How to choose a plastic manifold: number of circuits, diameter and type of connection
How to choose a plastic manifold: number of circuits, diameter and type of connection
A plastic manifold may seem like an unassuming component at first glance, easily underestimated. In practice, however, it determines whether your plumbing or heating installation operates reliably, quietly and for many years without problems – or whether you will be dealing with weak flow in one circuit, leaking connections and air blockages every season. Choosing the right manifold depends on three main parameters: the number of circuits, the diameter of the connection and the type of design (open vs. closed). Each of these parameters has technical reasons and specific consequences for the entire system.
In this article, we will go through all three decision-making criteria in depth, supplement them with practical examples from everyday customer practice and help you choose the right product right from the start – without unnecessary mistakes and costly modifications.
What is a plastic manifold and where is it used
A manifold (in professional practice also called a distributor, manifold or collector unit) is a hydraulic component that divides one incoming flow of water or heated liquid into several separate circuits. Each circuit can supply a different appliance, room or branch of the network. The same principle applies to return collectors – they collect water from all circuits back into one pipe.
Hepworth plastic manifolds are mainly intended for:
- supply of cold and hot utility water (domestic distribution, apartments, houses)
- distribution boxes with multiple branches
- low-temperature heating systems (floor heating, where the medium temperature does not exceed approx. 60–70 °C)
- garden and technological distribution systems
Unlike brass or stainless steel manifolds for high-temperature heating, plastic versions are more cost-effective, easier to install and resistant to corrosive media. Their disadvantage is the lower temperature and pressure limit – that is why it is important to know where exactly you can and cannot use them. The topic Water manifold vs. heating manifold – can I use the same one? is covered in a separate article in our Knowledge Centre.
Parameter No. 1: Number of circuits – how to determine it correctly
The number of circuits is the first thing you need to know before you look at any catalog. Each appliance or branch of the network that you want to regulate or control separately represents one circuit. A mistake in choosing the number of circuits is the most common reason for returning goods or unnecessarily buying an additional piece.
How to count circuits in practice
Let’s take a typical family house with three bathrooms, a kitchen, a garden tap and a boiler room. If you want to supply each bathroom separately (for pressure regulation or individual closure during repairs), you need 3 circuits just for the bathrooms. The kitchen adds another one, the garden another. Result: at least a 5-circuit manifold for cold water. If you include hot water with the same branches, you have another 5-circuit collector for hot water.
With floor heating, the logic is the same: each zone (room, hallway, bathroom) forms one circuit. Four zones = 4-circuit manifold. Five zones = 5-circuit. There is no universal number – it depends entirely on your floor plan and requirements for zonal regulation.
Golden rule from practice: Always consider one extra circuit as a reserve. Installing an additional branch after the distribution is completed is many times more expensive than buying a manifold with one more circuit. An unused circuit can simply be closed with a valve.
Two-circuit manifold – when it is sufficient
A two-circuit design is suitable for simple situations: one circuit for device A, another for device B. Typical use: a garden pit with one tap for the garden and one for the car, a small apartment with a central distribution for the bathroom and kitchen, or a small workshop with two branches. In the Hepworth range, you will find for example closed 2-circuit manifold 22x15 or open 2-circuit 22x15 – each for a different type of connection, which we will talk about more in the next section.
Three-circuit manifold – the golden middle for smaller systems
A three-circuit design covers most common applications in smaller apartments or studios: supply to the bathroom, supply to the kitchen and a third circuit for the WC or garden outlet. It works just as well in a small floor heating system with three temperature zones. closed 3-circuit manifold 22x15 is a typical example of a compact solution for such situations.
Four-circuit manifold – for more extensive distributions
Four circuits are standard for a medium-sized family house or a larger apartment. For plumbing: bathroom, kitchen, WC/guest WC, garden. For heating: living room + dining room, bedroom, children’s room, bathroom. 4-circuit manifold with valve 15×15×15×15 is interesting because it has a valve integrated on each output – each circuit can be individually regulated without the need to buy additional fittings.
Parameter No. 2: Connection diameter – 15 mm, 22 mm or other
Diameter is the second key parameter and it is precisely here that the most errors occur when placing orders. Plastic Hepworth manifolds are available in two basic configurations of cross-sections: with 15 mm and 22 mm connections (or their combination). What do these numbers mean in practice?
Hepworth system and its dimensions
Hepworth is a British piping system with metric dimensions. The number refers to the outer diameter of the pipe – not the bore (inner diameter). This is important: 15 mm outer diameter corresponds to a bore of approx. 12 mm, 22 mm outer diameter corresponds to a bore of approx. 18.6 mm. On the manifold, the number indicates the diameter of the corresponding neck into which the pipe is inserted (in the push-fit system) or onto which the fitting is screwed.
When to use 15 mm
15 mm necks are standard for individual circuits (outputs/branches). Most domestic appliances, taps and heating valves are dimensioned precisely for this diameter. In practice, this means: if pipes lead from the manifold to individual taps, valves or floor circuits, these outputs will be 15 mm.
When to use 22 mm
Diameter 22 mm typically appears on the main inlet/outlet of the manifold – that is, where the main water or fluid supply comes in before being divided into circuits. The logic is simple: the main pipe must have a larger cross-section in order to be able to supply all circuits simultaneously without hydraulic pressure loss. The combination 22×15 (inlet 22 mm, circuit outputs 15 mm) is therefore very common in 2- and 3-circuit manifolds.
If you are unsure which diameter you need, read our article Which manifold diameter do you need: 15 mm, 22 mm or larger? – there you will find more detailed flow capacity calculations.
Combination of dimensions on one manifold
For example, the closed 2-circuit manifold 22x15 has an inlet of 22 mm and outputs of 15 mm. This is a typical solution when the main supply is from a 22 mm pipe (larger capacity, smaller pressure losses over a long section) and each branch circuit runs through a 15 mm pipe. Similarly, the closed 3-circuit manifold 22x15 has the same principle, only with three outputs.
On the other hand, the 4-circuit manifold 15×15×15×15 has all the necks the same – 15 mm. This is suitable when the entire system is dimensioned for 15 mm pipe including the main supply (for example, in a compact apartment distribution where the inlet and outlets run from similarly dimensioned pipe).
Parameter No. 3: Connection type – push-fit, threaded or other
The Hepworth system is primarily based on push-fit (push-on) connections. The pipe is simply pushed into the fitting or manifold neck, where it is held by an internal serrated ring (grab ring) and sealed with an O-ring. The connection is instant and does not require soldering or special tools.
Push-fit connection – advantages and limitations
Push-fit is popular for its fast installation and the possibility of disassembly without damaging the material. In practice, this means: if you need to replace the manifold or add a circuit, you unlock the collar (release collar) and pull the pipe out without cutting. The disadvantage is that the pipe must be precisely round, without cuts and cleanly cut – deformed ends or burrs will cause leaks. Always use a pipe insert when working with PE/PEX pipes to prevent flattening of the inside.
Threaded connection
Some manifolds (or their adapters) have threaded necks – BSP thread (British standard, imperial). Note: when the manufacturer specifies, for example, a 3/4" thread, it refers to the nominal thread size, not the actual diameter in millimeters. 3/4" BSP has an outer diameter of approx. 26.4 mm. This is why metric and imperial necks should not be mixed directly – always use a transition fitting or reducer.
Combination of push-fit and thread
In practice, we often encounter manifolds where one end (main inlet) is threaded (for connection to old steel or brass systems) and the circuit outputs are push-fit (for plastic Hepworth pipes). This hybrid connection is very common in renovations, where an old metal distribution is combined with a new plastic one.
Open vs. closed type – a fundamental difference
Along with the number of circuits and diameter, there is another important property that many customers overlook: whether the manifold is open or closed. It refers to whether the main body is through-going (open) or blind-ended (closed) on one end.
Closed manifold has one end sealed with a plug or an integrated blind part. It is used where the manifold is an end component – the supply comes from one side and the water is divided only into side circuits. For example, closed 2-circuit 22x15 or closed 3-circuit 22x15 belong to this category.
Open manifold is through-going – it has necks on both ends of the main body. This allows multiple manifolds to be connected in series, or to leave the second end free for possible future expansion (to be sealed with a plug). open 2-circuit 22x15 is a typical example – you can connect it with another 2-circuit manifold to create, for example, a 4-circuit assembly without the need to buy a special 4-circuit unit.
You can find a detailed analysis of this difference in the article Open vs. closed manifold – what is the difference and when to use which in our Knowledge Center.
Mounting and installation of the manifold
Correct mounting of the manifold is just as important as its correct selection. A plastic manifold with pipe connections can weigh several kilograms after the system is filled. If it is not securely mounted, mechanical stress occurs in the connections – and this is the most common cause of leaks, not a sealing defect.
For Hepworth manifolds, special manifold carriers 3/4" are available, which allow secure mounting on the wall or into a distribution cabinet. The carrier functions as a bracket into which the manifold is inserted and locked – no direct screwing into the body of the plastic manifold, no risk of cracking brittle material. The correct installation of the carrier and placement of the manifold into the cabinet are described in detail in the article Manifold carrier: how to correctly mount the manifold on the wall or into a distribution cabinet.
The following basic rules apply during installation:
- The manifold must be accessible – valves and shut-off valves must not be embedded or covered with non-removable cladding.
- Always install a main shut-off valve (ball valve) before the manifold – during repair or replacement, you do not need to shut off the entire distribution system.
- Observe the minimum spacing between circuits – with push-fit systems, space is needed for pressing and possibly loosening the pipe.
- After installation, leave the system under pressure for at least 2 hours and visually and manually check each connection.
Manifolds with valves – when it pays off
Some manifolds have an integrated regulating valve on each circuit. This is an exceptionally practical feature – without it, you would have to install a separate valve or flow regulator for each circuit. In floor heating systems, flow regulation for each zone is almost essential: a zone with a smaller area must have a lower flow than a large room, otherwise the system becomes thermally unbalanced.
For example, the 4-circuit manifold with valve 15×15×15×15 has a valve on each of the four outputs, making it ideal for systems where you need hydraulic balancing of individual circuits. Without this regulation, water (or heating fluid) would always prefer the shorter or least resistant circuit – and the others would be supplied insufficiently.
Pressure and temperature limits of plastic manifolds
Hepworth plastic manifolds are certified for use with water and heating media up to certain combinations of pressure and temperature. Typical values for polypropylene (PP) or polybutene (PB) materials are:
- Maximum medium temperature: 70–80 °C at lower pressure (0.5–1 bar)
- Maximum temperature: 60 °C at normal operating pressure of 3–4 bar
- Maximum pressure at 20 °C: 10–16 bar (depends on the specific product)
- For cold water (20 °C): long-term operation up to maximum working pressure
Important note: these values apply to stationary operation. Hydraulic shocks (water hammer) can generate several times higher pressure for a short period. Therefore, it is always recommended to install an expansion tank and anti-hammer devices with plastic manifolds if the system is prone to shock operation (fast closing of valves, switches, etc.).
For high-temperature heating systems (boilers with temperatures of 80–90 °C and higher), plastic manifolds are not suitable – brass or stainless steel versions are required. More on this topic can be found in the article Water manifold vs. heating manifold – can I use the same one?
Most common mistakes in selection and their consequences
Over the years of customer experience, the same mistakes keep repeating. Here are the most common ones:
- Wrong number of circuits: The customer buys a 2-circuit manifold and after installation realizes they need three branches. Result: purchase of an additional unit, rewiring, time loss. Solution: always draw up a plan before going to the store.
- Confusing external and internal diameter: The customer measured the pipe from the inside and got 12 mm, ordered a 12 mm manifold – but Hepworth 15 mm means an external diameter of 15 mm. The pipe does not fit. Solution: always measure the external diameter with a caliper.
- Open where a closed one is needed: The customer installs an open manifold as an end element, forgets to blind off the other end, and ends up with a leaking plug. Solution: think through the network topology before selection.
- Missing carrier: The manifold is only attached with connected pipes. After years of operation, the connections fatigue and start to leak. Solution: always install a manifold carrier.
- Inadequate temperature control: A plastic manifold connected to a high-temperature boiler circuit. The material softens, the connections start to leak. Solution: always verify the maximum temperatures of the specific product.
Practical scenarios from real customer cases
Scenario 1: Apartment renovation, 3 rooms + bathroom
The customer renovated a 65 m² apartment. The new water supply system was to supply the bathroom, kitchen, and a separate toilet. The main supply from the riser was in 22 mm pipe. Solution: one 3-circuit closed manifold 22×15 for cold water and another for hot water from the water heater. On each of the three circuits, the apartment owner could independently shut off the supply without interrupting the supply to other rooms. The entire installation took 4 hours, including mounting the carriers.
Scenario 2: Floor heating, new house construction
A family house of 120 m² had 4 separate floor heating zones: living room, kitchen, bedroom, and bathroom. The designer proposed a 4-loop distributor with valves on the supply collector and a symmetrical 4-loop on the return. The valves enabled hydraulic balancing – the living room with the largest area had the valve fully open, while the bathroom with the smaller area had the valve partially throttled. Result: even temperature in all zones, no overheating or undercooling.
Scenario 3: Garden installation with two loops
The owner had a garden manhole with a 22 mm supply. He needed one loop for automatic irrigation (with a timer) and a second for a manual tap near the garage. Solution: an open 2-loop distributor 22×15 – open so that in the future he could connect a third loop (planned pool) without replacing the whole distributor. For now, he sealed the second end with a plug.
Decision table for quick selection
| Situation | Recommended type | Diameter |
|---|---|---|
| Small apartment, 2 water branches | 2-loop closed | 22×15 |
| 3-room apartment + bathroom | 3-loop closed | 22×15 |
| Smaller house, 4 heating zones | 4-loop with valve | 15×15×15×15 |
| Garden manhole + future expansion | 2-loop open | 22×15 |
| Chaining multiple distributors | Open type (in series) | According to the system |
| Hydraulic balancing of floor heating loops | 4-loop with valve | 15×15×15×15 |
Compatibility with other system components
Plastic distributors Hepworth are optimally combined with pipes, fittings, and valves of the same system. The reason is simple: the push-fit connection is calibrated for a specific pipe tolerance range. Pipes from other manufacturers may have a slightly different outer diameter or ovality, which can lead to leakage or a too loose fit.
If you connect Hepworth with another system (e.g., PPR, PE-X, or older flexible PE), always use certified transition fittings with the appropriate connection standard. Improvised solutions (e.g., wrapping with Teflon, forcing a wrong diameter) are common and always lead to the same result: a leak at the worst possible place and at the worst possible time.
Details about typical installation errors and how to avoid them can be found in the article Common problems with plastic distributors: leaks, air in the loops, and weak flow.
Air in the system – how to avoid it when selecting a distributor
Air locks in closed systems are a chronic problem. The solution is the correct installation position of the distributor (always horizontal, with a bleed valve at the highest point of the system) and the correct filling and bleeding procedure. Some distributor kits have integrated bleed valves on each loop – this is an advantage, but also a higher price.
When selecting a distributor, therefore, also consider where you will install it and what the pipe profile is: systems with a large vertical difference between the distributor and the consumers are more prone to air locks and require a more careful procedure during initial filling. The whole issue of bleeding is discussed in more detail in the article Maintenance and inspection of plastic distributors – what and how often to check.
Most frequently asked questions (FAQ)
Can I connect pipes to different sides of the same distributor – i.e., some from above and some from the side?
Yes, Hepworth plastic distributors are designed so that their side outlets are fixed in orientation, but in many cases the distributor can be mounted in different positions (e.g., outlets facing up or to the side) using a bracket. Always check whether the given position is compatible with bleeding – if the loops are oriented in such a way that air cannot naturally escape, a manual bleed valve is required.
Can I connect two 2-loop open distributors to get four loops?
Yes, that is exactly what open types are for. Two 2-loop open distributors can be connected via their free ends (push-fit or threaded connection). The result is a functional 4-loop collector. The advantage is flexibility – you can expand the system gradually. The disadvantage is slightly higher hydraulic losses at the connection point and a larger footprint. If you know from the beginning that you need four loops, a more direct solution is to buy a 4-loop unit right away.
What is the difference between a distributor with a valve and one without – is it just about convenience, or also about hydraulics?
It is mainly about hydraulics. In a system without valves, the fluid naturally flows into the loop with the lowest resistance – typically the shortest or unrestricted loop. Other loops may be supplied insufficiently. A valve on each loop allows throttling to balance the pressure loss (hydraulic balancing), so that each loop receives the correct amount of flow regardless of length. This is a necessity for floor heating, and an advantage, not a necessity, for simple water installations.
Will a plastic distributor last for decades in the wall without inspection?
If installed correctly, within the permitted temperature and pressure range, and with good mounting, yes – plastic materials such as polybutylene or PP do not have a tendency to corrode. The critical points are the connections, not the material itself. Therefore, distributors should always be accessible (via a maintenance hatch or cabinet) and visually inspected at least once a year. More about proper periodic inspection can be found in the article Maintenance and inspection of plastic distributors – what and how often to check.
How can I determine the outer diameter of my existing pipe if it is embedded in the wall?
Measure the visible parts (e.g., near valves, taps, or fixtures) using a caliper or a diameter gauge. If there is no free section, you can identify the pipe by the table of nominal diameters: in apartment installations, the most common are 15 mm (to fixtures), 22 mm (main branches to distributors), and 28 mm or larger (risers and main house lines). If you are still unsure, consult an installer who has experience with that system.
Can I use a Hepworth plastic distributor for a pressure test with air?
No – this is a safety risk. Plastics are dangerous during air pressure tests because compressed air stores a large amount of energy. In the case of a connection failure, the fitting can be violently ejected. Always perform pressure tests with water (hydropneumatic test), where the safety risk in case of failure is minimal. The correct pressure for testing Hepworth plastic systems is 1.5 times the maximum operating pressure, at least 6 bar, for 30 minutes.
Conclusion: three parameters, one correct choice
Selecting a plastic manifold is a logical and systematic process if you proceed correctly: first, count the circuits (each independent branch is one circuit, add one extra as a backup), then determine the diameters based on the outer dimensions of your pipes (15 mm, 22 mm or a combination of both), and finally decide on the type of execution – closed for a terminal unit, open for daisy-chaining or future expansion. If you need hydraulic balancing, choose the version with integrated valves.
Hepworth plastic manifolds are a well-designed, tested system with decades of history in British and European plumbing practice. Their advantage is the simplicity of push-fit installation, chemical resistance and long service life when used correctly. Observe the parametric limits, do not forget the carrier and always keep the system accessible for inspection – and your distribution system will serve you without problems for many decades.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.