Dimensions and connection thread pitch of Hepworth stainless steel manifolds

We are still preparing products.

But you can look at other categories.

Dimensions and Thread Spacing of Hepworth Stainless Steel Manifolds

When discussing stainless steel manifolds for floor heating, most customers focus on the number of circuits or the material. However, dimensions and thread spacing determine whether the manifold will fit into the cabinet at all, whether it can be connected to an existing system, and whether the entire installation looks and functions as it should. These technical details cause the most problems on construction sites – and it is precisely these we will examine in depth in this article.

This article is primarily intended for installers, designers, and technically skilled builders who need precise numbers and relationships – not just marketing phrases. You will find body dimensions, an explanation of thread standards, a comparison of different configurations according to the number of circuits, and practical notes from installation practice.

Basic Structure of a Stainless Steel Manifold – What It Is Made Of

Before we move on to the numbers, it is important to understand what a manifold is made of. A stainless steel manifold for floor heating typically consists of two parallel collector pipes (one for supply, the other for return), connected by structural consoles, with branches for individual floor heating circuits mounted on them.

Each pipe has threaded connections at both ends – on one side, the floor heating circuits are connected (outputs/inputs to the loops), and on the other side, there are connections to the main distribution and fittings (air vent, drain valve, possibly control elements). It is precisely the dimensions of all these threads and the mutual distances between them that determine what can be connected to the manifold and how the whole unit will take up space in the installation area.

Schematic: View of a typical stainless steel manifold – two parallel pipes (supply and return) with branches for individual circuits. The spacing of the branches and the axis distance of the pipes are dimensioned.

Key Body Dimensions of the Manifold According to the Number of Circuits

Hepworth stainless steel manifolds for floor heating are available in configurations from 2 to 6 circuits. Each configuration has different overall body dimensions, because the length of the collector pipe directly depends on the number of branches and their spacing. Understanding the relationship between the number of circuits and the length of the manifold is essential for choosing the right manifold cabinet or for assessing whether the manifold will fit into an existing space.

Branch Spacing – What It Is and Why It Matters

Branch spacing (in English, "port spacing" or "connection pitch") is the distance between the centers of two adjacent connection points on the collector pipe. For Hepworth stainless steel manifolds, this spacing is 50 mm. This value is standardized and compatible with most European fittings for floor heating.

Why is the spacing important? Because:

It determines which fittings (flow meter, control head, actuator valve) can be mounted – most fittings are designed for 50 mm or 40 mm spacing.
It affects the overall length of the manifold and thus the minimum dimensions of the cabinet.
With 50 mm spacing, the installed hoses are far enough apart to be comfortably turned and tightened without interfering with each other.

Overall Body Length – Table According to the Number of Circuits

Based on a 50 mm spacing between the centers of individual circuits and on construction edge dimensions (front ends of the pipes with connections), the following approximate overall body lengths apply to Hepworth stainless steel manifolds:

Number of circuits	Overall body length (mm)	Minimum recommended cabinet width (mm)	Typical weight (kg)
2-way	~195–210 mm	300 mm	~1.0–1.2
3-way	~245–260 mm	350 mm	~1.2–1.5
4-way	~295–315 mm	400 mm	~1.5–1.8
5-way	~345–365 mm	450 mm	~1.8–2.1
6-way	~395–420 mm	500 mm	~2.0–2.4

Note: Always verify the exact dimensions in the current technical specification sheet of the specific product, as the manufacturer may slightly adjust the dimensions during construction innovation. The values in the table are typical for the standard range of distributors and are used for preliminary dimensioning and selection of the cabinet.

In practice, this means that for a typical family house with 5 floor heating circuits (ground floor), you need to obtain a cabinet with an internal width of at least 450 mm, preferably 500 mm, to leave some space around the distributor for accessories, hoses, and convenient work. More about this issue is discussed in the article "Placement and Installation of the Distributor Cabinet – Where and How to Mount the Distributor."

Axial distance between supply and return collector pipes

The second critical dimension is the vertical distance between the centers of the upper (supply) and lower (return) pipes of the distributor. This value is also referred to as the "pipe pitch" and in the standard series of Hepworth stainless steel distributors it reaches 100 mm from center to center of the pipe.

Why is this distance important? Primarily because it determines the overall height of the distribution block including the mounted fittings (air vent, shut-off valves, flow meters). If you are considering adding actuator heads to the valves or an electric drive, you must account for the fact that the total height of the block with accessories can increase to 250–300 mm. Therefore, the installation cabinet must have an internal height of at least 350–400 mm to comfortably fit everything and leave space for connecting hoses leading into the floor.

Diagram: Cross-section view of a pair of collector pipes – axial distance 100 mm, outer diameter of the pipe approx. 30 mm, pitch of branches 50 mm.

Thread standards and sizes of connecting threads

This is where most installers make mistakes – if they cannot distinguish which thread is where, they may end up buying the wrong reducers or fittings that do not fit. Stainless steel Hepworth distributors use threads according to the British standard BSP (British Standard Pipe) – specifically conical or cylindrical threads marked as G (cylindrical) or R (conical). In practice, for heating system pipes, we most often encounter cylindrical G threads (Whitworth).

Branch threads for floor heating circuits

The branches for individual floor heating circuits (where the hoses or tubes are connected to the floor loop) have a thread of G 3/4" (external / internal depending on the construction). This is a standardized size that corresponds to hose fittings for tubes with a diameter of 16×2 mm and 17×2 mm, which are the most commonly used dimensions for floor heating pipes.

Specifically: the G 3/4" thread has:

External thread diameter: 26.44 mm
Thread pitch: 1.814 mm (14 threads per inch)
Relevant standard: ISO 228-1 / EN ISO 228-1

These branches are usually provided as an external thread (Male thread) or internal thread (Female thread) on the distributor – depending on the specific series. In the case of Hepworth stainless steel distributors, the branches of the circuits are standardly external G 3/4" thread, to which the corresponding female hose fitting is tightened.

Threads on the main supply and return circuits (inlet/outlet from the boiler)

On the ends of the collector pipes, where the distributor is connected to the main heating circuit (from/to the boiler, mixing group or pump group), the thread is G 1" (internal or external, depending on the series). This size is common and compatible with standard fittings for heating systems.

G 1" has the following parameters:

External thread diameter: 33.25 mm
Thread pitch: 2.309 mm (11 threads per inch)
Relevant standard: ISO 228-1

This thread is used to connect shut-off ball valves, check valves, drain valves and possibly pump units. When using copper or brass (brass alloy) as the opposite material, always use sealing paste compatible with stainless steel and a hemp gasket or Teflon.

Threads for the air vent and drain valve

At the top end of each collector pipe (or on a side outlet) there is an opening for the air vent. This thread is standardly of size G 1/2" (internal). A manual or automatic air vent is mounted here. The drain valve is usually connected via a G 1/2" or G 3/8" thread – depending on the specific distributor series.

Diagram: Overview of threaded connections on one pipe of a stainless steel manifold – three types of threads for three different functions.

Compatibility of threads with hose kits and fittings

One of the most frequently asked questions in practice is whether the G 3/4" thread on a manifold is compatible with standard hose kits for 16 mm and 17 mm pipes. The answer is yes – provided that the hose kit has a female side with a G 3/4" thread (which is standard for EVOH, PEX-a, PEX-b and PE-RT systems with an outer diameter of 16 or 17 mm).

With 20 mm pipes or 16×2 mm pipes with a thicker wall, sometimes reductions from G 3/4" to G 1/2" or vice versa are used, but this is not common practice in a properly designed system. In practice, we see that installers working on the reconstruction of older systems often encounter various non-standard dimensions – in such cases, it is necessary to use stainless steel or brass reducing couplings, always sealed with hemp and paste or Teflon.

Be careful with interchangeable markings: some products from Asia are sold with the marking "3/4 inch", but they refer to the NPT (National Pipe Thread) according to the American standard ANSI/ASME B1.20.1, not BSP. These threads have different pitch and diameter – NPT 3/4" has 14 threads per inch, the same as BSP G 3/4", but the diameter and thread profile are different. Mixing NPT and BSP threads may seem to work when manually tested, but will cause leakage under pressure. Always use fittings with BSP threads (marked G or Rp).

Practical examples from installation practice

Example 1: Single-family house, ground floor + upper floor, 8 circuits in total

There are 4 floor heating circuits on the ground floor and another 4 on the upper floor. The customer asked whether one 8-way manifold could be used for the whole house. This is not a good idea for several reasons: the length of an 8-way manifold would exceed 550 mm, which is problematic for standard cabinets, and in addition, temperature control for the ground floor and upper floor would be complicated, as these two zones usually have different desired temperatures. The correct solution: one stainless steel 4-way manifold for the ground floor and one stainless steel 4-way manifold for the upper floor, each with its own pump unit and its own thermostat.

Result: each manifold has a total length of about 310 mm, fits into a standard 600 mm wide cabinet with all the fittings, and in addition, the system is hydraulically independent for each zone.

Example 2: Apartment in an apartment building, renovation, 3 circuits

The customer had the following layout: living room + kitchen (one circuit), bedroom (one circuit), and bathroom (one circuit). A stainless steel 3-way manifold had three G 3/4" branches and the total length of the manifold was about 250 mm. The manifold was mounted in an installation cabinet 400×600 mm (width × height), which provided enough space for a bleed valve, a ball valve on the supply circuit, and a pump. The customer saved money compared to a brass manifold, mainly because stainless steel does not require any additional protection against corrosion and the system will be reliable for decades.

Example 3: New construction, floor heating throughout the house, 6 circuits

For a new construction with a larger floor area, a stainless steel 6-way manifold was selected. The total length of the body was about 410 mm. The cabinet had to be specially ordered with a width of 600 mm (internal width 550 mm), so that all the hoses could fit around the manifold and adjustments to the flow meters could be made without problems. The installer appreciated that all the G 3/4" branches were on the same vertical axis, which allowed for clear hose routing and easy identification of individual circuits after labeling.

Dimensioning of the manifold cabinet based on the dimensions of the manifold

Correctly oversizing the cabinet is critical for comfort during the initial setup and later maintenance. Based on practical experience, the following recommendations apply for the minimum internal dimensions of the cabinet:

Cabinet width: total length of the manifold + at least 90 mm (45 mm on each side for convenient connection of hose fittings and handling with a wrench).
Cabinet height: at least 350 mm for a manifold without actuator heads, at least 450 mm when installing thermostatic actuators, as these add about 80–100 mm in height above the branches.
Cabinet depth: at least 100 mm for a built-in installation, recommended 150 mm when installing a pump unit.

For more information on selecting and placing the cabinet, see the article "Positioning and installation of the manifold cabinet – where and how to install the manifold."

Diagram: Minimum internal dimensions of the installation cabinet for a 5-way distributor – width, height, required space around the body.

Why 50 mm pitch is an industry standard

The 50 mm pitch between the centers of the branches is not a coincidence or a manufacturer's whim. It is an industry standard that emerged as a compromise between several requirements:

Space for the wrench: With a G 3/4" thread, the outer hexagon of a hose kit end is typically 30–32 mm. With a 50 mm pitch, there is 18–20 mm of space between two adjacent hexagons, allowing a 30 mm wrench to be applied even in tight spaces.
Space for control heads: Thermoelectric actuators for valves on the distributor usually have a body diameter of 27–32 mm. With a 50 mm pitch, they can be mounted side by side without interfering with each other.
Flow meters: Standard rotary or float flow meters for G 3/4" have a body width of 40–48 mm. With a 50 mm pitch, they can be mounted without problems, each on its own branch.
Overall compactness: A smaller pitch (e.g., 40 mm) would shorten the distributor but would make it impossible to mount accessories. A larger pitch (e.g., 60 mm) would unnecessarily extend the entire block.

In practice, it is important to know that not all manufacturers strictly adhere to a 50 mm pitch. Some cheaper products from China or Poland may have a 45 mm or 55 mm pitch, which can cause problems when mounting standard European accessories. Hepworth, as an established European manufacturer of heating technology, maintains the 50 mm standard, which is one of the reasons for the reliability and longevity of their systems.

Compliance with EN standards and material technical specifications

Hepworth stainless steel distributors are made from AISI 304 steel (equivalent to 1.4301 according to EN 10088-1). This grade of stainless steel is standard for potable water and heating systems and meets the requirements of standards EN 13244 (plastic pipe systems for water supply) and EN 10312 (stainless steel pipes for piping systems). The following maximum operating values apply for pressure and temperature:

Max. operating pressure: 6 bar (typical floor heating systems operate at 1.5–2.5 bar)
Max. operating temperature: 80 °C (for systems with condensing boilers usually 35–50 °C)
Thread tightness: tested at 10 bar pressure test (1.67 times the max. operating pressure)

For a comparison of materials and their impact on system operation, read the article Stainless Steel vs. Brass Distributor – Which Material is Better for Underfloor Heating.

Most common mistakes when selecting a distributor in terms of dimensions

After years of working with customers, we repeatedly encounter the same mistakes that cause unnecessary problems on the construction site:

Underestimating the overall height of the block: The customer measures the distributor without accessories, buys a cabinet, and then finds out that with actuators, air vents, and flow meters, the whole thing does not fit in the height of the cabinet. Always calculate with accessories.
Interchangeability of G 3/4" and 3/4 NPT: As mentioned, BSP and NPT threads are not interchangeable. If you buy hose kits or accessories from foreign e-shops, verify the thread standard.
Selecting a distributor that is too short: The customer wants 2 circuits today, but plans an expansion. A stainless steel distributor is not a cheap device; replacing it with a larger one later is expensive. If there is a chance of expansion, choose a 4-way or 5-way distributor with blind connections for the currently unused circuits.
Ignoring the 100 mm axis distance when installing a front panel: If installers mount the distributor in a cabinet with a front panel and do not consider that the pipe axes are 100 mm apart, they may run into collisions with cabinet walls or other fittings.

This topic is discussed in more detail in the article Common Problems with Stainless Steel Distributors – Leaks, Weak Flow, Noise.

How to read a distributor technical drawing

If you have a technical drawing (datasheet) of the distributor, pay attention to the following dimensions:

L (overall length): The total length of the body from edge to edge, including the front threaded connections. This is the dimension you compare with the cabinet width (minus the reserve on each side).
A (branch pitch): The distance between the centers of adjacent branches – standard 50 mm.
H (pipe axis distance): The distance between the centers of the supply and return pipes – standard 100 mm.
D (pipe outer diameter): The outer diameter of the collector pipe – typically 28–32 mm for the series with a G 1" connection.
d (branch outer diameter): The outer diameter of the branch – typically 20–22 mm for G 3/4".

If you do not have the technical drawing directly on the product page, do not hesitate to contact us – for each product, we can provide the current datasheet from the manufacturer.

FAQ – most frequently asked questions about dimensions and threads of stainless steel distributors

What is the branch pitch of Hepworth stainless steel distributors?

The standard pitch between the centers of adjacent branches is 50 mm. This value is compatible with most European accessories for underfloor heating – hose kits, thermoelectric actuators, flow meters, and control valves.

What thread do the branches have for underfloor heating circuits?

The branches for individual circuits have an external thread of G 3/4" (BSP, ISO 228-1). This thread is compatible with standard hose kits for 16 mm and 17 mm pipes. Be careful with interchangeability with NPT thread (American standard) – these threads are not interchangeable without the risk of leakage.

What is the axis distance between the supply and return pipes?

The axis distance between the centers of both collector pipes is 100 mm. The overall height of the block, including mounted air vents, actuators, and flow meters, usually reaches 250–300 mm, so the cabinet must have an internal height of at least 350 mm.

How do I calculate the total length of the distributor, which I need to know for selecting a cabinet?

Approximate calculation: number of circuits × 50 mm (pitch) + approx. 100 mm for front ends and connections. For example, for a 4-way distributor: 4 × 50 = 200 mm + 100 mm = approx. 300 mm. Add at least 90 mm of reserve (45 mm on each side) for work with a wrench and pipe routing, which gives a minimum internal cabinet width of 390 mm – in practice, choose a cabinet with an internal width of 400 mm or more.

Can I connect a 20 mm pipe to the distributor via a reducer?

Yes, technically it is possible using a stainless steel or brass reducer G 3/4" → G 1" or the appropriate hose fitting for a 20 mm pipe with a G 3/4" thread. However, you must consider that a larger pipe diameter means a higher flow, so it is necessary to verify the hydraulic balance of the circuits by calculation. More about hydraulic balancing can be found in the article Hydraulic Balancing of Circuits via a Stainless Steel Distributor – Why and How to Do It.

Is a stainless steel distributor with a higher number of circuits suitable even if some circuits are not used?

Yes, unused branches are simply sealed with blind plugs (caps) with an internal thread of G 3/4" – these are commonly available in accessories. A sealed branch does not affect the function of the active circuits in any way. From a long-term perspective, it is therefore more advantageous to buy a distributor with a reserve, for example, a 6-way instead of a 4-way, if there is a chance of future expansion of the underfloor heating.

Conclusion – dimensions are the basis of the correct selection

The dimensions and thread pitch of the stainless steel manifolds from Hepworth are not just technical curiosities for meticulous designers. These are practical values that determine whether the installation will proceed without problems, whether the manifold will fit into the cabinet, whether the accessories will fit, and whether the system will be easy to maintain throughout its lifetime. Key numbers to remember: branch spacing 50 mm, pipe thread G 3/4", main supply thread G 1", pipe axis distance 100 mm.

If you are unsure about the selection – in terms of dimensions, number of circuits or cabinet dimensions – read other articles in our Knowledge Centre, for example, How to choose a stainless steel manifold for floor heating – number of circuits, diameter and flow rate or How many circuits does a manifold need – how to correctly determine the number of floor heating circuits. A correct decision at the beginning will save you a lot of trouble and unnecessary costs during installation and in the future.

Do you have a question about this topic?

Having trouble making a decision or dealing with a specific situation in your home? Write to us – we are happy to help.