Installation of an automatic air vent - procedure and placement in the system
Introduction – Why the correct installation of the air vent is just as important as its selection
The automatic air vent may seem like an inconspicuous fitting at first glance – a small brass valve with a float inside, often costing only a few euros. In practice, however, it determines whether the heating system operates quietly, efficiently, and without faults, or whether air keeps accumulating in it, the boiler makes noise, the circulation pump cavitates, and the radiators do not heat up. After dozens of implementations, service interventions, and complaints that technicians encounter in practice, one thing becomes clear: most of the problems with air venting are not caused by a faulty valve, but by its incorrect placement or careless installation. In this article, we will thoroughly examine where the valve physically belongs, how to correctly install it step by step, what errors are most commonly repeated in the field, and how to avoid them.
If you are also dealing with the selection of the appropriate type of valve, we recommend connecting this article with the topics "How to choose an air vent for the heating system" and "Automatic vs. manual air vent – differences and use," where the choice between automatic and manual types is discussed. Here we focus exclusively on installation and placement in the real system.
How an automatic air vent works – briefly, but important for installation
To make it clear why placement is so critical, it is necessary to understand the principle of operation. Inside the body of the valve is a float (a plastic or brass ball) that floats on the water surface. When air accumulates in the upper part of the valve body, the water level drops, the float drops with it, and mechanically opens a needle valve through which the air escapes into the atmosphere. When the body is filled with water again, the float rises and the valve closes, preventing water from escaping.
From this principle, two essential conditions for functionality follow:
- The valve must be mounted precisely in a vertical position (upright, with the neck upward) – otherwise, the float cannot move up and down properly and the mechanism will not function.
- The valve must be placed in a location where air physically accumulates in the system – that is, at the highest points of the system, because air is lighter than water and always rises upward.
These two conditions – orientation and placement – are the core of this entire article. Let's take a closer look at them.
Schematic diagram of the float valve operation principle
Exactly where the automatic air vent belongs in the system
In practice, the same scenario repeats itself: the investor or even an inexperienced installer mounts the air vent "somewhere on the pipe where it fits," for example, next to the boiler in the mechanical room, simply because it is easily accessible. The result is that the valve may look correct at first glance, but it never starts working in the system, because air simply does not reach it – air moves upward, not sideways or downward.
The correct locations for installing an automatic air vent in a typical heating system are:
- The highest point of the entire heating system – if the house has multi-story pipe distribution, it is usually the pipe on the top floor or in the basement.
- The upper part of collectors/distributors – in floor heating, the valve is mounted on the top edge of the distributor, where air accumulates from the circuits.
- The highest point of the boiler or on the boiler head – most modern boilers already have an air vent integrated by the manufacturer, but in older boilers or external bodies, it is added separately.
- The top part of the storage tank or accumulator – in systems with an accumulator tank, air accumulates precisely here.
- The heads of circulation pumps – many pumps have their own air vent in the head, which is important to check when starting up the system.
- The tops of rising pipes and local "air pockets" – places where the pipe changes direction from horizontal to vertical and creates local peaks (for example, bypassing door arches, beams, etc.).
Schematic diagram of air vent placement in a typical system
Step-by-step installation of an automatic air vent
The following procedure is based on common practice when installing on a new system as well as when adding to an existing layout.
1. Selecting the location and preparatory work
First, identify the highest point of the system or a local air pocket. In a new construction, this is already resolved in the design – air vent points are marked in the heating system diagram. In a renovation or additional installation, it is necessary to thoroughly inspect the system, ideally with the construction drawings of the piping, because the visually "highest point" in the interior may not be the same as the highest point of the piping (for example, if the piping runs in a ceiling cavity and rises elsewhere than expected).
2. Selecting the appropriate branch or fitting
A branch with an internal or external thread (most commonly a T-piece or a drilling fitting) is mounted on the pipe, onto which the valve is then screwed. In floor heating distributors, a prepared neck on the top edge of the distributor is used directly – this is usually standard equipment.
3. Mounting the valve in a vertical position
This is the most commonly underestimated step. The valve must be installed so that its vertical axis points exactly upward (a deviation of a few degrees is acceptable, but a valve lying on its side or at an angle will not function reliably, as the float will not move properly). It is recommended to use a spirit level during critical installations (e.g., in mechanical rooms with multiple circuits) to ensure accuracy.
4. Use of sealing compound
Teflon tape or hemp fiber with sealing paste is applied to the thread. Brass 1/2" threads are standardized, but the tightening torque must not exceed the appropriate limit—brass is a softer material than steel, and over-tightening may cause the valve body or branch to crack.
5. Installation of the cap and accompanying accessories
Most automatic valves are supplied with a plastic protective cap that covers the outlet. This cap must be installed—it prevents water from spraying during venting and also prevents dirt from entering the mechanism from the outside.
6. Initial filling of the system and functionality check
After installation, the system is filled with water, and the valve is left temporarily open during filling (on some types, it is necessary to manually remove the closure screw at the top) to allow free air escape from the pipe. During the first few days of operation, it is normal for the valve to react several times in automatic mode and release residual air, which gradually "shakes out" of the entire system.
Installation procedure – step diagram
Recommended sizing range and selection of a specific valve
For standard single-family homes and smaller apartment systems, a 1/2" thread is the norm, which suits the vast majority of applications on radiator circuits and underfloor heating manifolds. In the product range, you will find the Automatic air vent valve - 1/2", which is a typical solution for installation at the highest point of the system or on a manifold. If a more aesthetically pleasing finish is required—for example, in a visible installation in a living area, on a manifold in a bathroom, or in a neatly designed mechanical room—there is also the Automatic air vent valve - 1/2"; nickel available, which has a surface finish matching the appearance of nickel-plated fittings and complements, for example, nickel-plated manifolds or designer radiators.
When deciding between an automatic and a manual type—such as the Manual air vent valve - 1/2"—consider the accessibility of the installation location and the frequency of required intervention. An automatic valve is suitable where continuous and maintenance-free air venting is needed (e.g., in inaccessible areas, on floors, in shafts), while a manual valve is typically used directly on radiators, where the user manually vents it during commissioning or during service. A detailed comparison can be found in the separate article "Automatic vs. Manual Air Vent Valve – Differences and Applications."
Installation in the context of a thermostatic unit and solid fuel boiler
In systems with a solid fuel boiler (wood, coal, pellets in combination with a storage tank), other regulating valves are often addressed simultaneously—primarily the thermostatic unit, which protects the boiler from low-temperature corrosion by ensuring a rapid increase in the return water temperature to a safe level. In such configurations, air accumulates not only in the risers but also in the upper part of the valve itself and near the circulation pump, which is part of the thermostatic unit.
In the product range, you will find the Thermostatic unit with pump and check valve - DN 25; 55 °C and Thermostatic unit with pump and check valve - DN 25; 61 °C. During installation, it is advisable to place the air vent valve at the nearest higher point downstream of this unit in the flow direction, so that air released during the first filling or topping up of the system does not get trapped in the circulation pump, where it can cause dry running and damage to the bearing. Choosing the appropriate opening temperature of the thermostatic unit (55 °C, 61 °C, 72 °C, or 80 °C) is a separate topic covered in the article "What Thermostatic Unit Temperature Is Suitable for a Solid Fuel Boiler," and the choice of the correct size is explained in the article "DN 25 vs DN 32 – How to Choose the Correct Size of a Thermostatic Unit." The installation procedure for this fitting is then described in the article "Installation of a Thermostatic Unit with Pump and Check Valve."
Most common installation errors in practice
During service calls and complaints, essentially the same errors are repeated over and over. Here is an overview of the most common ones, listed in order of frequency in the field:
| Error | Consequence | Correction |
|---|---|---|
| Installation in a tilted or horizontal position | The float does not work properly, air is not vented | Reinstall the valve in a vertical branch |
| Placement at a lower point in the system | Air does not reach there at all, the valve is "useless" | Move to the actual highest point |
| Missing protective cap | Water spraying, dirt entering, corrosion of the mechanism | Install the original manufacturer's cap |
| Over-tightened thread during installation | Cracks in the body, leakage | Observe the appropriate tightening torque, use suitable wrenches |
| No shut-off valve below the air vent | Impossible to replace the valve without draining the entire system | Install a ball valve between the pipe and the air vent |
| Clogging of the mechanism by dirt from a new system | The valve constantly leaks or does not vent | Clean the system, or replace the valve |
If your valve appears to be non-functional—either constantly leaking or not reacting at all—we recommend the separate article "Why Does the Air Vent Valve Leak or Not Work," where this issue is discussed in depth, including the diagnosis of specific causes.
Why it is important to install a shut-off valve under the valve
A very practical but often neglected tip from the field: it is worth installing a small ball shut-off valve under every automatic air vent valve. The reason is simple—the valve mechanism has a limited lifespan (typically a few years depending on water quality and operating conditions), and without a shut-off element, its replacement would mean draining the entire system, which is unnecessary and time-consuming, especially in larger water volumes (e.g., systems with a 1000-liter or larger storage tank). With a shut-off valve, the replacement takes only a few minutes without affecting the rest of the system.
Schema of correct installation with a shut-off valve
Installation for different types of systems – practical scenarios
Scenario 1 – classic radiator system with a circulation pump
In a typical family house with radiator heating, an automatic air vent is installed at the highest point of the main riser, usually near the boiler or on the highest positioned radiator loop on the floor. If the system is also equipped with separate air venting at the boiler, it is advisable to combine this with the integrated air vent in the boiler body – it is not necessary to duplicate them unless the project specifically requires it.
Scenario 2 – floor heating with a manifold
In floor heating, the most common location is the top edge of the manifold (especially the return branch), where residual air from individual loops accumulates after the system is filled. It is important to bleed each loop individually using the regulating valves on the manifold during the first filling, while also having a functional automatic vent on the collector to maintain the system free of air in the long term.
Scenario 3 – system with a storage tank and solid fuel boiler
There are typically multiple points for air venting – on the storage tank itself (top part), on the boiler, and possibly near the thermostatic unit. In these larger systems with several hundred to a thousand liters of water, it is common for complete air venting to take several days of normal operation after the initial filling, during which the automatic vent repeatedly "works" and gradually releases residual air that is gradually released from the water during heating.
Scenario 4 – solar system and systems with antifreeze mixture
In solar circuits with a heat transfer fluid (glycol mixture), air venting is handled differently – here, the air vent is often manually closed using a ball valve during normal operation to prevent gradual evaporation and dilution of the antifreeze mixture by steam at high temperatures in summer months, when the collector can reach extreme temperatures above 100 °C. In these systems, air venting is carried out intentionally only during filling and service interventions.
Prevention of problems and long-term reliability
Correct installation is only the first step – long-term functionality also depends on the quality of the water in the system, regular inspection, and the condition of auxiliary valves. Poor or hard water causes deposits in the mechanism, which can cause the float to stick in the upper position (the valve then does not vent air) or, conversely, in the lower position (the valve constantly leaks). Regular visual inspection, at least once a year during the seasonal preparation of the system for heating, is a recommended standard. A detailed maintenance guide can be found in the article "Maintenance and inspection of control and safety valves in heating."
It is also worth noting that the quality of the water filled into the system directly affects the lifespan of not only the air vent, but also other valves – circulation pumps, thermostatic units, and the boiler itself. It is recommended to fill the system with treated water (softened or demineralized) where possible, which significantly extends the trouble-free operation of all components.
Comparison of inspection frequency by system type
Frequently asked questions about the installation of an automatic air vent
Must an automatic air vent be mounted exactly vertically?
Yes, this is a necessary condition for proper function. The float mechanism inside needs to move vertically according to the water level. If the valve is mounted at an angle or horizontally, the float does not respond correctly and the valve may not vent air or may leak uncontrollably.
Is it necessary to install a shut-off valve under the air vent?
It is not a strict norm, but it is strongly recommended in practice. Without a shut-off valve, it is not possible to replace the valve without draining the entire system, which is an unnecessary complication and time loss during a routine service replacement, especially in systems with large volumes of water (e.g., storage tanks).
How many air vents does a typical family house need?
It depends on the layout of the piping. In a simple radiator system, one automatic vent at the highest point plus manual vents on individual radiators is often sufficient. In a combined system with floor heating, a solid fuel boiler, and a storage tank, there may be a need for 2 to 4 automatic vents at different high points in the system.
Can an automatic vent be mounted directly on the boiler?
Most modern boilers already have an air vent integrated from the factory. In older boiler types or in external assemblies (e.g., a solid fuel boiler with a separate storage tank), a separate vent is mounted at the highest point of the boiler body or immediately after the boiler on the outlet pipe.
Why does a new vent drip water immediately after installation?
This is completely normal during the first filling of the system – the vent is releasing air that is present in large quantities in the system, and a small amount of water may also escape along with it until the system is fully bled. If dripping persists longer than the usual start-up period (a few days), we recommend checking the tightness of the mechanism according to the instructions in the article "Why the air vent leaks or does not work."
Is there a difference in installation between a new build and an additional installation in an existing system?
In principle, no – the installation procedure is the same. The difference lies in practical implementation – in an existing system, it is necessary to partially drain the system before installation or to use a tapping T-piece with a shut-off element to prevent a large water leak during the work. In new builds, bleed connections are planned during the design phase of the piping layout.
Conclusion
The installation of an automatic air vent is a simple operation that, however, requires respecting two basic rules – vertical orientation and placement at the actual highest point of the system or in a local air pocket. Neglecting these principles is the most common cause of a heating system remaining "air-filled," noisy, and inefficient even after the installation of a high-quality valve. We recommend combining the automatic vent with a shut-off valve for easy future replacement, paying attention to the quality of thread sealing, and in systems with a solid fuel boiler and a storage tank, also considering the simultaneous installation of a thermostatic unit, which protects the boiler from low-temperature corrosion. The full range of control and safety valves, including automatic and manual air vents, can be found in the category of control and safety valves, where individual types and surface finishes can be compared according to the specific requirements of your system.
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.