Open vs. closed splitter – what is the difference and when to use which

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Open vs. closed manifold – what is the difference and when to use which

When a customer first looks at the range of plastic manifolds, they will encounter one distinction that seems simple at first but in practice causes more confusion than you might expect: open vs. closed manifold. At first glance, both types look almost the same – the same plastic body, the same inlet and outlet diameters, the same number of circuits. However, the difference is fundamental and determines whether your system will operate reliably for decades or whether you will encounter flow problems, air in the pipes, and leaks from the very first season.

This article will explain what this difference means technically, where it arises, why it matters, and most importantly – in which specific situations to choose an open and in which a closed type. We will also look at how it works in practice in typical installations for single-family homes, apartment cores, floor heating, and water supply systems.

What is a manifold and what is it used for

A manifold is a hydraulic component that divides one incoming flow of fluid (water, heating water, or another medium) into several separate circuits and at the same time collects the return flows back into one collector. Without a manifold, you would have to connect each circuit directly to the source, which with a larger number of circuits leads to disproportionate and continuous branching of the piping, poor controllability, and difficult maintenance.

Hepworth plastic manifolds are primarily intended for systems operating with potable water (cold and hot water distribution in apartments and houses) or with low-temperature heating (floor heating, ceiling heating, cooling). They operate within temperature and pressure ranges typical for residential installations – usually up to 10 bar and up to 90 °C, depending on the specific model.

For more detailed information on exactly what a manifold is suitable for and when the difference between a heating and a water supply variant is important, see the article Water manifold vs. heating manifold – can I use the same one? in the Knowledge Centre.

Open manifold – what does it mean technically

The term "open" (sometimes referred to as "open ended" or "open flow" in English terminology) refers to a manifold whose body is open at one or both ends, without closures. This means that the medium can freely flow through the entire body of the manifold from one end to the other – the system is therefore through-flow.

Practical consequence: an open manifold can be connected to the inlet and outlet within one loop (e.g., the supply pipe enters on one side and exits on the other). This makes it possible to chain (connect in series) multiple manifolds together – for example, when you need to divide more circuits than one unit has, you can connect two open manifolds in series and double the number of outputs without having to connect an additional T-piece or bypass to the piping.

Another use case for an open manifold is a situation where the manifold itself is part of a larger hydraulic loop and must not block the end of the body to avoid creating a dead-end branch. In such connections, a closed end would cause circulation and water stagnation problems.

An example from our range is the 2-circuit open manifold 22×15 – it is a two-output unit with a 22 mm connection diameter on the inlet/outlet and 15 mm on the individual circuits. The open design means that both ends are fully through-flow and ready for further connection or chaining.

Closed manifold – what does it mean technically

A closed manifold has a single body, one or both ends of which are blind (sealed with a cap or built-in closure). The medium therefore flows only through the main inlet and is distributed to the outlet circuits – it does not flow further through the body of the manifold. It is a network node, not a through-flow pipe.

A closed manifold is designed as a terminal element – this means that it is at the end of a branch. You have one inlet, you have N outlets, and the other end of the manifold body is blind. Water cannot "flow through" the end of the body further into the system.

This is also the most commonly used type in typical residential installations, where one manifold serves all the circuits of a given room or zone and it is not necessary to connect it to another.

Good examples from our range include:

2-circuit closed manifold 22×15 – suitable for small distribution units, e.g., for two sinks or a small bathroom
3-circuit closed manifold 22×15 – typical for medium-sized bathrooms or kitchens with three circuits

When to use an open distributor – specific scenarios from practice

Over the years of working with customers, I have noticed that people often avoid using an open distributor simply because they are not familiar with it. However, in several situations, it is simply irreplaceable.

1. Chaining – when you have more circuits than one distributor can handle

Typical example: a developer is equipping an apartment building and wants to have a distribution for 5 circuits in each apartment, but in their stock system they only have 2- and 3-circuit distributors. Solution? One 2-circuit open + one 3-circuit closed (or vice versa). The open distributor is placed first in the series – its flow-through end is connected to the supply of the second piece. The result is 5 independent circuits served from one connection, without the need to buy a more expensive 5-circuit piece or cut extra pipe.

Important: when chaining, always make sure that the last piece in the series is closed – that is, you should not have an open end left without a closure. Blind ends of open pieces are solved either with a closed plug (if the manufacturer provides it) or simply with the last closed piece in the chain.

2. Installation in distribution boxes with double-sided input

In apartment buildings and professional installations, it is sometimes required that the distributor has input and output from both sides – for example, in a circular or branched distribution. An open piece allows this, a closed one does not.

3. Modularity – a system that will be expanded

If you are not sure how many circuits you will add in a year, it makes sense to start with an open distributor and leave the flow-through end accessible. Later, you can "plug in" another piece. You will appreciate this during renovations, where the project changes on the fly.

4. Hydraulic bypass

In some more specific connection schemes (e.g., in a mixed circuit with a control valve), water must be able to flow through the distributor body even when all output circuits are closed. An open version allows this without the need for additional bypass piping.

When to use a closed distributor – typical situations

A closed distributor is the right choice in the vast majority of standard home installations. Here are specific situations where I recommend it.

1. A separate distributor for one bathroom or kitchen

This is by far the most common case. You have one supply circuit from the main distribution, which you want to divide into two or three sub-circuits (e.g., sink, shower, WC). A closed distributor is perfectly suitable – one input, N outputs, a blind end. No unnecessary complications.

2. Floor heating in one room

If you have one room with two floor heating circuits (e.g., a larger living room divided into two temperature zones), a closed 2-circuit piece is ideal. Example: closed 2-circuit 22×15 can handle such a situation without the need for chaining.

3. End node in the hierarchy of distributions

In practice, a hierarchical arrangement is often used – from the main distributor, branches lead to secondary distributors for individual zones or rooms. Secondary distributors are always closed, as they are end nodes – the supply comes to them and is distributed, nothing continues beyond them.

4. Simple distributions in small apartments

In small layouts (studio apartments, one-room apartments, cottages, garden houses), one closed 2- or 3-circuit distributor is sufficient for the entire apartment. There is no need for chaining or more complex hydraulics. Here, a closed piece is the cheapest and simplest solution.

Differences in details – valves, dimensions and types of connection

In addition to the basic division into open/closed, plastic distributors also differ in other parameters that are equally important when choosing.

Valves directly in the distributor

Some models have a built-in valve (shut-off valve) on each output circuit, which allows you to close or limit the flow in a given branch without turning off the entire system. An example is 4-circuit water distributor with valve 15×15×15×15 – each of its four circuits has its own shut-off valve. This is a huge advantage for maintenance or in case of a breakdown: you do not have to turn off the entire distribution, only close the specific circuit.

Models without valves are cheaper, but require you to have shut-off devices located elsewhere in the system – either at the inlet or directly at the appliances.

Connection Diameter

The most common combinations in the Hepworth range are:

22×15 mm – main supply 22 mm, output circuits 15 mm. This is the standard for typical apartment cold and hot water distribution.
15×15 mm – all diameters the same, suitable especially for even distribution at lower flow rates or for underfloor heating with lower flow requirements.

If you are unsure which diameter you need, I recommend reading the article What diameter of manifold do I need: 15 mm, 22 mm or larger? in the Knowledge Centre – it is discussed in detail there with flow calculations.

Mounting bracket and fixing

Regardless of whether you choose an open or closed unit, always consider the method of fixing. Mounting bracket for 3/4" manifolds is compatible with manifolds with a 3/4" connection and allows for secure and aesthetic wall or cabinet mounting. A freely placed manifold shifts during handling of output hoses and pipes – and this is not only an aesthetic issue, but also a source of stress in the connections.

Common mistakes in selection and installation

Over the years, I encounter recurring mistakes in practice. Here are the most common ones that cost customers time and money:

Closed manifold where an open one is needed – the customer buys a closed one and then finds out they cannot connect another unit in the series. They solve it with T-pieces and reducers, and the system looks like spaghetti.
Open manifold without a cap at the end – they leave the end of an open manifold open without a cap. Result: a leak immediately after pressurization.
Wrong inlet diameter – they buy a 22×15 unit where they have a 15 mm inlet and solve it with a reducer. It is better to buy the correct manifold with a 15 mm inlet right away.
Mounting bracket omitted – the manifold is hung only on the output pipe loops. During the first opening or closing of a circuit, the connections are stressed and after a while they leak.
Confusing heating and plumbing manifolds – not all plastic manifolds are certified for potable water. This is a critical mistake especially in apartment buildings. More in the article Water manifold vs. heating manifold – can I use the same one?

Practical example from a real customer order: renovation of an apartment core

I illustrate this with a real type of order we regularly handle. An older panel apartment, renovation of the apartment core. The customer wants new piping for:

Bathroom: shower, sink, bathtub, washing machine – 4 circuits (cold and hot water)
WC: flush toilet – 1 circuit (cold water)
Kitchen: sink, dishwasher – 2 circuits (cold and hot water)

Overall, they need 7 circuits for cold water and 6 circuits for hot water. A solution using one large manifold would only be possible with the use of a larger metal unit – plastic manifolds are available in smaller sizes. The customer therefore gets:

For cold water: 1× open 4-circuit + 1× closed 3-circuit in series → 7 cold water circuits
For hot water: 1× open 3-circuit + 1× closed 3-circuit → 6 hot water circuits

Both series are mounted on brackets firmly in the distribution cabinet, the cold and hot water supplies are clearly distinguished by the color of the insulation braid. Each branch has a valve directly in the manifold or on the supply to the cabinet. Result: clean installation, each circuit individually closable, no extra T-pieces.

Hydraulic consequences of choosing the wrong type

If someone mistakenly bought a closed manifold where an open one was needed in the previous example and tried to compensate with a T-piece behind the manifold body, the following problems would arise:

Increased pressure loss – each T-piece adds resistance. With 4–6 T-pieces instead of one series, you can lose 0.2–0.4 bar of flow pressure, which at lower pressure in the network (common in older panel buildings) will cause weak water flow at the last appliance.
Harder air venting – air pockets form more easily in disorganized T-branches. With a compact series manifold, venting is straightforward.
Complicated maintenance – if something leaks after five years, you are looking for a leak among dozens of connections instead of just a few clearly arranged connections on the manifold.

These issues are discussed in detail in the article Common problems with plastic distributors: leaks, air in the circuits and weak flow.

Temperature, pressure and material limitations

Hepworth plastic distributors are made of polypropylene or polybutene materials with varying resistance to temperature and pressure. Here are the values you need to know:

Parameter	Standard heating circuits/Hot water	Underfloor heating
Max. operating temperature	60 °C (hot water)	45–55 °C (low temperature)
Max. pressure	6–10 bar	6 bar (typically)
Suitable medium	drinking water (certification required)	heating water, antifreeze mixture
UV resistance	Low – for indoor use	Low – for indoor use

Temperature has a direct impact on the physical properties of plastic: prolonged operation at the limit of the permissible temperature reduces the strength and rigidity of the material. Therefore, always check whether the specific model you are purchasing is dimensioned for the intended application – some cheap copies on the market have lower temperature limits than declared.

Open vs. closed in the context of installation

From an installation point of view, it is important to know that an open distributor has the same type of thread or socket on the through-flow end as on the inlet. This means that if you have a 22 mm socket on the inlet, you will need the same type of fitting to connect two open pieces. This connection is made either by a push-fit connection, welding or a compression ring – depending on the system used in the project.

A closed distributor has a permanently sealed body (molded during production) or a removable plug on the blind end. You will appreciate the removable plug when you want to expand the system later – it is enough to remove the plug and connect another piece.

A detailed installation procedure including steps for push-fit connections and pressure testing can be found in the article Installation of a plastic distributor Hepworth step by step and also in Mounting bracket for distributors: how to properly mount the distributor on the wall or into the distribution cabinet.

Number of circuits vs. type of distributor – which is more important?

Sometimes customers think about the open/closed type before considering the number of circuits. In reality, it should be the other way around. The correct selection process is:

Determine how many circuits you need in total.
Determine the inlet and outlet diameters you need.
Decide whether one piece is sufficient or if you need a chain – this is where you decide on the open/closed type.
Verify whether you need valves directly in the distributor.
Solve the mounting, bracket and fixing.

More about steps 1 and 2 can be found in the articles 2-circuit vs. 3-circuit vs. 4-circuit distributor – which one matches my system and How to choose a plastic distributor: number of circuits, diameter and type of connection.

Most frequently asked questions (FAQ)

Can I use an open distributor separately without chaining?

Yes, you can. An open distributor works just as well as a single piece – the through-flow end is simply sealed with a plug or closure. In this mode, it behaves exactly like a closed one. The only disadvantage is that you pay extra for a feature you do not use. If you know you will not be chaining, a closed piece will be cheaper and simpler.

How can I tell which end is closed and which is through-flow?

The closed end has either a blind, molded surface (without a hole) or a visible plug. The through-flow end has the same type of socket as the inlet – that is, a hole ready for a fitting. If you are unsure, check the technical documentation of the model, where this is always clearly indicated.

What happens if I leave the through-flow end of an open distributor open without a termination?

During pressure testing, the entire volume of water or medium will immediately flow out of the open end. This is the most common mistake when installing open pieces. Always terminate the through-flow end with a plug, closure or another piece in the chain before pressure testing the system.

Can I chain a closed and an open distributor in the opposite order – that is, with the closed one first?

No, that does not make sense. A closed piece has a blind end – it is not possible to lead a connection further into another distributor from it. The correct order when chaining is always: open (and another open if needed) → last closed or terminated with a plug. The inlet of the supply leads into the first open piece.

Are open and closed distributors equally resistant to pressure?

Yes, with the same material and dimensions, the pressure resistance is identical. Closing one end in no way affects the ability of the body to withstand internal pressure – in fact, the blind end can be slightly stronger in some cases because it does not have a weakened socket. The decisive factor is always the technical specification of the specific model.

Is there a significant price difference between open and closed distributors?

Not really. The price difference between an open and a closed variant of the same size and number of circuits is minimal in most cases – in the order of a few percent. Always decide based on the technical requirements of the project, not on price. Saving a few euros on a distributor and then dealing with a leak or weak flow is an unprofitable calculation.

Conclusion: a rule of thumb for selection

If you are to take away one thing from this article, let it be this simple rule: closed distributors for individual units, open ones for chaining or two-way flow. Most home installations can well manage with closed pieces. The open type is more suitable for experienced installers who deliberately decide that the system will be modular or through-flow.

And remember – the most important thing is to decide before you buy anything. Returning a distributor of the wrong type after you have already drilled and connected it to the system is difficult. If you are unsure, consult before ordering – a small detail in the type selection can save hours of work and unnecessary costs when reconfiguring the entire distribution unit.

Do you have a question on this topic?

Can't decide or are you dealing with a specific situation in your home? Write to us – we are happy to help.