How to choose an air vent for your heating system
Why the air vent is a critical component of the heating system
Air in the heating system is one of the most common reasons why radiators do not heat evenly, the circulation pump makes noise, and the boiler reports pressure errors. Despite the fact that it is a completely common physical phenomenon, many installers and end users underestimate the selection and placement of air venting components. The result is a system that is technically well executed in terms of piping and bodies, but suffers from air pockets, corrosion, and unnecessary pump wear.
Air enters the system in several ways - during filling and refilling with water, through microbubbles dissolved in water, which are released when heated (the solubility of gases in liquid decreases with increasing temperature), by diffusion through plastic pipes without an oxygen barrier, during system topping up, or through leaks on the suction side of the circulation pump. The air then accumulates at the highest points of the system, in radiators, in floor heating distributors, or in the heads of risers. This is precisely why every functional heating system needs a well-thought-out air venting strategy - not just one valve "somewhere on top", but a logically placed system of manual and automatic components.
In this article, we will examine the types of air vents that exist, how they work from a technical point of view, where they are mounted, what to pay attention to when selecting the size and material, and how to prevent the most common operational problems - from constant dripping to complete valve failure after years of operation.
Physical principle - why air accumulates precisely at the top
Air has a lower density than water, so it naturally pushes upwards in a liquid column. In practice, this means that the most critical points for air venting are:
- the highest positioned radiators in the building (upper floors, attic rooms)
- the upper part of vertical risers
- local "humps" on the pipe route, where the distribution goes through the ceiling or bypasses an obstacle upwards and then drops again
- the highest point of the boiler or heat source, provided it does not have its own integrated air vent
- floor heating distributors, which are usually placed in a box higher than the actual loops in the floor
If an air bubble gets stuck in such a place, it creates a local "blocking" effect - water cannot freely flow through it, the flow through that circuit decreases or completely stops. This is manifested by a cold radiator at the top and a warm one at the bottom, gurgling in the body, or noisy pump operation, as the pump "battles" the air pocket in the impeller.
Schematic representation of typical air pocket locations in the system
Types of air vents and their operating principle
Manual air vent
This is the simplest and cheapest type - a mechanical valve with a slot for a screwdriver or with a wing handle, which is manually opened and closed. Manual air vent 1/2" is typically mounted directly into the radiator (into the plug on the top of the body) or at the top of the riser. The advantage is simplicity, low cost, and practically zero failure rate - it has no internal moving parts except the closure itself. The disadvantage is that it requires human intervention - someone must regularly walk around the radiators and manually vent them, which is a problem in practice, especially in larger systems or for less technically skilled users.
Manual valves are proven especially in living spaces on individual radiators, where the user themselves can feel when the body is not heating evenly and can use a key or screwdriver to release the air. In professional practice, the manual type is also used as an additional valve next to an automatic one - for example, during the first filling of the system, when the amount of air is too large for a small automatic valve to handle without clogging.
Automatic air vent
The automatic type works on the principle of a float mechanism. Inside the body of the valve, there is a float that floats on the water surface. As long as the space is filled with water, the float is in the upper position and presses on a needle valve, which keeps the valve closed. As soon as air accumulates at the top of the body, the water level drops, the float drops with it, and releases the needle valve - air escapes through a small outlet into the atmosphere. When the space is filled with water again, the float rises back up and the valve closes automatically.
Automatic air vent 1/2" (also available in nickel version for visible applications, for example in boiler rooms with aesthetic requirements or when mounted on visible piping) operates completely independently without user intervention. It is mounted at the highest points of the system - on the boiler, in the distributor, at the top of the riser, or in the collecting head of heating circuits. Thanks to this, the system continuously vents air without anyone noticing.
Comparison of the principle of manual and automatic valves
A detailed analysis of the differences, advantages and disadvantages of both types, including recommendations on when to use which variant, can be found in the separate article "Automatic vs manual air vent - differences and use."
Criteria for selecting an air vent
Size and connecting thread
The standard for domestic and smaller commercial installations is a 1/2" thread (DN 15), which covers the majority of radiator and manifold applications. In larger systems or industrial boiler rooms, we also encounter larger sizes, but in those cases, air venting is usually handled through separate air separators or microbubble separators, not just through a simple float valve.
Material of the body and internal components
High-quality air vents have a body made of brass, which is resistant to corrosion even after long-term contact with heating water. Internal floats are usually made of plastic or stainless material, and seals are made of EPDM rubber, which can withstand operating temperatures of heating systems (typically up to 110-120 °C). When selecting cheap, unmarked valves from unknown sources, there is a risk that plastic components will not be sufficiently heat-resistant and may become brittle or deformed over time, leading to leakage.
Maximum operating pressure and temperature
Common automatic air vents for home heating are dimensioned for an operating pressure of around 6-10 bar and a temperature up to approximately 110 °C, which comfortably covers standard domestic heating systems that typically operate at 1.5-2.5 bar and temperatures of 45-80 °C. In specific applications (solar systems, industrial circuits with higher temperatures), it is necessary to verify whether the selected type corresponds to the actual operating parameters - solar air vents, for example, must be able to withstand temperatures above 150 °C and have a different construction with a closing safety device to prevent the escape of the heat transfer liquid when boiling.
Finishing - surface treatment
For visible installations, such as in boiler rooms with aesthetic requirements, bathrooms with design radiators, or when mounted directly on visible piping, it is worth opting for a nickel finish instead of the standard brass surface treatment. Functionally, both variants are identical; the difference is purely in aesthetics and the surface's resistance to fingerprints and finger smudges.
Where to install air vents in practice
Proper placement is just as important as the selection of the type of valve. In everyday practice, I encounter these typical positions:
- On each radiator - either a manual valve in the body plug (the most common solution in apartments and single-family homes), or an automatic valve for larger radiators and in buildings with more demanding operation.
- At the top of each riser in multi-storey buildings - an automatic type is almost always recommended here, as risers are often inaccessible without a ladder or are hidden in shafts.
- On the boiler - most modern boilers have an automatic air vent integrated directly in the factory, but for older boilers or in cascaded connections of multiple sources, it is recommended to add a separate valve at the highest point of the assembly.
- On floor heating distributors - an automatic air vent in the distributor head is now standard, as floor heating circuits are long, have low slope, and air is difficult to push out naturally.
- At the highest points of horizontal distribution - everywhere where the pipe "jumps" over an obstacle upwards and creates a local high point.
- At circulation pumps - some pumps have their own air venting system, but for older models, it is worth having an automatic valve just before the pump's suction side to prevent cavitation.
Recommended valve placement scheme in a multi-storey system
Air venting and its connection to the thermostatic unit
In solid fuel boilers, air vents are often combined with another important component - a thermostatic unit with a pump and check valve. This unit ensures that water is returned to the boiler only after reaching the set minimum temperature (typically 55 °C, 61 °C or higher), thus protecting the boiler from low-temperature corrosion caused by condensation of flue gases on the cold walls of the heat exchanger.
Connecting these two topics is natural - the thermostatic unit and the air vent are usually installed in close proximity to the boiler, at the highest and most exposed point of the system, where heat and air from the circuit concentrate. In the product range, you can find a thermostatic unit DN 25 with an opening temperature of 55 °C and a variant DN 25 with a temperature of 61 °C - the choice of a specific temperature depends on the type of boiler and fuel, which is discussed in detail in the article "What thermostatic unit temperature is suitable for a solid fuel boiler." The question of the correct size DN 25 versus DN 32 is covered in the article "DN 25 vs DN 32 - how to choose the correct size of the thermostatic unit."
Installation - basic principles
When installing an automatic air vent, a few proven rules from practice apply:
- The valve is always mounted in a vertical position, with the float mechanism facing upwards - otherwise, the float will not function correctly and will not respond to the water level.
- It is recommended to install a shut-off ball valve before the valve, so that the valve can be replaced or cleaned without the need to drain the entire system.
- The valve must be placed at the highest point of the given section, otherwise air will have nowhere to rise and the air venting will be ineffective.
- When installing in living areas, it is advisable to consider that the valve may occasionally make a slight noise - therefore, it is not recommended to install it directly above electrical wiring, sensitive furniture, or electronics.
- The valve cap (protective cover) is left loosely fitted, never tightened tightly - it must serve as a simple cover against dirt, not as a sealing element.
A complete installation procedure, including the recommended order of steps, required tools, and typical errors, can be found in the article "Installation of an automatic air vent - procedure and placement in the system." Similarly, when installing a thermostatic unit with a pump, I recommend studying a separate guide in the article "Installation of a thermostatic unit with a pump and check valve," as this component has specific requirements for flow direction and pump orientation.
Step-by-step installation procedure for an automatic air vent
Why the air vent is leaking or has stopped working
One of the most common service interventions I encounter in practice is a complaint about "a vent that constantly drips" or, conversely, "a valve that does nothing and the radiator is cold". Both problems usually have a simple explanation:
Constant dripping - most often caused by dirt or limescale deposits on the sealing needle, which prevents the closure from seating properly. The same can happen after long-term operation, when the rubber seal wears out. The solution is to disassemble, clean or replace the valve - given the low cost of automatic valves, it is almost always worth simply replacing the unit with a new one rather than spending time on complicated cleaning.
The valve does not release air at all - often caused by clogging of the outlet nozzle with impurities from the water (sediment, fragments of seals, deposits), or by mechanical blocking of the float, which "sticks" in the upper position and does not react to the drop in water level. In systems with a higher content of impurities (older systems without filtration, after major work on the piping), it is advisable to also install a dirt filter in the system, rather than relying solely on the air vent.
A detailed analysis of individual causes, including specific diagnostic steps, can be found in the article "Why the air vent leaks or does not work."
Regular maintenance and inspection
Although an automatic air vent is designed to operate without intervention, it is recommended to perform a visual inspection once a year (ideally before the heating season) - check for signs of significant water leakage around the valve, or slightly loosen the protective closure to verify that the mechanism is not stuck or clogged. For manual valves on radiators, a simple check by bleeding at the beginning and in the middle of the heating season is recommended - it takes just a few seconds with a key or screwdriver for each radiator.
As part of broader maintenance of control and safety valves in the system (pressure relief valves, expansion vessels, thermostatic units), it is advisable to proceed systematically - a detailed overview of recommended intervals and procedures can be found in the article "Maintenance and inspection of control and safety valves in heating."
Overview of recommended inspection intervals
| Component | Recommended inspection interval | What to check |
|---|---|---|
| Automatic air vent | 1x per year | Water leakage, float functionality, clogging |
| Manual air vent on radiator | 2x per season | Presence of air, tightness after closing |
| Thermostatic unit | 1x per year | Functionality of the return valve, pump operation |
| System pressure overall | monthly | Pressure drop indicating a leak or excessive air venting |
Practical scenarios from everyday installation practice
Scenario 1 - a two-story house with a domestic heating system. A typical situation: the owner complains that the radiators on the upper floor are not heating properly, while everything is fine in the ground floor. After an inspection, it turns out that the riser to the upper floor has no air vent at all and air gradually accumulates precisely in the upper part of the piping. Solution - installation of an automatic air vent at the top of the riser, after which the problem usually disappears immediately.
Scenario 2 - a solid fuel boiler without thermostatic control. Common in older installations where the boiler was replaced, but the original valves were left unresolved. The result is a boiler that after a few seasons shows signs of soot and corrosion on the heat exchanger due to low return temperature. Adding a thermostatic unit (for example, with an opening temperature of 61 °C for most standard wood-fired boilers) together with a properly placed air vent at the highest point of the installation significantly extends the life of the heat source.
Scenario 3 - underfloor heating with "cold loops". After installation of a new manifold, one of the loops does not heat evenly. The cause is almost always residual air trapped in the loop after filling, which did not escape due to insufficient flushing. The solution involves thorough flushing of individual loops through the closing valves of the manifold and adding a functional automatic air vent directly on the manifold head, rather than relying only on a single central vent in the boiler room.
Scenario 4 - noisy pump after refilling the system. After adding a large amount of water (for example, after repairing a leak), the pump starts to vibrate and "whine" loudly. This is almost always due to air being sucked into the impeller. In this case, temporarily increasing the flow, tapping the pump body, and especially a functional automatic air vent just before the suction side of the pump, which captures the air before it reaches the circulation mechanism, helps.
Common questions about air vents
Does every radiator need its own air vent?
Yes, in practice it is recommended that each radiator has at least a manual air vent, because even in a well-designed system, air can locally accumulate in individual radiators independently of the rest of the piping. In more modern or larger installations, automatic vents are increasingly preferred directly on the radiator, especially if the radiator is hard to access.
Is an automatic vent more expensive to operate than a manual one?
No, there is no difference in operation, since it is a mechanical, not an electrically powered component. The difference is only in the purchase price, which is slightly higher for the automatic type, but this investment quickly pays off in terms of saved time and comfort, since you do not have to worry about it.
Can an automatic vent cause water leakage in a place where you don't want it?
Yes, which is why proper placement is important - always keep in mind that the valve can occasionally drip slightly, especially when releasing a large amount of air after filling the system. For this reason, it is recommended to avoid installation directly above sensitive surfaces, electrical wiring or furniture, and in case of need, install a small drip tray under the valve or direct it into a drain pipe.
How often should an automatic air vent be replaced?
The lifespan of a quality brass valve with an EPDM seal typically ranges from 8 to 15 years under normal operation, depending on the quality of the heating water and the amount of impurities in the system. At the first signs of leakage or malfunction, it is recommended to simply replace it with a new one due to its low purchase price.
Can an air vent be installed in an existing system without draining all the water?
When installing on a new branch, it is necessary to drain that section or at least reduce the pressure and close the supply. If a T-branch with a closing valve is already installed in advance for future additions, it is possible to install the valve without major intervention into the rest of the system.
Is air venting in any way related to the thermostatic unit on a solid fuel boiler?
Directly not, these are two separate functions - the air vent deals with the presence of air, while the thermostatic unit protects the boiler from low-temperature corrosion. In practice, however, both valves are usually installed in the same area near the boiler, so it is recommended to address them together when planning the boiler room layout.
Summary - what not to forget when choosing
Choosing the right air vent is not about finding the "best" type, but about selecting the appropriate combination of manual and automatic components according to the characteristics of the specific system and placing them correctly in the highest points of the piping. For a typical family home, a proven combination is an automatic air vent on the boiler, risers and floor heating manifolds, supplemented by manual vents on individual radiators. For solid fuel boilers, the issue of air venting should be addressed together with the issue of protection against low-temperature corrosion using a thermostatic unit with the correct opening temperature and brightness.
When choosing a specific product, it is worth reaching for proven brass versions with high-quality EPDM sealing, dimensioned for standard domestic heating operating parameters (6-10 bar, up to 110 °C), and not to skimp on details such as a closing valve before the valve, which significantly simplifies any future service without the need to drain the entire system.
Do you have a question on this topic?
Having trouble making a decision or dealing with a specific situation in your household? Write to us - we will be happy to advise you.