Air venting and balancing of floor heating distributor

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Why air venting and balancing the manifold is key to the performance of underfloor heating

Underfloor heating is in its essence a simple system - warm water flows through pipes embedded in the floor and transfers heat to the room via a large surface with a low surface temperature. Precisely because it operates with low temperature differentials (typically 35/28 °C or 40/30 °C), it is much more sensitive to any shortcomings in hydraulic setting than traditional radiator heating. Air in the system, incorrectly balanced flows or a clogged filter can cause rooms to heat unevenly, the system to be noisy, the boiler or heat pump to operate inefficiently, and energy bills to rise without an obvious reason.

The underfloor heating manifold is the heart of the entire system - it distributes the heating water to individual circuits and collects it back into the return. If it is poorly vented or balanced, it will be noticeable throughout the entire house. In this article, we will take a detailed look at how to properly vent the manifold, how to set the flows on individual circuits and what to pay attention to so that the system works exactly as it was designed.

How air gets into the system and why it must be removed precisely through the manifold

Air gets into the heating system during filling, after each topping up of water, during shutdown and restart, and also gradually - dissolved oxygen is released from the water during heating, especially if the system lacks good water treatment or if the system is leaky and there is a constant addition of fresh water (which always brings dissolved air with it).

Air bubbles tend to rise against the direction of water flow upwards, so they accumulate at the highest points of the system - and these are precisely the manifolds, if they are placed higher than the floor heating circuits (which is common, since the manifold cabinet is typically embedded in the wall at a height of around 30-60 cm above the floor, while the pipes run under the floor lower, but loops rise and fall and air is trapped precisely at the top of the loop at the connection to the manifold).

Signs of air in the underfloor heating system

In practice, air in the underfloor heating circuits is manifested in several typical ways:

Bubbling or noise in the manifold cabinet, especially after turning on the circulation pump
One or more circuits are significantly colder than the others, even though the flow meter or valve appears open
Uneven heating of the room - part of the floor is warm, part is cold
The circulation pump runs noisier, or the dry run protection is triggered in electronic pumps
Slow temperature rise after starting the heating season
Flow fluctuations on the manifold flow meters - the needle or float wobbles instead of staying at a stable value

Step-by-step procedure for venting the manifold

Venting the underfloor heating manifold is not a complicated task, but it requires a systematic approach and a bit of patience. A system that is quickly vented often "makes noise" again after a few days, because air is hidden in the pipes in smaller bubbles, which are only released gradually.

Preparation before venting

Before the actual venting, it is necessary to:

Check the pressure in the system on the boiler manometer or in the technical room - it should correspond to the value specified by the designer, typically 1.5-2.5 bar in family homes depending on the height of the building and the expansion tank
Prepare a container or cloth to catch the escaping water
Close (reduce) the circulation pump to lower speeds, if possible - at lower flow rates, air bubbles separate and rise to the venting valves more easily
Have a filling and draining set at hand, if it is part of the manifold or the supply pipe

The actual venting

The procedure is as follows:

Close all circuits except one - using the regulating screws or valves on the manifold, close all loops except the first one you will be venting. This ensures that the entire flow from the pump goes only through this one loop and air is more effectively pushed out.
Open the venting valve on the supply and return branch of the manifold - quality manifolds have an automatic or manual venting valve on both main lines (supply and return).
Let air escape together with a small amount of water until a continuous stream of water without bubbles starts to flow. In larger systems, this can take several minutes per circuit.
Close the venting valve and move on to the next circuit - gradually open the second loop, close the first one, and repeat the procedure.
After venting all circuits, open all circuits simultaneously and let the system run for at least 24-48 hours, then check the pressure and, in case of a drop, top up the water and repeat the venting.

This "one circuit at a time" procedure is much more effective in practice than venting all circuits at the same time, because with several open circuits, the flow is divided and the flow speed in individual pipes drops to such an extent that air bubbles cannot be effectively carried to the venting point.

Ventilation scheme - procedure for a multi-circuit manifold

The entire flow from the pump thus goes only through one open circuit, which increases the flow speed and effectively pushes air bubbles towards the venting valve on the main line.

Automatic vs. manual venting valves on the manifold

Quality underfloor heating manifolds are often equipped with automatic venting valves located on both main lines. These valves contain a float that drops and opens a small channel when air is present, allowing the air to escape, and when the water level rises, the float rises and the valve closes.

An automatic venting valve is very practical especially during the first weeks of operation, when dissolved air is still being released from the system. It does, however, have some disadvantages:

Over time, the float mechanism can become clogged with dirt or microbubbles and freeze
If the pressure in the system drops, it may suck air in instead of out
Requires regular visual inspection to check for continuous slight dripping, which indicates worn sealing

A manual air vent (also known as a Flexi valve or needle valve with a square key) provides greater control over the process, but requires the presence of the operator. In practice, it is often combined - an automatic valve for regular operation and a manual one for thorough air venting during commissioning or after service work.

Adjusting the manifold - why it is essential and what happens if it is skipped

Adjusting (hydraulic balancing) the manifold means setting the correct water flow to each individual underfloor heating loop so that each room or zone receives exactly the amount of heat it needs - no more, no less. Without balancing, water will always prefer the path of least resistance, i.e., the shortest and hydraulically simplest loops, while longer or more complex loops will receive less water than needed.

In practice, this looks like this: in a family house, you have a living room with a 90-meter loop and a bathroom with a 35-meter loop next to it. If both loops are left completely open without setting the flow, the bathroom will overheat very quickly because the water passes through there more easily and with less resistance, while the living room remains underheated, even though it has the same or higher manifold capacity available.

How to calculate the required flow for individual loops

The correct flow for each loop is based on the heat loss of the room and the temperature drop of the heating water. A simplified formula looks like this:

Flow (l/min) = Heating power of the loop (W) / (temperature drop (°C) × 70)

For example, for a room with a heat loss of 1200 W and a temperature drop of 8 °C (e.g., 40/32 °C), the required flow is approximately 1200 / (8 × 70) = 2.14 l/min. This calculation should be part of the underfloor heating design, but in practice, we often encounter situations where the design is missing or the system was installed DIY, and balancing is done "by eye" based on the room temperature after the system is started.

Practical procedure for balancing a manifold with flow meters

If the manifold is equipped with flow meters (rotameters) on individual loops, the balancing procedure is relatively straightforward:

Ensure the entire system is vented and pressurized to the design value
Start the circulation pump at medium or higher speed
Open all loops fully and record the current flow on each flow meter
Gradually restrict the loops with a flow higher than the calculated or recommended value using the adjustment screw on the top of the flow meter
After setting, wait at least 15-20 minutes for the system to stabilize, as a change in one loop slightly affects the flows in the other loops (this is a coupled hydraulic system)
Repeat the fine-tuning until all loops reach the desired values within a tolerance of approximately ±0.2 l/min

Adjusting a manifold of type EK (without flow meters)

For manifolds of type EK, which do not have flow meters, balancing is done either using differential pressure and pre-calculated valve setting values (manufacturer's Kvs tables), or practically using a measuring device with temperature probes, or empirically based on the return temperature from each loop. In practice, we recommend at least roughly measuring the surface temperature of the return pipe for each loop with a non-contact thermometer in the absence of flow meters - if the temperatures on all returns are similar, the loops are approximately balanced. The differences between the outputs of individual loops and their characteristics are discussed in more detail in the article on the differences between EK manifolds and manifolds with flow meters.

Schematic - properly vs. improperly balanced system

Influence of the location and type of manifold cabinet on air venting

Proper placement of the manifold directly affects how easily the system can be vented and maintained. The manifold should always be located at the highest point of the respective heating loop (within the given zone or floor) so that air naturally accumulates at the headers and not somewhere in the middle of the loop under the floor, from where it is practically impossible to vent without draining the entire system.

When selecting and installing the manifold cabinet, sufficient space for service access must also be considered - the air vents, adjustment heads and flow meters must be easily accessible without the need to disassemble the surrounding piping. We therefore recommend choosing cabinets with sufficient reserve, for example Wall-mounted manifold cabinet N-MAX 1 - 450mm for smaller manifolds with 2-4 loops, or a larger model such as Wall-mounted manifold cabinet N-MAX 5 - 1200mm, which provides comfortable space for service work on manifolds with a larger number of outputs. An alternative with a classic look is, for example, Wall-mounted manifold cabinet N-KLASIK 2 - 535mm. More detailed selection criteria are discussed in a separate article How to choose a manifold cabinet - dimensions and placement.

The role of step insulation in the stable operation of the system

Although step insulation is not directly related to air venting, its proper placement affects the overall stability and efficiency of the system, which is then evaluated during balancing. Step insulation separates the heating pipe from the floor structure and prevents heat transfer downward and sideways into the surrounding structures instead of upward into the room. If the step insulation is insufficient or damaged, part of the heat is lost outside the heated area, which is expressed externally similarly to insufficient flow - the room is not heated, even though the flow meter indicates the correct value.

When dealing with complaints about insufficient heating of a room, we always recommend checking not only the hydraulics at the manifold, but also the condition and thickness of the floor insulation under the screed. Standard floor insulation has a thickness of 6 mm, available for example as Floor insulation - 1.5m; 6mm - bundle 75m or in a narrower version Floor insulation - 1.0m; 6mm - bundle 12.5m. More about its function and correct laying can be found in the article Floor insulation - what it is for and how to lay it correctly.

Typical mistakes during air venting and balancing from practice

Over the years of service interventions, the same mistakes keep repeating, leading to problems with floor heating. Here is an overview of the most common ones:

Venting all circuits at once

As mentioned above, venting all circuits at the same time is the most common mistake, because the low flow rate in individual pipes cannot effectively push out the air. The result is that the system appears to be vented, but after a few days, the gurgling and uneven heating return.

Adjusting before complete venting

If balancing is done before the system is thoroughly vented, the measured flow values are distorted by the presence of air bubbles, which change the effective cross-section of the pipe. After later re-venting, the flows can become unbalanced again and the balancing has to be repeated.

Ignoring the pressure in the system

Low pressure in the system (below the value recommended by the project, often below 1 bar) causes the air to release from the water much more intensively and the system "re-vents" itself repeatedly without the problem being resolved permanently. In such a case, the pressure must first be topped up to the correct value and the expansion tank should be checked.

Incorrectly set circulation pump

If the pump runs at too low a speed, the overall flow through the system is insufficient to effectively push the air to the venting points. On the other hand, excessively high speeds can cause noise and unnecessarily high pressure differences between circuits, which complicates precise balancing.

Missing documentation of settings

A very common problem is that after balancing, the values are not recorded anywhere. With any subsequent intervention (for example, replacing the pump, servicing the boiler, adding another zone), the settings are then lost and the balancing has to be done from scratch. We recommend always recording the flow values or valve settings for each circuit and attaching a label with this table directly to the inside of the manifold cabinet door.

When is it appropriate to call a professional

Basic venting and approximate balancing can be done by the house owner with a bit of skill according to the procedure above. However, there are situations when it is more appropriate to call a heating technician:

The system repeatedly vents and the problem returns within a few days, which may indicate a leak, undersized expansion tank or a problem with the automatic water make-up
The project with calculated flows is missing and it is necessary to perform an accurate calculation of heat losses for each room
The manifold is of type EK without flow meters and measuring equipment (differential manometer, temperature probes) must be used
Suspected major faults - leaking valve, damaged head, clogged pipe after renovation
The system combines multiple heat sources (for example, heat pump and boiler) and the balancing must also take into account switching between sources

Common faults and their solutions, including leaks, stuck heads or non-functional flow meters, are discussed in detail in the article Common faults of manifolds and their solutions, and regular maintenance of the system can be found in the article Maintenance and servicing of the heating system manifold.

Recommended inspection and maintenance interval

Floor heating is not a system of "set and forget". The recommended general inspection schedule looks like this:

Task	Recommended frequency
Pressure check in the system	Monthly during the heating season
Visual inspection of vent valves (leakage/dripping)	During each boiler visit, min. 1x monthly
Thorough venting of all circuits	At the beginning of the heating season and after each major water make-up
Check and possibly adjust flows in the circuits	1x per year, ideally at the beginning of the season
Cleaning of the filter before the manifold (if part of the assembly)	1-2x per year
Check tightness of threaded connections on the manifold	1x per year

Summary of complete maintenance procedure in one diagram

Why temperature drop directly affects the pipe diameter and circuit resistance

During balancing, it is often forgotten that not all floor heating circuits are hydraulically the same, even if they have the same length. Different pipe diameter (typically 16x2 mm or 17x2 mm), different laying spacing (for example 150 mm versus 300 mm in edge zones) and laying method (spiral vs. meander) change the hydraulic resistance of the circuit, and thus also how much water flows through it at the same valve opening. Therefore, two circuits of the same length may require completely different settings on the manifold if one is laid more densely (edge zone near windows) and the other more sparsely (central zone of the room).

For this reason, it is not recommended to adjust the distributor "according to the length of the circuit" indicated on the loop label - the length is only one input into the calculation. What is decisive is always the heat loss of a specific zone and the selected temperature drop, as described in the formula above. If the project also specifies the type of laying and spacing for each room, this is valuable information for fine-tuning as well, when trying to find out why two seemingly similar circuits require different flow settings.

Connection between the filter before the distributor and the stability of the balancing

The filter (sludge trap) installed on the supply pipe before the distributor has a direct impact on how long the balancing remains valid. A clogged filter increases the overall hydraulic resistance of the system and reduces the available differential pressure for the distributor - this is manifested externally by a gradual decrease in flow rates on all circuits, while the ratio between the circuits remains approximately preserved. This is therefore a different manifestation than an unbalanced system, where the ratio between the circuits is disturbed, but the overall flow may be fine.

When addressing a complaint about insufficient heating, we recommend distinguishing between these two scenarios: if the flows have decreased evenly on all circuits, the filter should be checked and possibly cleaned or the insert replaced; if the drop is uneven and affects only certain circuits, it is more likely an issue with balancing, air, or a partially clogged loop. Checking the pressure difference before and after the filter (if the system is equipped with measurement points) can clearly confirm this difference without the need for disassembly.

Frequently asked questions about air venting and balancing the distributor

How often should floor heating be vented?

A thorough air venting of all circuits is necessary at the first start-up of the system, then at the beginning of each heating season and after any major water refill (e.g., after repairing a leak). In a trouble-free, tight system with good water treatment, it is sufficient to perform only a routine check of the automatic air vents during the season.

Why is one room still cold after air venting?

If air has been successfully removed and the room is still colder than others, the problem is most likely due to incorrect flow balancing - the circuit is receiving less water than needed in relation to its length or the room's heat loss. The solution is to fine-tune the flow using a flow meter or the regulating valve of the respective circuit, or to check the step insulation and the condition of the screed above the pipes.

Can a distributor be balanced without flow meters?

Yes, but it is more difficult and less accurate. Either the Kvs values are calculated and set according to the manufacturer's tables, or the temperatures on the return of individual circuits are measured using an infrared thermometer. The differences between EK distributors and distributors with flow meters are explained in detail in a separate article in this category.

How can I tell that the system has low pressure?

Low pressure can be seen on the pressure gauge in the boiler room - the value should not drop below the minimum recommended by the boiler manufacturer or the designer (typically 1-1.5 bar in a cold system). Signs of low pressure also include frequent gurgling and repeated need for air venting, or error messages on the boiler.

Do I need to adjust the settings of the boiler or heat pump during balancing?

Directly, no. Balancing the distributor concerns only the flows in the individual circuits. However, a properly balanced system allows for a lower temperature of the heating water at the source, since heat is distributed evenly and there is no need to "overheat" insufficiently supplied circuits at the expense of the overall system efficiency.

Can I do the balancing and air venting myself, or must a professional do it?

Basic tasks - air venting individual circuits and approximate flow adjustment on a distributor with flow meters - can be done by a skilled homeowner following the procedure described in this article. For more complex systems, multi-zone control, or persistent problems, we recommend calling in a heating technician with the appropriate measuring equipment.

Summary at the end

Air venting and balancing the distributor is not a one-time task to be done once when the system is put into operation, but a recurring part of the operation of every floor heating system. A systematic approach - first thorough air venting of each circuit, then stabilizing the system, and finally precise flow balancing - is the only way to achieve even, quiet, and energy-efficient operation of the entire heating system. A quality cabinet with sufficient space for maintenance, properly laid step insulation, and regular annual inspections are an investment that pays off in the form of lower heating costs and fewer service interventions. If you are unsure about taking action yourself, do not hesitate to use our other articles from the Knowledge Center or to contact a specialist with the necessary measuring equipment.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us - we are happy to help.