Setting Flow on Individual Circuits
Setting flow on individual circuits of a brass manifold
A brass manifold is the heart of every branched heating system - underfloor heating, radiator distribution, and combined systems alike. However, even the best condensing boiler and the best-designed pipework can't guarantee even, economical heating if the individual circuits aren't hydraulically balanced. In practice this means each circuit must receive exactly as much heating water as it needs to cover the heat loss of the given room or area - no more, no less. In this article we explain in detail why flow setting matters, how it's calculated, the actual procedure for setting it on the manifold's flow meters, and the most common mistakes made along the way.
Why setting the flow matters
The manifold (supply branch) distributes hot heating water from the boiler to the individual circuits, while the collector (return branch) gathers it back from the circuits and returns it to the boiler. If all circuits had exactly the same length, the same pipe diameter and the same heat output, the water would distribute itself among them roughly evenly on its own. In a real house, though, this is almost never the case - a bathroom has a different heat output than a living room, a circuit running to a distant bedroom has a longer route and therefore higher hydraulic resistance than a circuit in a neighboring room, underfloor heating in the kitchen has a different pipe spacing than in the hallway. Without flow control, water always "takes the path of least resistance" - it flows preferentially through short, hydraulically easier circuits, while more distant and longer circuits get less water than they need.
The consequences of an unbalanced system are very concrete in practice: some rooms are overheated while others never reach the target temperature, the circulation pump works unnecessarily in its upper output range (higher electricity consumption and noise), the return temperature is higher than it should be, which for a condensing boiler significantly reduces its efficiency (the boiler condenses less, making poorer use of the flue gases' latent heat), and thermostatic heads or room thermostats then end up "fighting" the incorrectly distributed flow instead of just fine-tuning the temperature. A properly hydraulically balanced system, by contrast, allows for a lower operating temperature of the heating water, even comfort in all rooms, and real savings on gas or electricity consumption of roughly a few to several tens of percent, depending on the original condition.
How the required flow for a single circuit is calculated
Before setting anything directly on the manifold, you need to calculate for each circuit what flow of water it actually needs. This is based on the heat output the given circuit must deliver (the room's heat loss, or the output of the floor area) and on the temperature drop (the difference between the water temperature at the circuit's supply and return). A standard formula based on water's specific heat capacity is used:
Q [l/h] = 860 × P [kW] / ΔT [°C]
where P is the circuit's required heat output in kilowatts, ΔT is the temperature drop (the difference between the supply and return temperature of the circuit) in degrees Celsius, and the number 860 is a conversion coefficient derived from water's specific heat capacity (1 kWh heats/cools 860 liters of water by 1 °C). For practical reading on the manifold's flow meters, the result in liters per hour is usually converted to liters per minute by dividing by 60.
For underfloor heating, a lower temperature drop is typically used, usually 5 to 10 °C (for example 40/35 °C or 45/35 °C), because the large floor area requires a more even, gentler water temperature. For radiator circuits connected to the manifold, a higher drop is typically used, usually 10 to 20 °C (for example 55/45 °C or 70/50 °C in older systems). The smaller the chosen temperature drop, the higher the flow needed to achieve the same output - this is important to know especially when combining underfloor heating and radiators on a single manifold, where the circuits naturally have different flow requirements even at a comparable output.
Practical example: a bathroom circuit with a heat loss of 0.9 kW and a temperature drop of 10 °C needs a flow of Q = 860 × 0.9 / 10 = 77.4 l/h, which rounds to 1.3 l/min. A living room circuit with a heat loss of 2.2 kW at the same 10 °C drop needs Q = 860 × 2.2 / 10 = 189.2 l/h, i.e. approximately 3.2 l/min. It's exactly these calculated values that are then set directly on the flow meters of the manifold's individual circuits.
Step-by-step flow setting procedure
The flow meters themselves on a quality brass manifold are usually transparent (glass or clear plastic) cylinders with a movable float or ball inside and a scale showing flow in liters per minute. Below the flow meter there's a control ring or control valve, which changes the flow cross-section and thus the value at which the float settles. The setting procedure is as follows:
1. Venting the whole system. Before any flow setting, the system must be thoroughly vented - air bubbles in the circuits distort the reading on the flow meter and cause unstable, "jumping" float readings. Venting is done via the vent valves on both the manifold and collector, ideally with the circulation pump running and all circuits open.
2. Bringing the system into operation with the circulation pump at full or normal operating output. Flow setting is always done under real operating conditions, i.e. with the boiler/pump running as the system will normally operate, not "dry".
3. Opening all circuits fully. The control rings on all flow meters are first opened to maximum, so you can verify that the system as a whole achieves sufficient total flow (the sum of all circuits) matching the pump's and boiler's output.
4. Gradually throttling the circuits to the calculated value. By turning the control ring (usually clockwise reduces flow, counterclockwise increases it - the exact direction is always marked on the flow meter body or in the manufacturer's manual), the flow is gradually reduced until the float settles exactly at the calculated value in l/min on the scale. Depending on the flow meter type, you generally read either the middle of the float or the top edge of the ball - this information is usually on the label or in the specific manifold's technical datasheet.
5. Checking after temperatures stabilize. After setting all circuits, it's advisable to let the system run for at least several hours to a day, and then check the temperatures in individual rooms with a surface or room thermometer. If a room still doesn't reach temperature, or conversely is overheated, the flow is fine-tuned - slightly increased or decreased relative to the originally calculated value.
6. Recording the set values. It's recommended to note down the final setting for each circuit (for example with a marker on the manifold cover, or in a separate log), so that in the future - for example after a service intervention, a pump replacement, or a partial draining of the system - it can simply be restored without a lengthy re-tuning process.
Proportional balancing method for multiple circuits
If individual circuits are set completely independently, based only on the calculated target value, it can happen that the sum of the set flows exceeds the circulation pump's actual output - in that case all circuits influence each other, and fine-tuning one slightly changes the values on the others too. That's why, with a larger number of circuits (typically 4 or more), the so-called proportional balancing method is recommended:
Instead of setting the circuits in any order, you always start with the **critical circuit** - the one with the least favorable ratio of required output to available pressure (usually the longest or hydraulically most demanding circuit, for example the circuit running to the room farthest from the manifold). This circuit is left fully or almost fully open, because it's exactly this circuit that determines how much pressure the circulation pump has available for the remaining, hydraulically more favorable circuits. The other circuits are then gradually throttled, from the one closest to the manifold to the farthest (except for the already-set critical circuit), to exactly the calculated flow ratio relative to the critical circuit.
The advantage of this method is that you don't need to repeatedly re-tune in circles - the system is set once, in a logical order, and the resulting flow distribution remains stable even with changes in outdoor temperature or system load. In practice this method is combined with a calculation using a spreadsheet formula or a heating designer's design software, but even without it, the simple procedure described above (calculate the target l/min for each circuit and set them on the flow meters) gives a fully satisfactory result in the vast majority of family houses.
Measuring and verifying the setting
Reading the value on the flow meter is only the first step - it's also important to verify that the set flow actually results in the desired room temperature. A surface-mounted room thermostat, or a surface thermometer/thermostat installed directly on the circuit's pipe (for example on the return branch at the manifold or directly at the radiator), is excellent for this - such a device lets you monitor the pipe's surface temperature and infer from it whether the circuit is delivering enough heat, or is conversely unnecessarily oversupplied.
For both service interventions and routine annual heating system checks, it's worth having a surface-mounted thermostat with its own sensor installed directly on the manifold, which can monitor the temperature on a specific circuit over the long term and flag a deviation (for example after a partial system air-lock or after replacing a thermostatic head) before it shows up as an uncomfortably cold room. Combining a quality brass manifold with precise flow meters and supplementary measuring accessories significantly shortens the time needed to fine-tune the system and also makes it possible to quickly diagnose any future faults without having to remove the manifold cover.
 | AVANSA TH 2A surface-mounted thermostat - a simple, reliable solution for checking temperature directly on the circuit's pipe, an excellent aid for verifying that the set flow actually corresponds to the required temperature on a given manifold branch. Price from €11.81. |
Manifolds with multiple circuits (typically 2 to 12) are chosen based on the number of rooms/branches in the project and on whether it's a purely radiator, purely underfloor, or combined distribution system. Below are two specific sets from our range that are commonly used in practice as a complete manifold + collector solution with flow meters.
 | Manifold/collector set - without cabinet - 1"xEK; 3-way; brass - a complete brass set with three circuits, each with its own flow meter and control ring, ready exactly for the setting procedure described above. Price from €138.74. |
 | Manifold/collector set for heating elements - without cabinet - 1"xEK; 2-way; nickel - a two-circuit set suitable for smaller radiator distribution, nickel-plated finish, precise flow meters for easy hydraulic balancing of both circuits. Price from €182.04. |
Most common mistakes when setting flow
Even experienced installers sometimes make one of the following mistakes when setting flow, so it's worth paying extra attention to them:
Setting cold, without the circulation pump running. A flow meter shows a real value only when water is actually flowing through the system, driven by the pump at operating pressure. Setting "by eye" without the circulation running is imprecise and usually has to be redone entirely once the system starts up.
Ignoring air in the system. Even a small air bubble trapped at the top of a circuit causes the flow meter's float to "jump" or not settle at a stable value at all. Before setting, the system must always be thoroughly vented, ideally repeated after a few days of operation, since air can also be released gradually afterward.
Setting all circuits to the same value. A common but incorrect practice is to set all circuits "by eye" to the same flow regardless of output and length - the result is exactly the unbalanced state that correct setting is meant to prevent. Each circuit should have its own individually calculated value.
Forgetting a follow-up check after seasonal reconfiguration. If the system is partially drained between the heating and summer season, or the boiler's settings are changed (for example switching to a lower weather-compensation curve), it's worth checking after every major change whether the flows on individual circuits have stayed at their original values - a small shift is normal and quickly fine-tuned.
Throttling in the wrong place instead of using the thermostatic head. The flow meter's control ring is used for basic hydraulic balancing (set once at commissioning), while the thermostatic head or actuator on the manifold is used for ongoing control based on the current room temperature. Confusing these two functions (for example trying to control the daily temperature just by throttling the flow meter) leads to inefficient and slow control.
Frequently asked questions about flow setting
What flow should I set if I don't know the room's exact heat loss?
If you don't have project documentation with an exact heat loss calculation, you can roughly estimate based on the room's area and type (a bathroom or corner room has a higher loss per m² than an interior room) and on typical tabulated specific heat output values (approx. 60-100 W/m² for a typical insulated new build, higher values for an older, uninsulated building). It's always more accurate to have at least an approximate calculation done by a designer - an incorrectly estimated flow can be corrected later, but it unnecessarily prolongs the system fine-tuning process.
Can I set the flow without disconnecting the pump or draining the system?
Yes, quite the opposite - the setting must be done during normal operation, with the circulation pump running and water flowing through the system. Flow meter control rings are designed to be turned directly during operation, without needing to drain or shut down the system.
Why doesn't the float on my flow meter settle at a stable value and keeps fluctuating?
The most common cause is air trapped in the circuit - you need to thoroughly vent the system again, ideally repeatedly, a few days apart. Another possible cause is a fluctuating circulation pump output (for example with cheaper or undersized types), or the flow meter being placed too close to an elbow or another point with turbulent flow.
Is there a difference between setting flow for underfloor heating and for a radiator circuit on the manifold?
The principle and the procedure for setting are the same; only the input temperature drop used in the calculation differs (underfloor heating typically 5-10 °C, radiators 10-20 °C), so the resulting flow values in l/min will differ for the same heat output. If both types of circuits are combined on one manifold, it's important to calculate each type with its own correct drop, not a single value for the whole system.
How often does the flow setting need to be checked or repeated?
For a stable, unchanged system, a basic check once a year, ideally at the start of the heating season, is enough. After any major intervention in the system - a pump replacement, partial draining, replacing a thermostatic head, or building work that changes a room's heat loss (for example added insulation) - it's worth rechecking the setting and fine-tuning it if needed.
Can flow be set without flow meters, based only on room temperature?
Yes, as a last resort it can also be balanced empirically - by gradually fine-tuning the control rings and observing the resulting room temperature over several days. This method, however, is considerably slower and less precise than calculation and direct reading on flow meters, so it's recommended mainly as a supplementary fine-tuning step after the basic setting based on calculated values.
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