An automatic air vent, often shortened to AAV or auto air vent, is a self-acting bleed valve. It sits at points where air gathers and releases that air without anyone touching a radiator key. When it works you never notice it, and when it fails you get either trapped air or a slow drip.
What an automatic air vent is, and why air gets into heating systems
A sealed heating system should be full of water, but air and gas find their way in all the time:
- Fresh water brings dissolved air. Every top up through the filling loop adds water containing air, which comes out of solution as the water heats.
- Draining and refilling. Changing a radiator or valve lets air in, and some always stays behind.
- Corrosion. Steel radiators corroding from the inside give off gas as well as sludge.
- Small leaks. A weeping joint can draw air in as the system cools and contracts.
Air collects at high points and in the pump. In the pump it can stop circulation entirely: Worcester Bosch's Greenstar 8000 Life manual describes fault 281 as "pump stuck or running in air". Manufacturers take air seriously at start-up too. Ideal's Logic Combi2 installation and servicing instructions (UIN 236183) say the boiler must not run "before it has been fully vented of air", and Worcester's Greenstar i ErP user instructions (6 720 806 946) show status 357, "Primary heat exchanger air venting program active", which runs for about 100 seconds.
How an automatic air vent works
Inside the small brass or plastic body is a chamber with a float. The parts work together like this:
- When the chamber is full of water, the float rises and holds a needle valve shut at the top.
- When air bubbles collect in the chamber, the water level drops, the float falls and the needle valve opens.
- Air escapes through a tiny outlet under the cap until water fills the chamber again, lifting the float and closing the valve.
- Many vents sit on an isolating or check valve, so an engineer can remove the vent head without draining the system.
The small cap on top is not the valve itself. It is a dust cap over the outlet. If it is screwed down tight, air cannot leave, and the vent stops doing its job.
Where automatic air vents are fitted
- On the boiler pump. Most combi and system boilers have one on the pump body inside the casing. Baxi's EcoBlue System installation and service manual (document 7219708-03) shows a "pump with automatic air vent", and Ideal's Logic Combi2 manual says the vent head is retained in the pump body.
- At high points on pipework, where air naturally rises, such as the top of a vertical pipe run in a loft or airing cupboard.
- Near cylinders and on primary pipework in some system and regular boiler installations.
- On underfloor heating manifolds, usually one at the end of each manifold bar.
If you are looking for the boiler air vent on a combi, you will not see it from outside. It is behind the front panel, which only an engineer should remove.
Should the cap be open or closed?
In normal use the cap should be loose, so air can get out. Boiler manuals give that instruction to installers at commissioning: Baxi's EcoBlue System manual says to "ensure that the cap on the automatic air vent on the pump body is opened", and Ideal's Logic Combi2 manual tells the installer to "loosen the auto air vent dust cap" when filling. Some installers close the cap again if a vent starts weeping, as a stopgap.
On a vent inside the boiler, leave the cap alone. On an external vent on pipework, you can look at whether the cap is loose or screwed down and note it for your engineer. Only turn it if the instructions for that vent or your installer's handover notes say the owner may, and then only the small cap, never the vent body. If water appears, close it and call an engineer.
Signs of a faulty automatic air vent
| What you notice | Likely problem |
|---|---|
| Drips, a damp patch or white or green crust around an external vent | The needle valve is not sealing, often because of debris or scale |
| Pressure falling slowly with no leak at radiators or pipes | A vent weeping water, sometimes inside the boiler where you cannot see it |
| Gurgling or trickling sounds from the boiler or pipes | Air not escaping because the vent is stuck shut or the cap is closed |
| Radiators repeatedly cold at the top and full of air | Air building up faster than it is being vented |
| Pump noise or pump and circulation fault codes | Air sitting in the pump body |
A brief hiss of air from a vent is normal and means it is working. Water coming out is not normal, and the usual answer is a new vent head rather than a repair. Our guides on a gurgling boiler and a boiler losing pressure cover the other causes of those two symptoms.
Bleeding radiators versus automatic venting
An automatic air vent only clears air that reaches it. Air trapped at the top of a radiator stays there until you release it with a radiator key, so the two work side by side. If radiators are cold at the top, bleed them first using our radiator bleeding guide, then check the gauge and top up through the filling loop if it has dropped below the level in your manual (our boiler pressure guide explains the readings). If you find yourself bleeding the same radiators every few weeks, something is letting air in or making gas, and an engineer should look at the whole system rather than just the vent.
Replacement, cost and protecting your home meanwhile
The automatic air vent on a combi or system boiler pump sits inside the boiler casing, next to electrical and gas components. Opening its cap or replacing it there is work for a Gas Safe registered engineer. Do not remove the casing to reach it.
External vents on pipework can be swapped by a heating engineer, usually on the isolating valve beneath, and it is not a job that calls for draining a sealed system yourself. A vent head is a small part, so the swap is normally priced like a minor repair, typically £100 to £180, plus a call-out of typically £60 to £150 if it is a separate visit. If water from a vent has reached other parts inside the boiler, an internal leak repair is typically £100 to £300.
While you wait:
- Put a towel or shallow tray under a dripping external vent and check it daily.
- Keep an eye on the pressure gauge and top up to the level in your manual if it falls.
- If water is dripping from inside the boiler casing, switch the boiler off at its controls and the fused spur, keep electrics dry and call an engineer.