How many port manifold do I need – how to correctly determine the number of underfloor heating loops

How many manifold do I need – how to correctly determine the number of underfloor heating circuits

This is a question that almost everyone who is embarking on planning or renovating an underfloor heating system faces. At first glance, it seems simple – after all, it is enough to count the rooms, right? If you think that, in this article you will find out why it is not that simple, and on the contrary – why the process is not complicated at all when done correctly. The correct choice of the number of circuits and thus the number of manifold outlets is the basic prerequisite for the underfloor heating to work efficiently, without local cold spots, without excessive energy consumption and without unnecessary noise in the pipes.

This article will take you through the entire thought process – from measuring the area, through understanding why one room may need more circuits, to practical scenarios from real buildings and renovations, where we have seen what happens when the manifold is chosen only at the last moment.

What is actually an "underfloor heating circuit" and why does its length matter

Before we get to the actual counting, it is necessary to understand what a circuit is and why you cannot simply say "one room = one circuit". An underfloor heating circuit is a closed loop of pipe – usually PEX or MLCP – that comes from the manifold, runs under the floor at a certain spacing (usually 10 to 20 cm) and returns back to the collector. The water enters from the supply side (top row on the manifold), circulates through the loop and returns cooled back through the bottom row (return).

The key parameter is the maximum length of one circuit. For the most commonly used pipe with a diameter of 16 × 2 mm, a maximum circuit length of 80 to 100 meters is recommended, exceptionally up to 120 meters. With 20 × 2 mm pipe, you can go up to 120 meters without problems. Why is this important? Because the longer the circuit, the greater the hydraulic resistance, the greater the pump you need and the harder it is to hydraulically balance the entire system. In practice, we try to make all circuits in the system approximately the same length – or at least within a reasonable ratio (maximum 1:1.3 between the shortest and longest circuit). This is a topic we cover in the article Hydraulic balancing of circuits via a stainless steel manifold – why and how to do it.

How to calculate the pipe length for one circuit – the basis for determining the number of circuits

The calculation is not rocket science, but it does require a bit of methodical approach. The procedure is as follows:

1. Measure the area you want to heat – in each room separately, in m².
2. Choose the pipe spacing – typically 15 cm (i.e. 0.15 m). The greater the thermal output you need, the smaller the spacing. At 10 cm spacing, the output is higher, but the pipe consumption increases dramatically.
3. Calculate the loop length in the area: length [m] = area [m²] ÷ spacing [m]. For example, for 20 m² at 0.15 m spacing: 20 ÷ 0.15 = 133 m – this is just the loop itself!
4. Add the connecting pipes – the distance from the manifold to the entrance into the room (and back). If the manifold is 5 meters from the room, add 10 m (there and back).

In the example above: 133 m (loop) + 10 m (supply/return) = 143 m. This is too long for one circuit with DN16. The result? That room must be divided into two circuits. And it is precisely here that most people are surprised.

Why counting only by rooms is not enough – real examples from practice

Over the years of working with customers on the design of underfloor heating systems, we repeatedly come across the same mistake: a customer comes with a floor plan, counts the rooms and says, "I have eight rooms, I need an 8-way distributor." In reality, after calculating the lengths, it turns out that a distributor with twelve or fourteen outlets is needed.

Example No. 1: family house 140 m² – bungalow

A single-storey family house, total usable area 140 m², 7 rooms. At first glance, 7 loops. The distributor is located in a technical room in the middle of the house, the distance to individual rooms is from 3 to 8 meters. After calculation:

• Living room + dining room (open plan): 42 m², with a spacing of 15 cm → ~280 m loop + 8 m connection = 288 m → 3 loops
• Bedroom: 18 m² → ~120 m + 10 m = 130 m → 2 loops (with DN16), or 1 loop with DN20
• Child's room 1: 12 m² → ~80 m + 6 m = 86 m → 1 loop
• Child's room 2: 12 m² → ~80 m + 6 m = 86 m → 1 loop
• Bathroom: 8 m², with a spacing of 10 cm (higher performance) → ~80 m + 5 m = 85 m → 1 loop
• WC + corridor: 6 m² → ~40 m + 4 m = 44 m → 1 loop
• Kitchen: 14 m² → ~93 m + 7 m = 100 m → 1 loop (barely)

Result: 7 rooms, but 10 loops in total. Instead of the expected 7-way distributor, a 10-way one is needed, or a combination of two smaller ones. In this case, the customer ultimately chose separate distributors – one 6-way for the day zone and one 4-way for the night zone – which also brought the advantage of zoning (different temperature settings for daytime use and sleeping areas).

Example No. 2: apartment renovation, 3-room apartment 78 m²

Apartment renovation, an apartment on the 3rd floor of a panel building. The distributor is installed in the entrance (shaft). The rooms are smaller and regularly shaped, with reasonable distances (2–5 m). Most of the loops came out to 1 loop per room – a total of 5 loops. The customer ordered a stainless steel 5-way distributor for floor heating, which was the right choice. No overdimensioning, no unnecessary costs.

Example No. 3: two-storey family house, 220 m²

A two-storey house with a distributor on each floor (two cabinets). Ground floor 110 m² (living room, kitchen, bathroom, boiler room, corridor) → 8 loops. Upper floor 110 m² (3 bedrooms, bathroom, corridor, walk-in closet) → 6 loops. Total of 14 loops, two distributors. The customer considered one central distributor, but in a two-storey house, this technically complicates the connections and the lengths of the connections increase significantly. Two separate distributors – each closer to their own zone – were the obvious choice.

Factors affecting the number of loops – a complete overview

To summarize systematically, here are all the variables that determine how many loops (and thus how many-way distributor) you will need:

1. Room area and pipe spacing

The basic pair of parameters that determines the length of the loop. A smaller spacing (10 cm) means longer pipe on the same area – and thus more loops. A larger spacing (20 cm) shortens the total length, but reduces performance. In practice, 15 cm is used for regular living spaces, and 10–12 cm for bathrooms and areas with higher heat loss.

2. Pipe diameter

DN16 (16 × 2 mm) – most common, maximum recommended loop length 80–100 m. DN20 (20 × 2 mm) – allows loops up to 120 m, less hydraulic resistance, but a larger water volume in the system and thicker anhydrite. DN14 – exceptionally in special systems with thin floors, maximum length 60–70 m.

3. Distance of the room from the distributor

Every meter from the distributor to the room must be counted twice (supply + return). At a distance of 10 m, the connecting pipes take up 20 m of the maximum allowed loop length. In a large house or with an unfavourably located boiler room, this can become a dominant factor.

4. Room shape

In a regular rectangular room, you can lead the loop efficiently in a serpentine or spiral pattern. In L-shaped or irregular rooms, you have to divide into more loops, because in the "corner" areas the loop would be either too short or would cause hydraulic problems.

5. Zoning according to temperature requirements

Each regulated zone must have its own loop (or a group of loops with the same requirements). A bathroom with a hot water radiator (hand dryer) on a different loop than the floor. Areas with different uses (e.g. garage vs. living room) must always be on separate loops, even if the lengths are favorable.

6. Floor surface temperature and heat loss of the space

In rooms with higher heat loss (corner rooms, large glazing), a higher specific performance of the floor is needed. This is achieved by a smaller spacing – which again extends the loops and may require their division.

Practical Procedure: How to Determine the Number of Loops Step by Step

Now we move on to a clear, repeatable procedure that you can apply to any floor plan. We recommend creating a simple table in Excel or on paper.

Step 1: Draw a schematic floor plan and mark the areas

For each room, record: area in m², shape (regular/irregular), planned floor surface (tiles, wood, floating floor), planned use (bathroom, bedroom, living room). Subtract areas where pipes will not be installed (under the bath, under the kitchen line, under cabinets) – these areas are not heated and there is no need to run pipes there.

Step 2: Choose the spacing for each room

As a general rule:

• Bathroom, WC: 10 cm
• Living room, dining room: 15 cm
• Bedroom, children's room: 15–20 cm
• Corridor: 15 cm
• Garage area: 10–15 cm (depends on insulation)

Step 3: Calculate the loop length for each room

Use the formula: Loop length = (area ÷ spacing) × 1.05. The factor 1.05 accounts for corner turns and minor deviations in the route.

Step 4: Add the length of the connecting lines

Measure (or estimate from the floor plan) the distance from the planned location of the manifold to the pipe entrance in each room. Double this value (supply + return) and add it to the loop length.

Step 5: Divide long loops

If the total length for one room exceeds the maximum recommended limit (for DN16 → 100 m, for DN20 → 120 m), divide the area into two parts and treat each as a separate loop. Try to make both parts the same length (±10 %).

Step 6: Include a reserve

This is very important: always plan for 1–2 loops more than the calculation suggests. In practice, the wiring diagram changes slightly during implementation – an extra loop may be needed for a bathroom ladder, the need to separate a kitchen island may arise, etc. A manifold with unused outlets can simply be capped – and you will have the option to expand the system in the future without expensive reconstruction.

Available variants of stainless steel manifolds and when to use which

There are manifolds with different numbers of outlets on the market. In the category of stainless steel manifolds from Hepworth, you can find variants from 2 to 6 circuits (and for larger systems, a combination of multiple units is used). Here is a practical summary:

2-circuit manifold

Stainless steel 2-circuit manifold for floor heating is suitable for very small systems: a small apartment (studio, one-room apartment), summer kitchen, garden house or a technical room with a small area. In practice, we also encounter it when expanding an existing system – for example, when extending a bathroom or adding a room.

3-circuit manifold

Stainless steel 3-circuit manifold for floor heating covers a small apartment (2-room with smaller rooms) or part of a larger house – for example, the night zone of a floor with a bedroom, bathroom and corridor.

4-circuit manifold

Stainless steel 4-circuit manifold for floor heating is very popular for 3-room apartments and smaller single-storey houses up to about 80 m². It can also handle one floor zone of a larger house.

5-circuit manifold

Stainless steel 5-circuit manifold for floor heating fits medium-sized apartments (80–100 m²) or floors of a family house with an average layout. Very common in practice – five loops cover a living room, bedroom, two children's rooms and a bathroom.

6-circuit manifold

Stainless steel 6-circuit manifold for floor heating is the maximum in one piece and covers systems up to about 110–130 m² (depending on the layout). For larger houses, two units are used, each with its own pump and zone control.

Combination of two distributors – when and how to do it

If your calculation results in 7, 8 or more circuits, you basically have two options: use one larger distributor (10, 12-way – mostly brass or other types), or connect two stainless steel distributors in series. The second option has several advantages:

• Zoning: each distributor can operate at a different temperature. A day zone (living room, kitchen) can have a supply temperature of 35 °C, while a night zone (bedrooms) can have 30 °C – different surface temperatures, different thermal comfort.
• Closer placement to rooms: two smaller distributors placed in two different shafts shorten the connecting pipes and allow for longer loops in the rooms.
• Easier regulation: each distributor has its own pump and thermostat – lower hydraulic resistance, easier balancing.

For more information on the physical placement of distributor cabinets, see the article Placement and installation of the distributor cabinet – where and how to install the distributor.

Mistakes to avoid – what we have seen in practice

To conclude the technical part, we summarize the most common mistakes we see in underfloor heating system design:

• Too long circuits: The customer wanted to save money and combined two rooms into one circuit. The result? The hydraulic resistance was so high that the pump couldn't keep up, the distant part of the circuit was cold, and the whole system suffered from noise. Repairing after the concrete has been poured is not possible without expensive grinding of the screed.
• Too short circuits: A 10 cm spacing in a small bathroom (4 m²) gives a 40 m circuit – ideal. But if someone sets the same spacing in a 25 m² living room, they get a 250 m circuit – unsolvable for DN16. Moreover, in a short circuit (under 30 m), the flow speed is uneven and the system is noisy.
• Ignoring connecting pipes: The customer calculated only the loop from one corner of the house, forgetting about a 12 m supply pipe. The circuit was 24 m longer than expected.
• Undersized distributor: They bought a 4-way distributor because it seemed cheaper – the calculation showed the need for 6 circuits. They had to compromise on areas, and some parts of the house now have less performance than they should. Differences in thermal comfort are noticeable.
• No reserve: During the renovation, it turned out that the customer also wanted a bathroom ladder and underfloor heating in the wardrobe – which were not included in the original design. Without a free output on the distributor, it was not possible to connect an additional circuit.

More details on choosing a distributor in terms of material and flow can be found in the article How to choose a stainless steel distributor for underfloor heating – number of circuits, diameter and flow.

Reference table: how many-way distributor for which house/flat size

Type of building / area	Estimated number of circuits	Recommended distributor	Note
Studio / 1-bedroom flat (up to 35 m²)	2–3	2- or 3-way	Bathroom + living room (+hallway)
2–3-bedroom flat (40–70 m²)	3–5	4- or 5-way	Typically 1 circuit/room
3–4-bedroom flat (70–100 m²)	5–8	6-way or 2× smaller	Living room may need 2 circuits
Single-storey house (80–130 m²)	7–12	6-way + additional or 2× 6-way	Depends on layout
Two-storey house (over 150 m²)	12–20	Two separate distributors (per floor)	Ground floor / upper floor zoning
Commercial space / offices	Design	Multiple distributors	Requires professional HVAC design

Most frequently asked questions (FAQ)

Can I use fewer circuits than the calculation suggests and compensate with a larger pump?

Technically yes, but in practice it doesn't work well. Higher hydraulic resistance in an overly long circuit causes uneven heating (the entrance part of the loop is warm, the exit part is cold), increased pipe noise and energy inefficiency. The pump has to work at higher speeds, which increases electricity consumption. A properly designed system with appropriate circuit lengths works more quietly, evenly and economically.

What if my calculation results in 7 circuits – should I buy 6+2 or one 8-way?

It depends on the system. If you want to use stainless steel distributors from Hepworth (maximum 6-way), the combination of 6+2 (or 6+3 with a reserve) is the correct choice. You will buy two separate stainless steel distributors, each with its own cabinet, pump and regulation. The advantage is also in zonal control – two zones with different supply temperatures. If you want one unit, you have to go for another type (brass or multi-part system), but that is a different category of products and a different topic.

How many circuits can I have on one pump?

It depends on the hydraulic resistance of the system. A standard circulation pump for underfloor heating (e.g. Wilo Yonos Para 25/6 or Grundfos UPM3 25-75) can easily handle 4–8 circuits of appropriate length (60–90 m each) with a distributor of 1 inch diameter. With long circuits or a large number (over 8–10), it is better to divide the system into two pump groups. Specific pressure losses should be calculated by an HVAC designer.

Do I need a flow meter and thermostat on each circuit?

A flow meter (rotameter) is part of a professionally installed distributor and is used for hydraulic balancing – setting the flow for each circuit individually. It is an absolute necessity for proper system function – without it, some circuits take the flow at the expense of others. A thermostat (actuator) on each circuit is needed if you want to regulate each room separately. For example, if you combine a hallway and a WC into one zone, one actuator for both circuits is sufficient. The topic of regulation and balancing is covered in a separate article Hydraulic balancing of circuits via a stainless steel distributor – why and how to do it.

Can I add a circuit later, after the floor has been poured?

Adding a floor circuit after pouring is not possible without breaking the floor – the pipes are embedded in concrete and any change requires excavation. What you can do easily, however, is to use a free port on the manifold, if you have left one there in advance (blinded). That is why we always recommend leaving at least 1–2 spare ports. If the manifold does not have a spare port and you want to add another circuit, you will have to replace the entire manifold – which is technically possible (the manifold is not embedded in concrete), but unnecessarily complicated.

Is there a difference in how circuits are arranged on the manifold – does the order matter?

From a hydraulic perspective, no – all ports on the supply are connected the same way (common bar) and the same on the return. The order of circuits on the manifold therefore does not affect hydraulics. From a practical perspective, however, it is advisable to organize circuits logically – for example, from left to right according to the position of rooms in the house, or group them by zones (day, night). This makes identification easier in case of a fault or regulation. Each circuit should be labeled – for example, with labels or tags on the manifold.

Conclusion: accurate calculation is the basis, reserve is wisdom

Determining the number of floor heating circuits is not a matter of guesswork or a simple rule of "one room = one circuit". It is a systematic calculation of pipe length based on area, pipe spacing, and distance from the manifold – and this calculation must be done for each room separately. In practice, we see that the result is almost always higher than the customer expected, and it is precisely here that the decision to save on a cheaper (smaller) manifold leads to problems that can only be resolved at great expense.

If you are unsure about the calculation, create a simple table according to the procedure described in this article and verify the result with an HVAC designer or contact us – we are happy to review your design. Do not forget that the selected manifold should always have at least one more port than the current need. This small "insurance" can save you from an expensive and time-consuming renovation in the future.

Before placing your final order, also read the related topics in the Knowledge Center: Dimensions and connection thread pitch of stainless steel manifolds Hepworth for technical details on dimensions, Installation of a stainless steel floor heating manifold – step-by-step procedure for installation instructions, and Stainless steel vs. brass manifold – which material is better for floor heating for a comparison of material properties. Choose the right manifold from the start – floor heating is an investment that is very difficult to correct.

Do you have a question about this topic?

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

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