatibility with other system components of wall heating

Fittings never work alone – they are part of a larger system. In wall heating systems, it is important to ensure that all components are mutually compatible:

Distributor (manifold): fittings must fit the manifold ports – verify the type of connection (threaded, crimped, screw-on) and diameter
Control valves: if there are control or shut-off valves on the branches, they must have the same diameter as the branch port of the T-fitting
Flexible hoses (meanders): individual wall heating circuits are made of meander-shaped hoses; their outer diameter must exactly match the diameter of the fittings
Insulation: in wall or floor conduits, it is important that the fittings are insulated the same way as the pipe to prevent thermal bridges and condensation

Step-by-step selection of fittings – summary

For clarity, we summarize the entire selection process into a simple sequence of steps that you can use for every wall heating project:

Determine the pipe diameters in the entire system – main line, secondary lines, circuits
Draw a schematic of the piping layout with each connection and each branch marked
For each branch, select the correct T-fitting according to the combination of diameters (main line x side branch)
For each extension, select a straight or reduced coupling
Verify compatibility with the selected type of pipe and other system components
Count the number of pieces and add a reserve of 10–15%
Order the material and before installation, check that you receive exactly the types you ordered

If you are unsure about any step, we recommend consulting with a technical advisor or reading more articles in the Knowledge Center – for example, Installation of Hepworth fittings step by step or Couplings and T-fittings Hepworth: differences between straight and reduced fittings.

Most Frequently Asked Questions (FAQ)

Can I use a 10 x 10 x 10 T-piece in a place where a 15 x 10 x 15 T-piece should be used?

No, this is an error that unfortunately occurs in practice. A 10 x 10 x 10 T-piece has all three outlets with a diameter of 10 mm, which means you would have to reduce the main pipe from 15 mm to 10 mm. This will cause a significant increase in pressure loss and reduce the flow for all other circuits downstream. Always use a fitting with a diameter matching the actual pipe diameter you are connecting.

What is the difference between a 22 x 10 x 22 T-piece and a 22 x 10 x 10 T-piece?

Fundamental. A 22 x 10 x 22 T-piece has both the inlet and outlet of the main run with a diameter of 22 mm and a 10 mm side branch – this is a T-piece in the middle of the run, where the main run continues at full diameter. A 22 x 10 x 10 T-piece has an inlet of 22 mm, but both outlet branches are 10 mm – here the main run of 22 mm ends and splits into two 10 mm branches. It is typically used at the end of the main run or at a circuit distributor.

How many T-pieces do I need for a typical family house with 4 rooms and underfloor heating?

It depends on the layout and design of the piping, but as a rough estimate: with a main run of 15 mm and circuits of 10 mm, you will need at least 2 T-pieces per room (supply and return branch) – so for 4 rooms, that is 8 T-pieces as a base. You also need to add any additional branches (bathroom, hallway, utility room) and a spare. Real projects for a 4-room house usually have 12–20 T-pieces in total.

Can I connect PEX pipe with Hepworth fittings designed for PERT?

Mechanically yes, if both types of pipe have the same outer diameter and wall thickness – because the fittings grip the outer diameter. However, always check the technical data of the specific products, as some types of fittings are certified only for certain types of pipe. If you have any doubts, contact the manufacturer's or supplier's technical support.

Where should the fittings be placed in the wall – and where not?

Ideally, the fittings should be accessible – either before plastering (during installation before plastering), behind removable covers, or at inspection points. If the fittings must be embedded in the wall or plastered in (which is not recommended, but sometimes necessary), they must be thoroughly pressure tested before plastering and wrapped in protective tubing or flexible insulation to allow possible replacement without demolition work. More on this topic can be found in the article Causes and solutions for leaks at Hepworth fitting joints.

How can I make sure I ordered the correct fitting – is there any way to check before installation?

Yes. Always physically measure the internal diameters of the fitting's outlets and compare them with the outer diameter of your pipe – the pipe should fit into the outlet easily, but without play. Also check the marking on the fitting (it is usually engraved or printed directly on it) and compare it with your order list. If you have the slightest doubt, do not install the fitting and verify it with the supplier – repairs in the wall after plastering are much more expensive than a few extra minutes of checking.

Conclusion: Choosing the right fittings is the foundation of reliable underfloor heating

Selecting Hepworth fittings for underfloor heating is not rocket science – but it requires a systematic approach, accurate measurement of diameters, and a clear piping plan. As we have shown in this article, the basic decisions revolve around three questions: what are the pipe diameters in the system, where are the branches and extensions, and what are the flow directions in each T-piece.

With the correct selection and installation, Hepworth fittings will operate reliably for decades without the need for intervention. With incorrect selection or installation, you may face leaks, hydraulic imbalance, or the need for costly repairs behind the plaster. Therefore, it is always worth investing time in preparation – including studying other articles in our Knowledge Centre, such as Maintenance and inspection of underfloor heating systems with Hepworth fittings or Common questions about Hepworth fittings for underfloor heating.

If you are now more confident in your selection after reading this article, we recommend browsing the specific range of fittings for Hepworth underfloor heating and choosing exactly what your project needs.

Do you have a question about this topic?

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