What distributor diameter do I need: 15 mm, 22 mm or larger?
What diameter of manifold do I need: 15 mm, 22 mm or larger?
One of the most common questions we encounter when selling plastic manifolds is: "What diameter should I actually choose?" This seemingly simple matter – after all, it's just two numbers on a label – in practice determines whether your system will work reliably for years, or whether you will constantly be dealing with weak flow, leakage and uneven temperatures in individual circuits. In this article, we will look at the topic of manifold diameters in depth: what these numbers mean, how the applications for 15 mm and 22 mm differ, when to go for a larger diameter, and how to make the right decision for a specific situation in your home or on a construction site.
What does the "diameter" mean in a plastic manifold?
When we talk about the diameter of a manifold, we mean the diameter of the connecting nozzles – that is, the size of the pipe or fitting that connects to the manifold. In the Hepworth plastic manifold catalogs, you commonly encounter two basic values: 15 mm and 22 mm. These dimensions correspond to metric pipe sizes in the PEX, MLCP (multilayer pipes) or copper pipe systems according to the British standard, which has long been established here.
It is important to distinguish three different dimensions that can appear on a single manifold at the same time:
- Collector diameter (central body): This is the internal diameter of the main pipe of the manifold, through which water flows for all circuits together. It is usually larger than the diameter of the branches.
- Inlet/Outlet diameter: The size of the connection to the source circuit – that is, to the boiler, to the storage tank or to the main distribution.
- Outlet diameter to circuits: The size of the nozzles for individual branches (e.g. floor circuits, radiators, sanitary circuits).
For example, the closed 2-circuit manifold 22X15 has an inlet/outlet of 22 mm and outputs to circuits of 15 mm. This is a very typical solution for medium-sized home heating systems.
Why diameter matters: hydraulics in practice
Behind each diameter stands the physics of flow. The larger the internal diameter of the pipe or collector, the lower the hydraulic resistance at the same flow rate. This has direct consequences:
- Smaller diameter = higher resistance = higher pump losses. When the inlet is too narrow, the pump has to work against higher pressure, which increases energy consumption and shortens the pump's lifespan.
- Smaller diameter on the collector = uneven flow distribution. If the collector is too narrow in relation to the number of circuits, the first circuits take a disproportionately large share of the flow and the last ones are "hungry". In practice, this looks like the first radiator burning and the last one being barely warm.
- Larger diameter = larger water volume in the system. This is not always an advantage – a larger volume means slower system response to temperature demands and can disrupt the condensation mode in condensing boilers.
Choosing the right diameter is therefore not about "the bigger, the better", but about finding the optimal point for a specific system.
When is 15 mm enough for everything?
A manifold with 15 mm connections on all nozzles – inlet, outlet and outputs – is a solution that makes sense in really compact, simple systems. Typical examples from practice:
Small apartments and studios
In an apartment with an area of up to 40–50 m², where you have 2–3 radiators and perhaps one towel dryer, 15 mm on the inlet is usually sufficient. The water volume is small, the pumps are dimensioned for low flow and the overall hydraulic resistance of the system is manageable. Here we encounter, for example, a configuration where a 4-circuit manifold with valve 15 x 15 x 15 x 15 is used, i.e. the same diameter on all nozzles including the inlet. This variant makes sense precisely when the inlet from the boiler is implemented with a 15 mm pipe and the flow in each circuit is low.
Hot and cold water distribution in an apartment
In sanitary distribution, 15 mm is very common and at the same time sufficient for most apartment applications. If you are distributing water to a sink, a shower, a WC and a kitchen, the flows are short-term and not simultaneous – it is not realistic that all taps will be open at full capacity at the same time. Therefore, 15 mm on the outputs to the circuits will cover the real needs.
Single and two-circuit systems with low flows
If you have only two smaller heating circuits – for example, two panel radiators in a smaller space – the entire manifold in a 15 mm version can be functional. In practice, it is, however, more advantageous to have an inlet in 22 mm, because this ensures a reserve in case of system expansion.
When is a 22 mm diameter necessary?
A 22 mm diameter on the supply and return of a manifold is much more common in practice and is the correct choice for most single-family homes and larger apartments. Here are specific situations where you cannot do without it:
Single-family home with 3 or more heating circuits
When you have a manifold through which water flows for 3, 4 or more circuits at the same time, the total flow on the supply must cover the needs of all branches. If each circuit is dimensioned for a flow of 0.3–0.5 l/min (which is common for medium-sized radiators), with four circuits we are talking about a supply requirement of 1.2–2 l/min. This is the threshold where a 15 mm supply starts to generate unacceptable pressure losses.
Typical configuration for a single-family home: 3-circuit closed manifold 22x15 or 2-circuit closed 22x15 – supply/return 22 mm, outputs to circuits 15 mm. This version allows you to connect a 22 mm pipe directly from the boiler or from the main manifold in the boiler room to the supply and at the same time lead a standard 15 mm pipe to individual rooms.
Floor heating systems
Floor heating places specific demands on the manifold: circuits are long (typically 60–120 m per loop), hydraulic resistance is high and flows must be precisely balanced. The supply should be at least 22 mm, ideally even larger, so that the manifold is not a hydraulic limitation of the system. Outputs to loops are usually 15 mm, which corresponds to a standard 16 mm PE-RT or PEX pipe (with sealing on a 15 mm neck).
Combined systems – heating + domestic hot water
In practice, it often happens that the customer wants to solve both heating and hot water distribution through one manifold. In that case, a 22 mm supply is practically essential, because the flow requirements for hot water are short-term but high – turning on a shower faucet to full means a flow of 0.1–0.2 l/s, i.e. 6–12 l/min, which simply cannot flow through a 15 mm neck without a dramatic drop in pressure. More about this type of application can be found in the article Water manifold vs. heating manifold – can I use the same one?
Long pipe runs with a larger pipe diameter
If pipes with a diameter of 22 mm are running from the boiler to the manifold (which is common practice at distances over 5–8 m from the boiler), it is illogical and hydraulically harmful to reduce these pipes to 15 mm with a reducer. The connecting necks of the manifold should always match the diameter of the supply pipe – otherwise you are creating an unnecessary hydraulic throttling area exactly where the resistance should be the lowest.
When should you reach for a diameter of 28 mm, 35 mm or larger?
In the category of plastic manifolds Hepworth, larger diameters are less common, but in practice you should know how to identify them. A larger manifold or supply neck comes into consideration in these situations:
Manifold for 5 or more circuits in a larger building
In small guesthouses, commercial spaces or large single-family homes over 200 m², the total flow through the main manifold can be such that even a 22 mm supply is not enough. In that case, manifolds with 28 mm connections or with a 1" (internal thread) neck on the supply are installed.
Connection to the primary circuit of a heat pump
Heat pumps work with higher flows and lower temperature differences than classic boilers. Heat pump manufacturers specify minimum diameters for primary circuits – usually 28 mm or 32 mm – and the manifold must have corresponding necks on the side of the primary circuit.
Solar thermal systems
Solar collectors are usually connected via mixing units and storage solutions, where flows and pressures require larger dimensions. Here, a plastic manifold with 28 mm and larger necks is standard.
Combination of different diameters on one manifold – what is standard?
If you have looked at the product range, you have certainly noticed that most manifolds have different diameters on the supply and on the outputs. For example, the type 22x15 has a supply/return of 22 mm and branches to circuits of 15 mm. This is the most common and hydraulically most logical combination in practice, and that is for a simple reason:
- Supply and return carry the total flow for all circuits → they need a larger diameter.
- Each output carries only the flow for one circuit → a smaller diameter is sufficient and corresponds to the pipe size to that circuit.
For example, with the 2-circuit open manifold 22x15 – each circuit has a flow of say 0.4 l/min, so the supply must handle 0.8 l/min. Through a 22 mm neck it is a detail for hydraulics, through a 15 mm it would generate unnecessarily large pressure loss. 15 mm outputs are again ideal for standard 15 mm copper pipe or 16 mm PEX/MLCP pipe with compression on 15 mm.
Practical Procedure: How to Determine the Required Diameter?
Instead of guessing, I recommend a method that reliably works even without a hydraulic calculation:
Step 1: Determine the Diameter of Existing Supply Pipes
If you are renovating an existing system, measure the outer diameter of the pipe that runs from the boiler to the manifold. For copper pipe: outer diameter 15 mm = nominal size 15 mm (English standard, same as metric), outer diameter 22 mm = nominal size 22 mm. For plastics (MLCP, PEX): outer diameter 16 mm connects to a 15 mm socket, outer diameter 20 mm connects to a 20 mm or 22 mm socket depending on the fittings.
Step 2: Count the Circuits and Estimate Total Flow
Each medium-sized radiator (1000–1500 W) requires a flow of approximately 0.08–0.12 l/s, i.e., 5–7 l/min, at a temperature drop of 10 K. Note – these are maximum values; actual flows in a well-adjusted system are lower. A simpler rule from practice: if you have more than 2 circuits, you need a 22 mm diameter at the manifold inlet.
Step 3: Check the Pipe Diameter for Each Circuit
For each circuit separately, determine the pipe diameter you plan to use. A standard 15 mm copper or MLCP pipe connects to a 15 mm socket. If you plan to use a larger diameter (22 mm) for the circuits – which is justified in the case of very long circuits – you will need a manifold with the corresponding outputs.
Step 4: Do Not Forget the Mounting Bracket
When choosing a manifold, also consider the mounting method. Mounting bracket for 3/4" manifolds allows secure mounting of the manifold on the wall or in a distribution cabinet. The correct bracket is just as important as the correct diameter – the manifold must remain in place under pressure and without vibrations. More on this can be found in the article Mounting bracket for manifolds: how to properly mount a manifold on the wall or in a distribution cabinet.
Most Common Mistakes in Diameter Selection – Examples from Practice
Over the years in the field and with hundreds of customers, the same mistakes keep repeating. Do you recognize them? You might find yourself in one of them:
Mistake No. 1: Purchasing a 15 mm inlet manifold where there is a 22 mm pipe
A customer is renovating an older family house. The boiler has a 22 mm supply, the distribution pipes are 22 mm, but the customer buys a manifold with a 15 mm inlet because it was "cheaper". Result: a 22/15 reducer has to be added, which creates unnecessary pressure loss, the system operates at higher pressure than necessary, the pump wears out faster, and the outer circuits are only slightly warm. The solution is simple – choose a 22×15 manifold, where the inlet matches the existing pipe.
Mistake No. 2: Choosing a collector with an unnecessarily large diameter for a small system
The opposite extreme: a customer installs a manifold with a 28 mm inlet for a small system with only two circuits. Hydraulically, this does not interfere, but the water volume in the manifold is unnecessarily large, the system reacts slowly, and in the case of a modern condensing boiler, it disrupts the proper condensing mode (the boiler requires a low return temperature). In addition, the price of such a manifold is significantly higher without any real benefit.
Mistake No. 3: Ignoring the output diameter to the circuits
A customer orders a 22×15 manifold but forgets that their PEX pipes for floor heating are 20 mm in diameter (outer). They cannot connect them to a 15 mm socket without a reducing fitting. The correct solution: always check the outer diameter of the pipe and the appropriate fitting solution before ordering the manifold.
Mistake No. 4: Correct diameter selected, but the type of termination (open vs. closed) was forgotten
Selecting the correct diameter is only part of the decision. Choosing the correct type of termination is equally important. An open manifold allows for future expansion with additional circuits, while a closed manifold is a final solution. More about this choice can be found in the article Open vs. closed manifold – what is the difference and when to use which.
Table: Quick overview of recommended diameters by application
| Application | Supply/return | Outputs to circuits | Note |
|---|---|---|---|
| Small apartment, 1–2 circuits, low flow | 15 mm | 15 mm | Only if the entire distribution is in 15 mm |
| Apartment or small house, 2–3 circuits | 22 mm | 15 mm | Most common case, type 22×15 |
| Family house, 3–5 circuits, radiators | 22 mm | 15 mm | Standard solution for FH |
| Underfloor heating, 3–6 loops | 22–28 mm | 15–16 mm | Depends on total flow |
| Sanitary distribution, 3–5 branches | 22 mm | 15 mm | Note: different type of distributor than for heating |
| Heat pump, primary circuit | 28–35 mm | 22–28 mm | Always according to the manufacturer's instructions for HP |
| Larger commercial building, 6+ circuits | 28–42 mm | 22 mm | Hydraulic calculation essential |
Relationship between distributor diameter and pressure losses – numbers
For those who like specific numbers: pressure loss when flowing through a pipe increases with the square of the flow velocity and is inversely proportional to the diameter (Darcy-Weisbach equation). In practice, this means:
- Flow 1 l/min through 15 mm port → flow velocity approx. 0.09 m/s → minimal pressure loss
- Flow 3 l/min through 15 mm port → velocity approx. 0.28 m/s → loss approx. 4× higher than at 1 l/min
- Flow 3 l/min through 22 mm port → velocity approx. 0.13 m/s → pressure loss roughly 10× lower than at the same flow through 15 mm
This numerical comparison clearly shows why increasing the supply diameter from 15 to 22 mm is such a significant improvement when there are more circuits and higher flows. The difference is not 20 or 30 percent, but an order of magnitude change.
Connection with the choice of number of circuits and type of distributor
Distributor diameter and number of circuits are two parameters that must always be considered together. It makes no sense to have a distributor with an ideal diameter but the wrong number of outputs – and vice versa. A detailed guide on how to combine these parameters can be found in the articles How to choose a plastic distributor: number of circuits, diameter and type of connection and 2-circuit vs. 3-circuit vs. 4-circuit distributor – which one suits your system.
It is also important to know that even if the port diameters on the distributor are the same, the actual construction (open vs. closed) affects the possibility of further system expansion. If you are considering adding circuits in the future, always choose the open version, even if you need only 2 circuits at the moment.
Most frequently asked questions (FAQ)
Can I use a reducer and connect a 22 mm pipe to a 15 mm port on the distributor?
Technically yes, but it is not a hydraulically suitable solution. A 22/15 reducer creates a throttling point exactly where the resistance should be lowest – at the distributor inlet. The result is unnecessary pressure drop, uneven flow in the circuits, and increased pump load. The correct solution is to choose a distributor with a 22 mm inlet (e.g., type 22×15) instead of installing a reducer.
What if my existing pipes are 15 mm, but I want to connect 4 circuits?
This is a situation where you need to consider the actual flow you really need. If all four circuits are small (e.g., four small radiators in a smaller house), the total flow may be manageable even through 15 mm. In the opposite case, it is cheaper and more appropriate to modify the boiler supply to 22 mm and buy a matching distributor, rather than having an underperforming system.
Is the distributor diameter the same as the diameter of the pipe I want to connect to it?
For copper pipe 15 mm: yes, directly to the 15 mm port. For PEX/MLCP pipe with an outer diameter of 16 mm: these pipes are usually connected via compression fittings, which have a 15 mm port on the distributor side. For PEX pipe 20 mm: you need a 22 mm or 20 mm port, depending on the fittings. Always check the outer diameter of the pipe and the type of fittings you plan to use before ordering.
Does it make sense to choose a larger distributor diameter "for the future" in case of system expansion?
On the supply/return side: yes, if there is a real chance that you will expand the system and need more flow, it is reasonable to dimension one step higher. On the side of outputs to circuits: no – unnecessarily large output diameter without a corresponding pipe leads only to fitting complications. For future expansion, it is better to choose an open distributor that allows you to add a circuit without replacing the entire body.
What diameter do I need for a combined distributor – heating and hot water?
For a combined distributor for hot water and heating, the supply diameter must cover both types of load. Sanitary hot water has short-term flow peaks (6–12 l/min), heating has long-term low flow (1–3 l/min). The supply should be at least 22 mm, ideally 28 mm, if you have more sanitary branches. At the same time, remember that the distributor for hot water and for heating must be certified for potable water or for heat transfer medium – these types differ and must not be mixed.
What if I don't know what flow my system needs?
In most common domestic situations, a simple rule of thumb from practice applies: if you have heating with 3 or more circuits in a family house, choose a 22 mm supply. For 1–2 circuits in a smaller apartment, 15 mm may be sufficient, but 22 mm will never hurt. If you are dealing with a larger or non-standard system, ask for a hydraulic calculation from a designer or seller – an investment in a proper design always pays off in the form of a reliable, energy-efficient system.
Conclusion: diameter is not just a number on a label
Choosing the diameter of the distributor is one of those decisions that seem trivial until problems arise. Then the customer finds out that some circuits are cold, the pump runs all day, and the boiler unnecessarily cycles. In most cases, it is enough to follow simple rules: the distributor supply must match the diameter of the existing supply pipe or be larger, 22 mm for 3 or more circuits, and the outputs to the circuits must match the diameter of the pipes in those branches.
A properly chosen, installed, and maintained distributor is an unobtrusive but key element of any heating or sanitary system. If you are unsure about the choice, take a look at other articles in this Knowledge Center – for example, Installation of a plastic distributor Hepworth step by step or Common problems with plastic distributors: leaks, air in circuits and weak flow – where you will find more specific practical advice.
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