How to Design a Pipe Layout for Wall Heating Using T-Pieces
How to Design a Pipe Distribution for Wall Heating Using T-Pieces
Designing a pipe distribution for wall heating is not just a matter of connecting two pipes together. It is a systematic discipline, in which decisions made on paper (or in your head) directly influence whether the system will function correctly for decades, or whether you will find yourself cutting into walls, sealing, and searching for where exactly the leak is. Over the years of practical work, I have seen projects where the plumber made an excellent soldered joint and hydraulic insulation, but designed the branches in such a way that one wing of the building was cold all winter. And I have also seen the opposite – modest pipe runs with Hepworth fittings installed with common sense, which have functioned without a single problem for fifteen years.
This article focuses specifically on the topology of pipe distributions using T-pieces – that is, how to plan the branching, where and why to use a specific type of T-piece, what the sizing rules are, and what errors in design can push problems into the future like a time bomb.
What is "wall heating" from the perspective of pipe distribution
Wall heating (sometimes also called capillary wall heating or surface wall heating) uses a network of thin pipes embedded in the wall structure – most often in special plaster or adhesive bed. Water flows through the network of these capillary pipes and transfers heat evenly over a large wall surface. The result is pleasant radiant heat without convective currents, without dust carried by air, and with low water temperature (typically 28–40 °C on the supply, 24–35 °C on the return).
From the perspective of pipe distribution, this means one essential thing: the pipe branches from the main distribution (usually 22 mm or 15 mm) into many smaller branches (typically 10 mm capillary loops). Precisely at these branching points, T-pieces are a key element. Choosing the right T-piece is not just about being watertight – it is primarily about the hydraulic balance of the entire system.
Basic pipe distribution topologies – series, star, tree
Before we get into the selection of specific T-pieces, we need to clarify which topology we are using for the distribution. In practice, we encounter three basic schemes with wall heating:
- Serial (chain) connection – loops are arranged one after another; the temperature on each subsequent loop is slightly lower, hydraulically the simplest, but thermally unbalanced.
- Star connection (distributor/collector) – all loops are connected directly from one distributor; ideal hydraulic balance, but requires a central location for the distributor.
- Tree connection (with T-pieces) – a combination of both; the main distribution gradually branches using T-pieces into smaller branches; a compromise between the simplicity of installation and hydraulic balance.
Exactly this tree connection is the most common in wall heating in apartment and single-family houses, as it allows the main pipe to be routed through the rooms and to branch off into individual rooms or walls without the need for a central distributor.
Hydraulic basics – why pipe sizing matters more than it seems
Before selecting T-pieces, we must understand hydraulics. Each T-piece is a place where the flow is divided (on the supply) or merged (on the return). If the T-piece is improperly sized – for example, a too large branch from a small pipe – hydraulic resistance is created, which can significantly reduce the flow in the continuing branch. This is not just theory: in practice, it is manifested by the last wing of the distribution being noticeably colder than the first.
The basic rule of hydraulics for wall heating is: the flow velocity should not exceed 0.5 m/s in the main 22 mm line and 0.3 m/s in 15 mm branches. In 10 mm capillary pipes, we usually move between 0.1–0.2 m/s. If these values are exceeded, noise and increased pressure losses occur.
Practically, this means that one 22 mm branch can supply significantly more loops than a 15 mm branch. That is why T-pieces come in several reduced variants – they allow a smooth transition from a larger to a smaller diameter, without sudden narrowing and hydraulic shocks. More about the correct selection of diameters can be found in the article What pipe diameters do I need for wall heating in this Knowledge Center.
Overview of Hepworth T-pieces for wall heating and their use
Hepworth fittings designed for wall heating are designed for work with low-temperature systems and plastic capillary piping. Their designation directly indicates the dimensions in order: main line × branch × continuation. Let's look at specific types:
T-piece 10 × 10 × 10 – the basis of the capillary network
T-piece 10 × 10 × 10 is a universal T-piece for the smallest diameter. It is used exclusively in the capillary network – that is, directly in the wall, where one capillary pipe branches into two. Typical situation: you have a loop that runs through a wall, and you want to divide it into two parallel strips within the same wall or into an adjacent room. This T-piece allows it without changing the diameter.
Warning: Since two 10 mm pipes continue after the branch instead of one, you must check whether the flow source (pump and supply branch) is capable of supplying both circuits. If not, one of the branches will be insufficiently supplied with heat.
T-piece 15 × 10 × 15 – branching from the distribution pipe
T-piece 15 × 10 × 15 is one of the most commonly used components in wall heating. The main 15 mm line continues without restriction, and one 10 mm capillary loop branches off from it. This is an ideal T-piece for situations where you are distributing heat within a single room – for example, from a 15 mm distribution pipe running along the room’s perimeter, you can branch off several capillary loops one after another.
Hydraulically, the advantage of this T-piece is that the 15 mm line remains undisturbed – the flow in the continuing branch is reduced only by the amount taken by the 10 mm capillary loop, which is relatively small.
T-piece 22 × 10 × 22 – branch from the main line
T-piece 22 × 10 × 22 is used to branch off one 10 mm capillary loop directly from the main 22 mm line. This is a very elegant solution when you want to branch off individual capillary loops directly from the main pipe supplying an entire floor or wing of a building – for example, a bathroom loop when the main line passes through the bathroom.
Practically: the 22 mm line remains fully functional, and the 10 mm branch is small enough to not significantly affect the pressure in the main line.
T-piece 22 × 10 × 10 – transition to a distribution branch
T-piece 22 × 10 × 10 is a special case: the main 22 mm line is reduced to 10 mm and at the same time another 10 mm loop is branched off. This T-piece is used at the end of the main line when you want to divide the remaining flow of the 22 mm pipe into two final 10 mm loops at once. It is a terminal element in the distribution tree.
Warning: since here the 22 mm is reduced to 10 mm plus a 10 mm branch, the hydraulic losses are more significant. This T-piece should never be installed in the middle of a long distribution line – it belongs at the end or in a position where the remaining flow is actually low.
Design process step by step
Designing a wall heating system using T-pieces is a task that is worth doing systematically. Here is a proven practical approach:
Step 1: Mapping heated areas and calculating heat demand
We start with the building plan and identify which walls are to be heated. For each wall, we calculate the required thermal output (W/m²). In capillary wall heating, typical values range from 40–80 W/m² of effective area, depending on the water temperature and wall construction. Based on the output and the flow rate of the medium (usually water with additives), we calculate the flow rate for each loop in l/h.
Each loop (one capillary panel or one wall) will have one 10 mm end of a T-piece on the supply and return. The total number of loops determines how many T-pieces you will need and which ones.
Step 2: Determining the location of the main line
The main line (usually 22 mm) comes from the heat source (boiler, heat pump, storage tank) and leads to individual rooms or building wings. Routing of this line should be planned to be as short as possible while still accessible to all heated walls. The main line usually runs in the floor, in a technical crawl space, or in utility rooms.
Golden rule: the main line should have a symmetrical route – that is, ideally, equally long paths to loops of comparable output. If one branch of the main line is three times longer than another, we must compensate with balancing valves, otherwise the shorter branch will be disproportionately favored.
Step 3: Sizing the branches and selecting T-pieces
After planning the route, we calculate the flow in each section of the pipe. General rule: sum up the flows of all loops connected behind a given point in the route. Compare the result with the maximum flows for the given diameters:
- Diameter 10 mm: maximum flow about 30–40 l/h at speed 0.15–0.2 m/s
- Diameter 15 mm: maximum flow about 80–120 l/h
- Diameter 22 mm: maximum flow about 200–350 l/h
Based on these values, you can determine where to reduce the diameter. It is precisely at these points that the correct T-piece must be selected. A more detailed comparison of straight and reduced fittings can be found in the article Hepworth fittings and T-pieces: differences between straight and reduced fittings.
Step 4: Assembling the distribution scheme
Draw the scheme: mark each point where the pipe branches with the type of T-piece. The scheme does not need to be architecturally accurate – it is sufficient if it clearly shows where the T-pieces are, what the dimensions of the inlets/outs are, and in what order they follow. This scheme will be your lifeline in the event of a repair after five years.
Practical examples of pipe distribution from field experience
Example 1: Single-family house, ground floor, 4 rooms
The customer had a single-family house with a ground floor of approximately 120 m², and wanted to heat the living room, bedroom, children's room, and bathroom using wall heating. The total thermal output of the system was 7.2 kW, which at a temperature drop of 35/29 °C corresponds to a flow rate of approximately 1,030 l/h – no, that is too much for one circuit, you would need a 40 mm diameter. In reality, the rooms were smaller, and each wall had its own capillary loop with a flow rate of 25–40 l/h. The total flow rate of the system was approximately 320 l/h.
Solution: from the boiler room, we ran two main supply lines Ø22 – one towards the living room + kitchen, the other towards the bedrooms + bathroom. On each of these supply lines, we used T-fittings 22×10×22 at each location where a branch into a capillary circuit was needed, and at the end of each supply line, a T-fitting 22×10×10. Result: the system worked without problems from the first winter, with balanced temperatures in all rooms.
Example 2: Apartment renovation, complex layout
In this case, the apartment was on the third floor, and the supply pipe came from a technical room in a vertical shaft. The main supply line was only 15 mm (limited by existing piping). From this, we branched individual capillary circuits into four walls using T-fittings 15×10×15. Since the 15 mm pipe could not handle all flows at once, we had to divide the circuits into two groups with balanced flows and connect each group through a separate control valve. The T-fitting 15×10×15 was indispensable here.
Example 3: Commercial space, large area
A showroom with an open layout, long walls, and the need for even thermal comfort. Here, we used a star connection with a central distributor, from which 22 mm branches led to each wall. At the branching points within each wall, we used T-fittings 15×10×15 and 10×10×10. This project was the most complex, but precisely for that reason, we documented it in detail – each T-fitting had its own number, diameter, and position in the floor plan.
Most common mistakes in designing pipe layouts with T-fittings
Over the years of practice, I have seen mistakes that people repeatedly make when designing systems. Here are the most important ones:
- Too many T-fittings on one branch in a row – each T-fitting adds hydraulic resistance. If you have 8 T-fittings in a row on one 15 mm pipe, the last circuit may have only a fraction of the required flow. Solution: divide the route into multiple branches from the main supply.
- Mixing up the direction of the main flow and the branch – a T-fitting is not symmetrical in terms of hydraulics. Always follow the flow direction according to the system diagram; a reversed T-fitting may work, but it increases pressure losses.
- Incorrect reduction at the end of the supply line – using a T-fitting 22×10×10 in the middle of a long supply line, where other circuits continue, is a typical mistake. After this T-fitting, there is not enough diameter for the rest of the system.
- Omitting the return line – the supply and return lines must have identical T-fitting topologies. If you have 10 T-fittings on the supply line, you must also have 10 T-fittings on the return line – otherwise, the hydraulic loop will not be properly closed.
- Ignoring the height of the supply line – T-fittings at the highest points of the system can collect air. Always design air vents at the highest points of the supply line.
This topic is discussed in more detail in the article Common mistakes in the installation of T-fittings and fittings for wall heating in this Knowledge Center.
Documentation and review of the design
Experienced plumbers know that half of the work in designing a pipe layout is documentation. When you embed pipes into the wall, you lose physical access to the T-fittings for decades. If you do not have a record of exactly where each T-fitting is located, you will end up with a sledgehammer and a lot of uncertainty in case of a problem.
Minimum documentation should include:
- A floor plan with marked pipe routes and the position of each T-fitting (including type designation: 22×10×22 etc.)
- An axonometric diagram or a simple tree diagram of the system with diameters and flow rates
- A list of all fittings used with production numbers or product references
- Photo documentation before embedding
This documentation is invaluable not only for possible repairs, but also when selling real estate, when the buyer or an expert requires proof of the system's technical condition.
Fittings and T-fittings: when to use which
It is important to know when to reach for a T-fitting and when to use a straight fitting. Straight fitting 10 × 10 is used to extend a capillary pipe without changing direction or diameter – that is, in places where you need to connect two pipes in a row, for example, when the material length was insufficient or during repair of a damaged section. Conversely, a T-fitting is always used for branching.
Rule of thumb: never replace a T-fitting with two fittings and pipe pieces. Although it may be technically possible, you will create a hydraulically unbalanced solution with two potential leak points instead of one. A T-fitting is one piece, one sealing surface in addition to a straight section – and that is the optimal solution.
Hydraulic balancing in a tree distribution system
Even with the best design of a tree-type distribution system, shorter branches will have less hydraulic resistance than longer ones. This means more water will flow through the shorter branch than desired, while the longer branch will be supplied with less. The solution has two aspects:
Design measures: design the system so that the distances of the branches from the source are as similar as possible. If this is not possible (which is usually the case), extend the shorter branches with artificial resistance—such as regulating valves.
Regulating elements: each capillary loop should have a regulating or balancing valve at its inlet. This valve is set during the first start-up of the system (hydraulic balancing) so that the flow through each loop corresponds to the design value. Without this step, even a perfectly designed distribution system can be unbalanced.
Material compatibility – what to mix and what not to
Hepworth fittings for wall heating are typically made of polypropylene or similar polymers compatible with low-temperature systems (up to 90 °C, typically 60 °C). If you are mixing different manufacturers in the system, always check the following:
- Compatibility of the outer diameter of the pipe with the inner diameter of the fitting (manufacturers can differ by tenths of a millimeter, which at capillary dimensions can mean the difference between a tight and a leaky connection)
- Temperature and pressure parameters: Hepworth fittings are dimensioned for a specific system; do not insert pipes from other manufacturers without verifying compatibility
- Chemical compatibility: if the system contains corrosion inhibitors or other chemical additives in the water, verify that they are compatible with the material of the fittings
For more on the correct way to connect pipes of different diameters and manufacturers, see the article How to correctly connect pipes of different diameters using reducing couplings.
Pressure testing and commissioning
After the distribution system is completed and before embedding, a pressure test must be carried out. For plastic capillary systems, a pressure test at 1.5 times the maximum operating pressure is standard, at least 6 bar, for a minimum of 30 minutes. Monitor the pressure gauge—any drop in pressure indicates a leak.
Practical tip: always perform the test before covering the pipe, even if you are 100% sure of the quality of the connections. Pressure tests have revealed leaks even on jobs where an experienced plumber worked with maximum care. It is an insurance measure, not a sign of distrust.
The step-by-step installation of fittings including T-pieces is described in detail in the article Installation of Hepworth fittings step by step.
Frequently asked questions (FAQ)
Can I use a T-piece 22 × 10 × 22 in a place where I need a T-piece 22 × 10 × 10?
No, without further reduction. A T-piece 22×10×22 maintains the 22 mm diameter in the continuing run—this is suitable when there are further loops after it. A T-piece 22×10×10, on the other hand, reduces the continuing run to 10 mm, which is intended for the last branching. If you use a 22×10×22 at the end of the distribution, you will be left with a blind 22 mm pipe that you must close with a blanking plug—this is an unnecessary complication and a potential leak point.
How many T-pieces 15 × 10 × 15 can I connect in a row on one pipe?
It depends on the total flow, but practically: if each branching 10 mm capillary loop consumes 30 l/h and your 15 mm pipe can handle 120 l/h, you can connect three T-pieces in a row (3 × 30 = 90 l/h, leaving another 30 l/h for a possible fourth loop after the last one). With a fourth T-piece, you would have only a small amount of flow left for the last loop, which may be insufficient. Always calculate from the end to the source and check the remaining capacity.
What to do if I am missing the correct size T-piece on site?
Never improvise with an incorrect T-piece or a homemade connection. Wall heating is a closed system, and once embedded, you will lose access to it for years. Rather wait for the correct part. If you need it urgently, in the wall heating category you will find a complete range of Hepworth fittings including all T-pieces, or contact the technical department for fast reordering.
How do I find out if my distribution is hydraulically balanced?
The simplest test: if the system is in operation, measure the surface temperature of the walls in different rooms with a non-contact thermometer. If the differences are greater than 2–3 °C despite the same set temperature, the system is not balanced. A more accurate method is to measure the flow through each loop with a flow meter during hydraulic balancing. The difference between the measured and design values tells you by how much you need to tighten or loosen the regulating valve of the respective loop.
Is it necessary to use fittings from the same manufacturer for the entire distribution?
Technically, no, but we strongly recommend it, especially for the capillary network. Different manufacturers may have slightly different dimensions of the outer diameter of the pipes and the inner diameter of the fittings. At a 10 mm diameter, even a difference of 0.1–0.2 mm can cause an incorrect sealing of the joint. Hepworth fittings are designed for Hepworth capillary pipes—when mixing with another manufacturer, always test the joint before embedding it in the wall.
Where in the run should a T-piece be and where a straight coupling?
A T-piece always where the pipe branches or turns off. A straight coupling 10 × 10 only where you are extending a straight section without branching—for example, when connecting two pipe pieces when the material length is insufficient or during a repair. If you are unsure whether a point requires a T-piece or a coupling, draw a diagram: if the pipe splits into two directions at that point, it is a T-piece; if it continues in one direction only, it is a coupling.
Conclusion: A distribution system that works for decades is designed with care
Designing a wall heating distribution system using T-pieces is not a routine task that can be done in half an hour. It is a system decision with long-term consequences. The correct choice of T-piece—whether 22 × 10 × 22 for branching from the main distribution, 15 × 10 × 15 for a distribution branch, or 10 × 10 × 10 in the capillary network—directly affects the hydraulic balance, system performance, and reliability.
A systematic approach—mapping areas, calculating flows, dimensioning branches, selecting T-pieces, documentation, and pressure testing—is an investment that pays off in the form of a system that works without problems and does not require costly repairs. If you have any doubts about the design, we recommend consulting a technical specialist or referring to other articles in this Knowledge Center, where you will find details on each installation step, fitting selection, and problem solving.
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.