How many circuits and outputs do I need on the manifold
Why the number of circuits and outlets on the manifold is a key question
When designing or making additional modifications to underfloor and radiator heating, almost every technician and investor sooner or later faces the question: "How many circuits do I actually need and which manifold should I buy?" This is one of the most important decisions in the entire system, because the manifold is a central point where the entire house's hydraulics meet - from the heat source to individual rooms. An undersized manifold means that later you will have nowhere to connect an additional loop, when you decide to finish the attic or convert the garage into a living room. On the other hand, an unnecessarily large manifold with empty outlets takes up space in the cabinet, complicates balancing and costs more money without any real benefit.
In practice, we encounter two extremes. On one side are builders who have a manifold installed "by eye" without calculation, and after two years they are dealing with the fact that they are missing one circuit for a new bathroom in the attic. On the other side are projects where an oversized 12-circuit manifold is installed in an apartment with three rooms - an unnecessary expense and space in the cabinet. The goal of this article is to show how the number of circuits is actually calculated, what to pay attention to and how to avoid the most common mistakes we encounter in practice when implementing single-family homes, apartments and renovations.
Basic principle - one circuit, one loop, one outlet
The underfloor heating manifold operates on the principle of dividing one input branch (supply and return from the heat source or from the main manifold) into several separate circuits. Each circuit represents one closed loop of pipe, which starts at the manifold, runs through the floor or wall of a given room and returns to the manifold. Each such circuit requires its own outlet on the supply and return strip of the manifold.
It is important to understand that the number of circuits is not determined by the number of rooms, but by the area to be heated and by the maximum length of one loop that is hydraulically and technically possible to use. In a larger room you can have two or even three circuits, while in a small bathroom, on the other hand, one circuit is often sufficient or it can even be combined with an adjacent room.
How the number of circuits is actually calculated
In practice, we start from three basic inputs: room area, maximum recommended length of one loop and pipe spacing. These three values together determine how many circuits the room will need.
Maximum length of one loop
As a general rule, one underfloor heating loop should not be longer than 80 to 100 meters with a 16 mm diameter pipe (the actual limit depends on flow characteristics and pressure loss that the manifold or pump can overcome). With thicker 17 or 20 mm pipes, longer loops can be used, typically up to 120 meters, because the pressure loss per meter is lower. In practice, however, most designers stick to the rule that the optimal loop length is 60 to 90 meters - at this length the system is well adjustable and pressure losses between circuits are not extremely different, which simplifies balancing the flows.
Calculation from area and spacing
The second way to verify the number of circuits is to calculate from the room area and laying spacing. With a standard spacing of 150 mm, we need approximately 6.7 meters of pipe per m² of area. With a spacing of 200 mm, it is approximately 5 meters per m². With a spacing of 100 mm (for example in bathrooms or in edge zones with higher heat loss), we need up to 10 meters per m².
| Room area | Spacing 150 mm | Required pipe length | Recommended number of circuits |
|---|---|---|---|
| up to 12 m² | 150 mm | up to 80 m | 1 circuit |
| 12 - 18 m² | 150 mm | 80 - 120 m | 2 circuits |
| 18 - 25 m² | 150 mm | 120 - 170 m | 2 - 3 circuits |
| 25 - 35 m² | 150 mm | 170 - 235 m | 3 circuits |
| over 35 m² | 150 mm | over 235 m | 4 or more circuits |
This table is only approximate - in practice, a specific project is always calculated according to the heat losses of the given room, type of floor and required surface temperature. For example, a kitchen with a large area covered by a kitchen unit will have a lower effective heating area than an equally large living room, and thus may require fewer meters of pipe, but shorter loops for better regulation in exposed areas (e.g. near large windows).
Why not just count by rooms
A common mistake we encounter with inexperienced installers is the simplification "one room = one circuit". This rule works only for small spaces up to about 15 m². In larger rooms, such as living rooms with an area of 30-40 m², one loop simply cannot cover the entire area without exceeding the recommended length. The result would be too large a pressure loss, uneven heating (the more distant parts of the loop would be colder) and problems with balancing.
On the other hand, in very small spaces, such as a WC, a closet or a corridor, it is often combined with several such rooms into one circuit, provided the heat losses are similar and the rooms are hydraulically close. This is one of the areas where it is worth getting advice from a designer or experienced technician - we will deal with this topic in more detail in the article on how to choose a manifold for underfloor heating.
Boiler circuits on the manifold
Manifolds are not used only for floor heating - in many single-family homes, floor heating is combined with boiler circuits, for example in bathrooms where a panel radiator is also needed, or in adjacent rooms such as a garage or utility room. In such cases, a simple rule applies: each radiator or each radiator branch connected to separate regulation requires its own circuit (output) on the manifold.
When designing combined systems, it is important to allow for a reserve in the number of circuits, as boiler circuits usually have different hydraulic requirements (higher water temperature, different flow) than floor circuits. Some designers therefore choose separate manifolds for floor and boiler circuits, while others use a common manifold with thermostatic valves for separate temperature regulation of the boiler branch.
How many outputs to choose - practical recommendation with a reserve
The basic question is: should the manifold be bought exactly for the number of required circuits, or with a reserve? Based on many years of experience with implementations, we recommend always allowing for at least one or two reserve outputs. The reasons are as follows:
- Future expansion - finishing the attic, garage renovation, extension, balcony glazing and its heating.
- Error in calculation or change of design - during construction, room layouts often change, partitions are added or the purpose of the space changes.
- Compensation for failure - if one circuit is damaged in the future (for example, during drilling into the floor), a reserve output allows for a simpler solution without the need to replace the entire manifold.
- Lower cost of expansion now rather than later - replacing an entire manifold with a larger model in a built-in cabinet with embedded connections is a much more expensive and complicated operation than buying a manifold with 1-2 extra outputs from the start.
Common practice is therefore: if the calculation shows a need for 6 circuits, order a 7 or 8-circuit manifold. This reserve pays off significantly in new builds, where changing the manifold after it has been built into the structure is costly and often means tearing down plaster or drywall.
Typical scenarios from practice according to house size
Smaller apartment or studio (40-55 m²)
In apartments of this size, we commonly encounter the need for 3 to 5 circuits - a living area with a kitchen (1-2 circuits depending on size), a bedroom (1 circuit), a bathroom (1 circuit), and optionally a corridor/entrance (1 circuit or shared with another room). A manifold with a reserve should have 5 to 6 outputs. A compact cabinet is suitable for such a space, for example Wall-mounted manifold cabinet N-MAX 1 - 450mm, which can accommodate a smaller manifold and its accessories.
Medium-sized family house (100-150 m²)
Here we typically deal with a range of 7 to 10 circuits on one manifold (in larger houses, two manifolds for different floors or zones are sometimes used). This category includes a standard house with a living room with a kitchen (2-3 circuits), 3 bedrooms (3 circuits), a bathroom and WC (1-2 circuits), a corridor and entrance hall (1 circuit). With such a number of circuits, it is worthwhile to go for a larger cabinet, for example Wall-mounted manifold cabinet N-KLASIK 2 - 535mm, which provides sufficient space for valve manipulation and future maintenance.
Larger family house with an attic (over 180 m²)
In larger houses with two floors, two separate manifolds are commonly used - one for the ground floor, the other for the attic or upper floor, each with 8-12 circuits. The total number of circuits in the house can thus exceed 18-20. In such cases, a larger cabinet is appropriate, for example Wall-mounted manifold cabinet N-MAX 5 - 1200mm, which comfortably accommodates a manifold with a high number of circuits and accessories such as a circulation pump, mixing valve or valves for system filling and draining.
How the number of circuits relates to the choice of manifold cabinet
The number of circuits directly affects the width of the manifold, and thus the size of the cabinet into which it is to be built. Each circuit means approximately an additional 50 mm of width on the manifold body (the exact value varies according to the manufacturer and type of fittings, for example, manifolds with flow meters or larger valves may have a greater spacing). When planning construction preparation - that is, when building a partition or installing a recessed cabinet into drywall - it is necessary to know the number of circuits in advance, because modifying the opening in the masonry is always more complicated than proper measurement at the beginning.
We therefore recommend following this order: first, count the required circuits (with a reserve), then select a specific type and width of manifold according to this number, and only then decide on the size and placement of the cabinet. We discuss this topic in detail in a separate article on how to choose a manifold cabinet - dimensions and placement, where you will also find specific tables with cabinet size ranges.
Specifics when combining different types of floor coverings
The number of circuits also affects the type of floor covering in individual rooms. With ceramic tiles, heat transfer is better, so the pipe spacing can be greater (for example, 200 mm) and you need fewer meters of pipe for the same area than with wooden flooring, where a denser spacing of 100-150 mm is recommended due to the higher thermal resistance of the covering. This ultimately means that a room with parquet may require one more circuit than a similarly sized room with tiles.
Therefore, in homes with combined floorings (for example, tiles in the kitchen and laminate in the living room, even though it is one open space visually), it is common to choose a solution with two separate circuits within one visual space, because each part has different performance and spacing requirements.
Flow meters and EK valves - do they affect the number of outputs?
The type of regulating elements on the manifold (classic EK valves with manual flow setting versus manifolds with flow meters, which allow precise reading and setting of liters per minute) does not affect the number of required circuits - this calculation is always determined by the area and heat losses. However, it does affect the practicality of regulating a larger number of circuits. For systems with 8 or more circuits, we clearly recommend a manifold with flow meters, as manual regulation of many circuits based only on valve opening is very imprecise and time-consuming in practice. The difference between these two types of manifolds is discussed in detail in the article "EK manifolds vs. manifolds with flow meters - differences."
What happens if you choose the wrong number of circuits
Underdimensioning the number of circuits is expressed in practice in several ways. Most often, it results in uneven heating of the room - parts of the loop closest to the manifold are significantly warmer than the more distant parts, because the water gradually loses temperature along the long route. In the case of a too long loop (over 100-120 m), the pressure loss also increases significantly, which requires a more powerful circulation pump and increases the electricity consumption for its operation.
The opposite extreme - too many circuits with short loops - also complicates balancing, because short circuits have low pressure loss and "take" too much flow at the expense of longer circuits, unless the system is carefully balanced. In practice, we most often see this in amateur implementations, where floor heating was divided only by intuition, without a real hydraulic calculation.
Practical steps to verify the number of circuits before ordering
We recommend the following procedure for the average investor:
- List of all rooms with floor and radiator heating, including their area.
- For each room larger than 15 m², plan for division into 2 or more circuits.
- Consider combining small spaces (hallways, toilets, storage rooms) into one circuit, if they are adjacent and have similar heat losses.
- Add separate circuits for radiators, if they are part of the system.
- Add 1-2 spare outputs to the resulting number.
- Choose a specific distributor according to the resulting number and then the corresponding cabinet.
This procedure is approximate and does not replace an accurate heating project, which should also calculate the actual heat losses of the building according to the current standard. For larger or more complex buildings, we always recommend having a hydraulic calculation prepared by a designer, who will also take into account the simultaneity of heat consumption (not all circuits run at maximum capacity at the same time) and the dimensioning of the heat source.
Relationship between the number of circuits and expansion joints
When implementing multiple circuits in the floor, it is important not to forget the proper separation of individual pipe sections when crossing expansion joints and along the room perimeter. This is indirectly related to the number of circuits - the more circuits and longer loops laid in the floor, the more expansion transitions need to be addressed. For this purpose, Expansion joint - 1.5m; 6mm - pack 75m or smaller packaging Expansion joint - 1.0m; 6mm - pack 12.5m is used, depending on the extent of expansion joints on the specific construction site. More information on the correct laying of expansion joints can be found in the separate article "Expansion joints - what they are for and how to lay them correctly."
Depressurization and balancing with a higher number of circuits
The more circuits the distributor contains, the more important it is to pay attention to proper depressurization during the first system filling and subsequent balancing of flows in individual circuits. In systems with 8 or more circuits, air can "hide" in some loops and cause bubbling or cold spots even after apparently successful filling. We recommend gradual depressurization, one circuit at a time, never all at once, while the other circuits should be temporarily closed. A detailed procedure can be found in the article "Depressurization and balancing of a floor heating distributor."
Most frequently asked questions about the number of circuits and outputs
Do I need a separate circuit for each room?
No, it is not a requirement. Smaller and adjacent rooms with similar heat losses (e.g., toilet and hallway) are commonly combined into one circuit. Larger rooms over 15-18 m², on the other hand, require division into 2 or more circuits to avoid exceeding the recommended loop length.
How many extra circuits should I have in reserve?
It is commonly recommended to have 1 to 2 spare outputs. In new buildings, where changing the distributor later is complicated (built-in cabinet, finished plaster), this reserve is practically essential - replacing it with a larger distributor after the building is completed is always more expensive than the original purchase with a reserve.
Can I combine floor and radiator circuits on one distributor?
Yes, it is a common practice, especially in bathrooms with a ladder radiator. However, you should consider that radiator circuits have different hydraulic requirements (higher flow, higher temperature), so it is advisable to take these differences into account during planning, or use separate regulation.
How do I know if one distributor is enough, or if I need two?
If the total number of required circuits in the house exceeds the capacity of commonly available distributors (typically up to 12 circuits) or if some parts of the house are too far from the distributor (e.g., attic), it is more appropriate to use two separate distributors placed closer to the respective zones. This shortens the supply branches and simplifies regulation.
Does the number of circuits affect the performance of the boiler or heat pump?
Directly, no. Indirectly, yes - a higher number of circuits means a higher total flow in the system at full load, which must be considered when dimensioning the circulation pump and sometimes also when selecting a hydraulic separator between the heat source and the distributors.
What if the number of circuits turns out to be insufficient in the future?
If the cabinet is built-in or embedded, the solution is either to install a second smaller distributor next to the original one (if space in the cabinet allows), or, in the worst case, to replace the entire distributor. Therefore, it is important to plan for a reserve from the beginning - for more information on this topic, see the article "Common distributor faults and their solutions."
Conclusion
The correct number of circuits and outputs on the distributor is not a matter of guesswork, but a simple yet thorough calculation based on the area of the rooms, the maximum recommended loop length, and the type of floor covering. In practice, it always pays to add a reserve of one to two outputs, because changing the distributor after it is built into a cabinet or wall is always more complicated and expensive than properly planning the capacity from the start. When selecting a specific distributor and the corresponding cabinet, we recommend proceeding systematically - first calculate the circuits, then choose the type of distributor, and only then address the dimensions and placement of the cabinet, as well as other accessories including expansion joints for proper floor pipe expansion. If you are unsure about the calculation for a larger or more complex house, do not hesitate to consult an experienced designer or technician - a mistake at this stage is always more difficult and expensive to correct than proper planning from the beginning.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your home? Write to us - we are happy to help.