What pipe diameters do I need for wall heating

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What pipe diameters do I need for wall heating?

Selecting the correct pipe diameter is one of those seemingly simple decisions that actually determines whether your wall heating system will operate efficiently, quietly and reliably – or whether you will be dealing with insufficient heating, noise in the walls and unnecessary energy costs. In practice, I often see that people either underestimate the diameter and make the distribution too narrow, or on the contrary, buy unnecessarily thick pipes, which increases the cost of both material and labor. This article will help you make the right decision even before you look at the price list.

Why is the pipe diameter for wall heating so important?

Wall heating works on a different principle than a conventional radiator system. The heat transfer medium – usually water – circulates through the distribution pipes directly inside the wall or in close proximity to the wall surface. The system usually operates at a lower water temperature than conventional heating (it can be as low as 30–45 °C instead of 70–80 °C with traditional radiators), which places special requirements on the flow and pipe diameter.

If the diameter is too small, the hydraulic resistance increases, the pump has to work harder, the system becomes noisier, and in the worst cases, some parts of the wall simply do not receive enough heat. If the diameter is unnecessarily large, the pipes take up more space in the wall, the material costs increase, and in addition, the whole system reacts to changes more slowly – which is a disadvantage especially in modern regulation systems.

From the diagram it is clear that hydraulic resistance drops dramatically with increasing pipe diameter. For wall heating, this has a direct impact on the choice of pump and the overall electricity consumption of the system.

Standard pipe diameters used in wall heating

In Hepworth wall heating systems, we most commonly encounter three basic pipe diameters: 10 mm, 15 mm and 22 mm. Each of them has its place in the distribution and its correct placement in the system is key to the hydraulic balance of the entire system.

10 mm pipe – for loops and branches into walls

Ten-millimeter pipe is typically the thinnest used in wall heating. In practice, it is used mainly for individual heating loops – that is, for the distribution lines that run along the wall surface and transfer heat to it. This is the pipe that the customer usually does not see at all, as it is embedded in the plaster or under the cladding.

The typical length of one loop made of 10 mm pipe should not exceed 60–80 meters under normal operation. If the loop is longer, the hydraulic resistance increases to such an extent that water circulation becomes problematic and distant parts of the wall are cold. In practice, this means that for a larger room (e.g. 20–25 m² with wall heating on several walls), it is necessary to design several shorter loops rather than one long one.

For connecting and branching 10 mm pipes, for example, you can use the Straight coupling 10 x 10 – simple, reliable and absolutely tight when installed correctly. If you need to branch off from a 10 mm distribution, you will use the T-piece 10 x 10 x 10, which allows one flow to be split into two directions at the same diameter.

15 mm pipe – for distributors and shorter main lines

Fifteen-millimeter pipe forms a kind of intermediate link in the distribution hierarchy. It is used where a higher flow is needed than a 10 mm pipe can handle, but there is no need to introduce a full 22 mm main line. Typically, it is used for:

horizontal distributors on a floor, where 2–3 loops are supplied from one point,
supply and return pipes to a local wall heating distributor,
distributions in smaller apartments, where the total length of the system is not large,
connections between the distributor and the wall inlet in less powerful systems.

In practice, I have encountered several cases where a customer in a family house with two rooms on a floor wanted to save money and designed the entire system using 10 mm pipes, including the supply pipes to the distributor. The result? The pump was constantly running at full capacity, the system was noisy, and the distant room did not reach the desired temperature. It was enough to replace the supply and return pipes with 15 mm and the problems disappeared.

22 mm pipe – for main horizontal and vertical lines

Twenty-two-millimeter pipe is the backbone of most medium to large wall heating installations. It serves as the main supply and return line from the boiler or heat pump towards the distributors on the floors or in the rooms. From the 22 mm pipe, branches are fed towards individual circuits, using reduced T-pieces.

For example, the T-piece 22 x 10 x 22 is ideal for situations where you want to branch off one 10 mm pipe from the main 22 mm line towards the wall or distributor, while the main line continues in the same diameter. Alternatively, the T-piece 22 x 10 x 10 is used where the main line ends and you want to create two 10 mm branches at once.

For branching from 22 mm to 15 mm and continuing the main line in 22 mm, there is no better choice than a reduced T-piece, just like the T-piece 15 x 10 x 15 solves an analogous situation in a 15 mm main line with a 10 mm branch.

How to determine the correct diameter for a specific installation?

The selection of diameter is not a matter of chance or intuition – it is a hydraulic calculation that takes into account several key parameters. An experienced designer or technician will work with the following values:

Thermal output and required flow

The basic formula states that the thermal output Q (in watts) depends on the flow rate m (in liters per hour), the specific heat capacity of water c (4 187 J/kg·K), and the temperature difference Δt between the supply and return. For floor heating, a temperature difference of 5–10 °C is typically used (e.g., supply 40 °C, return 35 °C).

To cover the heat loss of a standard living room with an area of 15 m² (assuming a heat loss of 1 000 W) at a temperature difference of 10 °C, the required flow rate is approximately 86 liters per hour. For a single 10 mm pipe, the maximum recommended flow rate without excessive noise and wear is about 60–80 l/h, for 15 mm it is 100–150 l/h, and for 22 mm up to 200–300 l/h.

From these figures, it is clear why a 10 mm diameter cannot be used throughout the system for higher thermal outputs – it simply cannot carry the required amount of heat transfer medium without an excessively high flow velocity, which causes noise, erosion, and increases resistance.

Water flow velocity

The recommended water flow velocity in floor heating pipes is 0.3–0.8 m/s. At higher velocities, noise (hissing, knocking) occurs and the pipes wear out faster. At lower velocities, there is a risk of sedimentation of impurities and air bubbles. This velocity is directly related to the diameter and flow rate – for the calculation, a simple formula applies:

v = Q / A, where v is the velocity [m/s], Q is the flow rate [m³/s], and A is the pipe cross-section [m²].

For a 10 mm pipe (internal diameter approx. 8 mm): at a flow rate of 60 l/h, the velocity is about 0.33 m/s – acceptable. At a flow rate of 100 l/h, the velocity would rise to 0.55 m/s, which is still acceptable, but at 150 l/h it would be almost 0.83 m/s and would start to make noise. That is why it is necessary to switch to a 15 mm or 22 mm diameter at higher flow rates.

Total loop length and number of branches

Another factor that influences the choice of diameter is the total length of the distribution loop. Every meter of pipe adds hydraulic resistance, and every fitting (connector, T-joint, elbow) adds so-called equivalent length. For example, a T-joint on a 10 mm pipe increases the resistance by a value corresponding to about 0.5–1 m of straight pipe.

If you are designing a distribution system with 40 meters of main pipe and 6 branches, the hydraulic resistance of the system is significantly higher than with 20 meters and 3 branches. A larger diameter of the main distribution compensates for this resistance – and it is precisely here that it is logical to switch from 15 mm to 22 mm for the main branch.

Typical configurations for different building sizes

Small apartment or one room (up to 30 m² of heated wall area)

In this case, the entire system is usually realizable using a combination of 15 mm supply/return pipes and 10 mm loops. The supply and return pipes of 15 mm diameter run from the heat source to the manifold, and 10 mm loops run from the manifold to the walls. The total flow rate of the system usually does not exceed 120 l/h, which the 15 mm main distribution can handle without problems.

Typical example: a two-room apartment in a panel building, where floor heating is installed in the living room and bathroom. The manifold is in the hallway, from which 15 mm pipes (supply and return) run to the heat source, and from the manifold four 10 mm loops run to the walls.

Single-story family house (100–150 m²)

Here you cannot do without a 22 mm main distribution. The main distribution runs from the boiler in 22 mm pipes to the manifold located in the technical room or in the central part of the house. From the manifold, 15 mm pipes run to individual rooms or to local manifolds, and from there 10 mm loops run to the walls.

In practice, for a family house with 5 rooms and a heat loss of around 8–10 kW, we recommend:

Main distribution: 22 mm
Supply/return pipes to manifolds in rooms: 15 mm
Heating loops in walls: 10 mm, each max. 60 m in length
Number of loops: according to the required output, typically 2–4 for a larger room

Two-story family house or larger building

For larger buildings with multiple floors, a 28 mm or 35 mm main vertical distribution between floors is sometimes considered, but in Hepworth floor heating the limit is usually 22 mm in most standard installations. The vertical riser in 22 mm supplies horizontal manifolds on each floor, from which 15 mm and 10 mm distributions run to individual rooms and walls.

Reductions and combining diameters – where are the critical points

In practice, one of the most frequently underestimated topics is the location of the transition from one diameter to another. Every reduction is a potential leak point (if improperly done), and also a place of sudden flow velocity change, which can cause turbulent flow and noise.

Rule number one: always reduce the diameter from the source towards the consumer (i.e., from a larger to a smaller diameter), never the other way around in the direction of flow. If you were to increase the diameter from the source, you would create pressure losses where you do not want them, and the system would behave unpredictably.

Rule number two: always make sure that the fittings are properly dimensioned for the given diameter and pipe material. Hepworth fittings are designed to ensure a solid, durable, and tight connection – but only if you use the correct fitting for the correct diameter. More on this topic can be found in the article How to properly connect pipes of different diameters using reduced fittings.

Rule number three: minimize the number of reductions. Each reduction means costs for fittings, installation time, and potential risk of leakage. If you can design the piping in such a way that you have fewer transitions between pipe diameters, do so.

Practical examples from installation practice

Example 1: Senior apartment, wall heating in kitchen and living room

In a project in an older apartment building, we installed wall heating for a senior citizen, where the requirement was even heat without radiators due to space constraints. The heat loss in the kitchen (14 m²) was 650 W, in the living room (22 m²) 900 W. The total power was approximately 1,600 W.

We designed: supply and return piping from an electric boiler (6 kW, new) in a 15 mm diameter, a distributor with two circuits also in 15 mm. From the distributor, 10 mm loops were run – two to the kitchen wall (each 45 m), three to the living room (each 50 m). The total system flow was 138 l/h, which the 15 mm main piping easily handled at a flow speed of approximately 0.27 m/s – well within the recommended minimum, so we slightly increased the circulation pump to the medium setting. The system runs quieter than the customer had expected.

Example 2: New single-family house construction, full wall heating

A larger project – a single-family house of 160 m² with two floors, an air-to-water heat pump. The total system power was approximately 9.5 kW, 14 wall heating circuits in various rooms. There was no question here – the main piping had to be 22 mm, the horizontal piping on each floor 15 mm, and all loops in the walls 10 mm.

To transition from the 22 mm main piping to 15 mm, we used reduced T-fittings, specifically an analog of a T-fitting with a central outlet in a smaller diameter – exactly the principle of, for example, a T-fitting 22 x 10 x 22, only with the appropriate transition fittings for 15 mm. The installation of the fittings went smoothly, and after pressurizing the system to 6 bar for 24 hours, there were no leaks.

Example 3: Renovation – adding wall heating to existing radiators

An interesting case: the customer had an existing system with radiators on 22 mm piping (an old copper installation) and wanted to add wall heating to one room. Solution: from the existing 22 mm piping, we branched off with a T-fitting to 10 mm and connected two wall loops in parallel. Since the existing system operated at a higher temperature (70/55 °C), we had to install a mixing group for the wall section to reduce the temperature to 40/35 °C. A 10 mm diameter for the loops was sufficient – each loop was 38 and 42 meters long. The customer was satisfied, the room was heated evenly instead of the previous localized overheating under the radiator.

Most common mistakes when selecting pipe diameter

Over the years of practice, I have seen several types of mistakes that repeat themselves again and again. These include:

Whole system in 10 mm: An installer or customer decides to save money and runs the entire piping, including the main piping, in 10 mm. Result: the system cannot transport enough heat, the pump is overloaded, distant rooms are cold.
Unnecessarily large diameter for loops: Some installers run 15 mm or even 22 mm loops in the walls "just to be safe". This is unnecessary, complicates routing in the wall, and increases system inertia – the room heats up slowly and cools down slowly, regulation is worse.
Ignoring equivalent lengths of fittings: During hydraulic calculation, the resistance of T-fittings, elbows and connectors is overlooked. The resulting resistance is higher than expected and the pump cannot keep up.
Wrong location of reduction: A reduction from a larger to a smaller diameter is made before the supplied circuits, not after them. This increases resistance at an inappropriate location.
Long loops in 10 mm: A loop longer than 80 meters in a 10 mm diameter is problematic. The resistance is so high that the end of the loop is cold. It is better to divide one long loop into two shorter ones.

Other typical mistakes in installation can be found in the article Common mistakes when installing T-fittings and connectors for wall heating.

Table of recommended diameters according to system power and piping length

Part of piping	System power	Recommended diameter	Max. flow
Wall heating loop	up to 1,200 W/loop	10 mm	up to 80 l/h
Supply/return to distributor	up to 3,000 W	15 mm	up to 150 l/h
Supply/return to distributor	3,000 – 8,000 W	22 mm	up to 300 l/h
Main vertical piping (multi-floor)	over 8,000 W	22–28 mm	over 300 l/h
Length of one 10 mm loop	—	max. 60–80 m	—

Pipe material and its influence on diameter selection

In Hepworth wall heating systems, multi-layer pipe (PEX, PE-RT or aluminium-plastic) or copper pipe is primarily used. Each material has slightly different internal diameters for the same nominal external diameter – and it is the internal diameter that determines the flow and hydraulic resistance.

For example, a 10 mm multi-layer pipe typically has a wall thickness of 1–1.2 mm, giving an internal diameter of 7.6–8 mm. A 10 mm copper pipe has a wall thickness of 1 mm and an internal diameter of 8 mm. The difference is small, but it accumulates over long loops. For typical Hepworth wall heating installations, both materials are suitable – it is important that the fittings are always correctly dimensioned for the specific external diameter and pipe material.

If you are unsure which fittings to use with your pipe, read the article How to choose the right Hepworth fittings for wall heating, where this topic is covered in detail.

Pressure losses and their calculation – the basis of correct dimensioning

Pressure losses in the pipe are the sum of friction along the pipe and local losses in the fittings. For standard wall heating installations, it is recommended that the total pressure losses in one circuit do not exceed 100–150 Pa/m of equivalent length. If the losses are higher, you must either increase the pipe diameter or increase the pump power – which is less energy efficient.

Approximate values of specific pressure losses for various diameters and flows:

10 mm pipe, flow 60 l/h: approx. 90–110 Pa/m
10 mm pipe, flow 80 l/h: approx. 150–180 Pa/m (limit value)
15 mm pipe, flow 100 l/h: approx. 50–70 Pa/m
15 mm pipe, flow 150 l/h: approx. 110–130 Pa/m
22 mm pipe, flow 200 l/h: approx. 30–45 Pa/m
22 mm pipe, flow 300 l/h: approx. 80–100 Pa/m

From these values, it is clear that a 10 mm pipe at a flow of 80 l/h is at the upper limit of capacity and for long circuits it would be better to switch to 15 mm or shorten the loop.

Hydraulic balancing of the system with different diameters

When combining different diameters in one system (which is common in wall heating), it is essential to hydraulically balance the system. This means that each circuit must receive its prescribed amount of heat transfer medium – not more, not less. Without balancing, most of the water will flow into the circuits with the least resistance (usually those with larger diameters and shorter lengths), while the long circuits in 10 mm will be under-supplied and cold.

Hydraulic balancing is achieved by using adjustable valves on the manifold. Each circuit is set to the desired flow using flow meters or pressure gauges. This is why it is important to know the designed flows for each circuit – and these directly depend on the correctly chosen pipe diameter.

For more information on how to design the overall piping layout, see the article How to design a piping layout for wall heating using T-fittings.

Where to buy the right fittings and what to look for when choosing them

When purchasing fittings for Hepworth wall heating, always check the following: nominal external diameter (this is the number you find in the fitting designation, for example "10" in Straight coupling 10 x 10), type of connection (compression, crimped, push-fit), pressure and temperature range, and certification for potable water or heating systems.

For wall heating, combined diameters are often needed – that is, reduced fittings. For example, T-fitting 15 x 10 x 15 is a classic reduced T-fitting for situations where you branch off a 10 mm loop from a 15 mm main, while the main continues in 15 mm. And T-fitting 22 x 10 x 22 performs the same function when the main is 22 mm.

For more information on the differences between straight and reduced fittings, read the article Couplings and T-fittings Hepworth: differences between straight and reduced fittings.

Most frequently asked questions (FAQ)

Can I make the entire wall heating system only in 10 mm pipe?

For small systems (one or two circuits, shorter lengths, low power up to 1 500 W), it can work if the supply/return pipes are short and the pump power is sufficient. For larger installations, it is not recommended – hydraulic resistance increases disproportionately, the system is noisy, distant parts of the wall remain cold, and the pump is constantly overloaded. Main distribution lines should always be at least 15 mm, ideally 22 mm for larger buildings.

What is the difference between external and internal diameter and which one is used when selecting a fitting?

Hepworth fittings are dimensioned according to the external diameter of the pipe. So when you are looking for a fitting for a 10 mm pipe, it means the external diameter is 10 mm. The internal diameter depends on the wall thickness and material – for a 10 mm external diameter, the internal diameter is usually 7.6–8 mm. Flow and hydraulic resistance are calculated from the internal diameter, but when selecting a fitting, always refer to the external diameter.

How many wall heating loops can branch off from one T-fitting?

Exactly one branch can come from one T-fitting (that is the nature of a T-fitting). If you need to divide the distribution into multiple loops, either connect T-fittings in series (cascading), or use a manifold. Cascading installation of T-fittings is common, but for more than 4–5 branches, a manifold with adjustable valves is more practical and hydraulically advantageous. Each series of branches also increases the total resistance, which must be considered when selecting the diameter of the main distribution.

Can I combine old copper and new plastic pipe in one system?

Yes, it is possible, but it requires transition fittings suitable for both materials. The main problem is not the diameter (both materials can have the same nominal external diameter), but the way of joining and material compatibility. Some fittings are designed only for plastic, others only for copper – always check whether the fitting is certified for both materials, or use a transition fitting. A pressure test after installation is absolutely essential in this case.

How long can a loop made of 10 mm pipe be without the end being cold?

A practically verified limit is 60–70 meters under normal operating conditions (flow 50–70 l/h, temperature drop 5–10 °C). With an optimal pump and correct setting, it is possible to reach 80 meters, but this is a real limit. Longer loops have such a high hydraulic resistance that even at full pump capacity, the end of the loop does not receive enough hot water. The solution is either to shorten the loop and add another one, or to switch to a larger diameter – but 15 mm loops in the wall are unusual and impractical in terms of routing in plaster.

What pipe diameter do I need if I am switching from radiator heating to wall heating and want to use existing piping?

Existing piping for radiators is usually 15 mm or 22 mm – which is ideal for main distribution in wall heating. You will, however, need to add 10 mm loops for wall circuits, with the branch from the existing piping realized by a reduced T-fitting (for example, T-fitting 22 x 10 x 22 from an existing 22 mm distribution to a new 10 mm loop). It is also important to check whether the existing pump can handle the higher hydraulic resistance of the new loops, and possibly replace or add a secondary pump for the wall heating section.

Conclusion: The correct diameter = the basis of reliable wall heating

Selecting the pipe diameter for wall heating is not a matter of chance or habit – it is a technical decision that directly affects performance, comfort, energy consumption, and the lifespan of the entire system. The principle is simple: main distribution lines must be sufficiently dimensioned (22 mm for larger buildings, 15 mm for smaller systems), transitions between diameters must be made with the correct reduced fittings, and heating loops in the walls are standard 10 mm with a maximum length of up to 80 meters.

If you are unsure about the calculations when designing the system, consult an HVAC designer or an experienced installer – an investment in a properly designed system is many times cheaper than later repairs of an undersized system after it has been plastered in. And if you need the right fittings for your system, browse the full range in the category wall heating and heating.

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