How to choose a stainless steel manifold for underfloor heating – number of circuits, diameter and flow
How to choose a stainless steel manifold for floor heating – number of circuits, diameter and flow rate
The manifold is the heart of any floor heating system. It is through it that hot water flows into individual loops in the floor, and it is it that determines whether the entire system will work quietly, evenly and efficiently – or whether you will have warmth in one room and cold in another. Choosing the right stainless steel manifold is not rocket science, but it does require knowing a few key parameters: how many circuits you will be heating, what water flow rate will go through them, what diameter of collector you need, and how to properly design the whole thing even before the first trowel of anhydrite concrete is laid.
This article covers the entire topic from the basics to the details of project practice. If this is your first time here, we recommend also checking out related topics in the Knowledge Center – for example, How many circuits does a manifold need – how to correctly determine the number of floor heating loops or Stainless steel vs. brass manifold – which material is better for floor heating. These articles together form a complete guide to designing a floor heating system.
What is a stainless steel manifold and why stainless steel?
A manifold (or collector, collectively also "manifold") is a hydraulic component that distributes hot water from one common source – boiler, heat pump, storage tank – into multiple separate circuits. Each circuit supplies one or more floor heating loops. At the output, the water from the circuits is again combined in the return and returned back to the heat source.
Stainless steel (usually AISI 304 or AISI 316) has become the standard in practice for several excellent reasons. It is resistant to corrosion even if the system contains oxygen or slightly aggressive water, which is common in open or semi-open systems. Stainless steel is mechanically strong, outlasting brass in environments with lower pH values, and at the same time does not release heat as quickly as metal – which also plays a role in regulation. In terms of durability, a stainless steel manifold is practically maintenance-free for the entire lifetime of the house when installed correctly.
Number of circuits – the foundation of everything
The first and most fundamental question when choosing a manifold is: how many floor heating circuits will you have? The number of circuits directly determines how many-way manifold you will buy. The rule is simple: one circuit = one pair of outlet/return on the manifold.
In practice, we encounter two approaches to dividing circuits. The first is the room-based principle – each room has its own circuit. This approach allows for individual temperature control in each room (e.g., bathroom 24 °C, bedroom 20 °C, hallway 18 °C). The second approach is the space-based principle – larger areas are divided into multiple circuits according to the length of the pipe loops, not according to rooms. Typically, one loop should not be longer than 80–100 meters (with a 16×2 mm pipe) or 100–120 meters (with a 20×2 mm pipe), otherwise the pressure loss increases so much that the pump cannot keep up and the flow rate in the circuit drops.
This means that a large living room with an area of 40 m² can have up to three separate circuits, while a small bathroom of 6 m² usually suffices with one. The final number of circuits is therefore the sum of all loops from all rooms. A typical family house with an area of 120–150 m² most often has 6 to 10 circuits, while an apartment unit of 60–80 m² usually has 4 to 6 circuits.
How to not underestimate the number of circuits when planning
A very common mistake we see in customer practice: the customer buys a manifold with exactly as many outlets as they have circuits today – and when during implementation it turns out that one circuit needs to be split or another added (e.g., for a bathroom underfloor heating, a sauna entrance or a terrace), there is nowhere to connect it. Professional tip: if you are not sure, better choose one or two circuits more. An unused outlet on the manifold can be easily capped off – that is much cheaper and less stressful than replacing the entire manifold after the pipes are embedded in anhydrite.
More details on how to correctly calculate the number of circuits can be found in the article How many circuits does a manifold need – how to correctly determine the number of floor heating loops in the Knowledge Center. Here we focus more on the hydraulic side of the selection.
Overview of available manifolds by number of circuits
For floor heating systems, manifolds are available from 2 to 6 circuits. Each of them has the same diameter of the collector, the same spacing of the outlets and the same type of connecting thread – they differ only in the length of the body:
- Stainless steel 2-way manifold for floor heating – ideal for small apartments, studios or for a secondary circuit manifold (e.g., only bathroom + hallway)
- Stainless steel 3-way manifold for floor heating – suitable for apartments with 3 rooms or smaller ground-floor houses
- Stainless steel 4-way manifold for floor heating – the most sold for apartments and smaller family houses (3+1 or 4-room apartments)
- Stainless steel 5-way manifold for floor heating – for medium-sized houses, common when floor heating is installed in the entire ground floor
- Stainless steel 6-way manifold for floor heating – for larger ground-floor houses or floors with a larger number of rooms
If you need more than 6 circuits, the solution is two manifolds connected in parallel (in a larger manifold cabinet), or a manifold with a higher number of outlets on special order. In practice, we often encounter the connection of two six-way manifolds in family houses over 200 m².
Collector diameter – why it matters
The internal diameter of the collector (i.e., the distributor pipe itself) is a hydraulic parameter that determines the total load the distributor can handle without excessive pressure loss. For stainless steel distributors Hepworth, which you can find in the category stainless steel distributors, the collector diameter is designed so that at typical floor heating flows (total flow through the distributor 0.5 to 3 l/min per circuit), the water velocity in the collector is low – usually below 0.3 m/s.
Why is this important? If the water velocity in the collector is too high, hydraulic losses occur, noise (hissing, flow noise) appears, and the flow distribution between circuits becomes uneven. Moreover, at high water velocity, air bubbles are carried further into the system, making air venting more difficult.
Practical calculation: how much water flows through the distributor?
The total flow through the distributor is the sum of the flows of all circuits. For an approximate calculation: one floor heating circuit for a standard room (e.g., 15 m², circuit thermal power around 1.0–1.5 kW) has a flow of approximately 0.8–1.3 l/min at a temperature drop of 5 K. For a standard 6-way distributor with all circuits fully open, the total flow can be 5–8 l/min (0.083–0.133 l/s).
Are you also interested in the pipe dimensions of the supply/return – this depends on the total flow and the desired velocity. More information can be found in the article Dimensions and connection thread pitch of stainless steel distributors Hepworth in the Knowledge Center.
Flow – how to set and check it correctly
The flow in each floor heating circuit is a key parameter for the proper functioning of the system. Too low a flow means that the pipe will heat up at the inlet, but cold water will reach the end of the circuit – the floor temperature will be uneven. On the other hand, too high a flow leads to excessive heat transfer (the floor overheats above 29 °C, which is a hygiene limit), noise, and unnecessary loading of the circulation pump.
Calculation of the required flow for a circuit
The basic formula for calculating the mass flow of a circuit:
q = Q / (c × Δt)
where: q = flow [l/s], Q = circuit thermal power [kW], c = specific heat capacity of water ≈ 4.18 kJ/(kg·K), Δt = temperature drop of the circuit [K]
In practice, in a low-temperature floor heating system, we typically work with a temperature drop of 5 to 10 K (e.g., supply 35 °C, return 30 °C = ΔT 5 K, or supply 40 °C, return 30 °C = ΔT 10 K). A concrete example: a circuit with a power of 1.2 kW and a drop of 5 K requires a flow of q = 1.2 / (4.18 × 5) = 0.057 l/s = 3.4 l/min. If we choose a drop of 10 K, the flow drops to 1.7 l/min.
On stainless steel distributors Hepworth, flow rotameters (flow heads) are integrated directly into the collector body, so you can visually set and check the flow without measuring. Each circuit is adjusted individually by rotating the regulating head to the desired value in liters per minute. This operation is part of hydraulic balancing – you will learn more in the article Hydraulic balancing of circuits via stainless steel distributors – why and how to do it.
What affects flow in practice
Along with the setting of control heads, the flow in the system is influenced by these factors:
- Length and diameter of pipe loops – a longer loop has higher hydraulic resistance, and the flow naturally decreases
- Performance of the circulation pump – a weak pump will not push sufficient flow into long loops
- Degree of opening of control heads – hydraulic balancing ensures the same flow into both short and long loops
- Air in the system – air bubbles dramatically reduce flow in the affected loop
- Clogging of filters and strainers – a mechanical filter (ball valve with a strainer) should always be installed on the supply side of the manifold
Outlet placement and connection thread spacing
The standard spacing of outlets on stainless steel manifolds for floor heating is 50 mm (center-to-center). This spacing is designed to allow the installation of control actuators (thermo-electric actuators) next to each other without mutual contact. The connection thread of the outlets is typically 3/4" or 1" – depending on the specific product and spacing; always check the technical data sheet or the article Dimensions and connection thread spacing of stainless steel Hepworth manifolds.
The inlet and outlet of the main pipe (supply and return to the collector) have a 1" or 1 1/4" thread – again depending on the version. This thread is connected directly to the primary distribution from the boiler or heat pump. It is important to have a ball valve installed on the supply branch (a shut-off valve allows you to isolate the manifold without draining the entire system), an air vent at the highest point, and a pressure gauge for pressure monitoring.
Thermo-electric actuators and room temperature control
A stainless steel manifold for floor heating is designed to accommodate thermo-electric actuators (actuators) directly on the outlet valves. The actuator responds to a signal from a room thermostat or controller and mechanically opens or closes the supply of hot water to the respective loop. This achieves individual temperature control for each room – and this is one of the main reasons why floor heating with a manifold is so energy efficient.
Actuators are usually two-position (open/closed) or proportional (0–10 V). For standard installations, two-position actuators are fully sufficient. They are controlled via room thermostats or intelligent control systems (Wi-Fi thermostats, KNX, smart home systems). It is important that the power supply to the actuators (24 V AC or 230 V, depending on the manufacturer) is routed through a suitable control module that also coordinates the signal for the circulation pump.
Manifold placement – where to install it
The ideal location for a manifold is the hydraulic center of the area it serves, so that the loops to individual rooms are as similar in length as possible. In practice, this means: in a central hallway, in a technical room, or in a closet. The manifold is installed in a manifold cabinet, which is either recessed into the wall or mounted on the wall. The installation height is usually 30–50 cm from the floor (top edge of the cabinet).
A detailed installation procedure, including how to install the cabinet in different types of walls, can be found in the articles Placement and installation of the manifold cabinet – where and how to install the manifold and Installation of a stainless steel floor heating manifold – step by step.
Typical scenarios from practice – how to decide
Scenario 1: 2+1 apartment, 55 m²
Living room (24 m²), bedroom (12 m²), bathroom (6 m²), hallway (8 m²), kitchen part of the living room. The living room, due to its size, requires 2 loops (loops of about 70 m each), the bedroom 1 loop, the bathroom 1 loop, the hallway 1 loop. Total of 5 loops. Recommendation: 5-way stainless steel manifold, or a 6-way with one spare.
Scenario 2: Single-family house, ground floor 90 m²
Kitchen + dining room (30 m²) – 2 loops, living room (25 m²) – 2 loops, hallway + utility room (12 m²) – 1 loop, bathroom (8 m²) – 1 loop, study (10 m²) – 1 loop. Technical room without heating. Total of 7 loops. Solution: one 6-way manifold plus one 2-way manifold for the remaining area, connected in parallel in one cabinet.
Scenario 3: Bathroom as an addition to an existing radiator system
The customer wants floor heating only in the bathroom (7 m²) and in the hallway (5 m²) – a total of 2 loops, connected to the existing boiler via a mixing valve (reducing temperature from 70 °C to 35 °C). A 2-way stainless steel manifold is sufficient, installed directly in the bathroom cabinet or on the wall.
Hydraulic balancing – the most commonly underestimated step
After installing the manifold and filling the system with water, it is necessary to hydraulically balance the loops. Without balancing, water will "flow" into the shortest loop with the lowest hydraulic resistance, and only a minimal flow will reach the longer loops. Result: some rooms will be overheated, others underheated, the pump will operate at an incorrect point, and energy consumption will increase.
Balancing is carried out by setting the calculated flow for each loop on the flow rotameters (or control valves). Control heads are throttled on shorter loops – thus increasing their hydraulic resistance to the same level as the longer loops. This way, the pump works against the same load in each loop and the flow is uniform.
Material, corrosion and long service life of the stainless steel manifold
Stainless steel AISI 304 withstands normal heating water with a pH value of 7.0–9.0, which is the recommended range for closed heating systems. If you have doubts about water quality or are working with softened water (low hardness, low pH), choose AISI 316, which is more resistant to chloride corrosion. For most common installations in family homes, however, AISI 304 is fully sufficient.
Important note: the stainless steel manifold must not come into direct contact with cement-based mortars, lime plaster or concrete. These materials are alkaline and can damage the stainless steel over a long period of contact. Therefore, the manifold surface must always be protected by a protective film or soft wrapping when pouring the floor or plastering walls near the cabinet.
Another important topic is galvanic corrosion when combining different metals. If the manifold is connected to a copper pipe, it is advisable to use transition fittings made of bronze or to use dielectric couplings. In practice, most floor heating systems today are implemented with plastic PEX or PEX-Al-PEX pipes, where this issue does not arise.
System air venting after filling
Every new floor heating system must be thoroughly vented after the initial filling. Air in the system is the most common cause of problems – from noise (gurgling, flow noise) to zero flow in some circuits. On the Hepworth manifold, an automatic air vent or a manual air vent is mounted at the highest point of the collector (usually at the end of the body).
Ventilation procedure: after filling the system with pressure (0.5–1.0 bar above operating pressure), gradually open each circuit and let the water flow until it runs without bubbles. Then check the pressure and, if necessary, add water. The automatic air vent on the manifold will continuously release microbubbles for the first few days – this is normal. A detailed procedure is described in the article Maintenance and air venting of a stainless steel floor heating manifold.
Common mistakes in selection and installation – what we have seen in practice
Over the years of customer experience, we have recorded several typical mistakes that are repeated:
- Undersized manifold – the customer buys a 4-way manifold for 5 circuits. The solution is to buy an additional manifold and connect it in parallel, which is unnecessarily complicated.
- Too long circuits – loops longer than 100 m with a 16 mm pipe have a hydraulic resistance of 10–15 kPa, which a standard pump can handle, but it is at the limit. We recommend not to exceed 80 m.
- Missing filter on the supply pipe – without a filter, the screen inside the manifold gets clogged with impurities from the pipe, which reduces flow.
- Hydraulic imbalance – without setting flow regulators, the system is unreliable, some rooms are uncomfortably warm, others cold.
- Missing shut-off valves – without valves on the supply and return lines, it is not possible to isolate the manifold without draining the entire system.
- Installation of the manifold below the anhydrite level – the cabinet must be installed so that the outlets are at least 20–30 cm above the finished floor, otherwise it is not possible to connect the pipe fittings.
Most frequently asked questions (FAQ)
Can I connect more circuits than the manifold has outlets?
It is not directly possible without a manifold with more outlets. The solution is to buy a larger manifold (if you are still pre-installation), or to add a second smaller manifold in parallel. Two manifolds are connected to a common supply and return – for example, one 6-way and one 3-way for a system with 9 circuits. This solution is technically fully functional, but it requires a larger manifold cabinet.
What pipe diameter is best for loops – 16 mm or 20 mm?
For standard floor heating in family homes, the most commonly used pipe is PEX 16×2 mm (outer diameter 16 mm, wall 2 mm). This pipe is laid with a spacing of 10–20 cm and a loop length of up to 80 m. A 20×2 mm pipe is used for longer circuits (up to 120 m), larger spacing and higher performance. A larger pipe diameter reduces hydraulic resistance, but increases material consumption and floor thermal inertia.
Is it necessary to hydraulically balance the circuits if they are all the same length?
If all circuits are the same length and dimensioned identically, the hydraulic resistance is the same and balancing is not technically necessary. In practice, however, identical loop lengths occur very rarely – different sized rooms mean different length circuits. Therefore, we recommend always performing balancing, at least a basic check of flows on the rotameters.
What to do if one circuit has no flow (rotameter shows zero)?
Most common causes: air in the circuit (solution: vent), regulating head turned to zero (check the setting), thermostatic actuator in the "closed" position (check the signal from the thermostat), or mechanical impurities in the screen (clean the filter). If flow is still not achieved after these steps, it may be a mechanical damage to the valve or an intentional closure during hydraulic testing that was forgotten. The troubleshooting procedure is described in detail in the article Common problems with a stainless steel manifold – leakage, weak flow, noise.
What pressure must the system withstand during pressure testing?
A pressure test is always carried out before the pipes are embedded in concrete. Stainless steel Hepworth manifolds are designed for a maximum operating pressure of 6 bar (some versions up to 10 bar). The test pressure is usually 1.5 times the operating pressure, i.e. 6–9 bar, for at least 1–2 hours. The pressure must remain stable without a drop. After a successful test, the pressure is reduced to operating pressure (1–2 bar for standard family homes) and the pipes can be embedded. Never pour anhydrite over pipes without a prior pressure test – any subsequent repair is extremely costly.
How many years will a stainless steel manifold last?
With proper installation, recommended water quality (pH 7–9, without aggressive additives) and regular venting (once a year), the service life of a Hepworth stainless steel manifold is 30 years or more – practically the same as the life of the house itself. Brass components (shut-off valves, flow meter inserts) may require replacement sooner, but the stainless steel body of the manifold is practically indestructible. Unlike brass manifolds, they are not prone to dezincification or microbial deposits at higher temperatures.
Conclusion: practical recommendations when choosing
Summary of the main rules when choosing a stainless steel manifold for floor heating:
- Count all circuits (loops) and choose a manifold with the same or 1–2 more outlets as a reserve.
- For standard apartments and smaller houses up to 80 m², a 4-way or 5-way manifold is sufficient, for larger houses go for a 6-way manifold, or a combination of two manifolds.
- Calculate or estimate the flow for each circuit and set the rotameters when commissioning.
- Always perform hydraulic balancing – this is the step that determines whether the system will work properly.
- Before embedding the pipes in concrete, do not forget the pressure test and check the tightness of all manifold connections.
- Installation in a manifold cabinet, correct height and sufficient space for actuators and operation are conditions for comfortable long-term operation.
Floor heating with a stainless steel manifold is a system that, with proper design and installation, works for decades without problems and provides exceptional heating comfort. An investment in a high-quality manifold, professional installation and careful hydraulic balancing pays off in reduced operating costs and the absence of faults throughout the life of the house.
Do you have a question on this topic?
Having trouble making a decision or dealing with a specific situation in your home? Write to us – we are happy to help.