Automatic air vent on a boiler or underfloor heating: how it works, when it is faulty, cap open or closed, and which one fits

Short answer: an automatic air vent is a small brass or plastic pot with a float, fitted on top of the boiler, the pump or the underfloor heating manifold. When air collects in it, the float drops and opens a needle valve that lets the air escape; when water comes in, it closes again. The cap must be open one turn, otherwise nothing can get out. When it fails, it either leaks water (float or valve fouled with limescale) or air stays in the system (valve scaled up, cap closed). A replacement costs € 9 to € 26 and you can fit it yourself, provided it is accessible from the outside; the air vent on the pump inside the boiler is often behind the casing.

How it works

Inside the housing is a float that operates a small valve at the top via a lever. When the housing is full of water, the float pushes the valve shut. When air collects at the top, the water level drops, the float sinks and the valve opens: the air escapes through the cap, water flows in, the float rises and the valve closes. This happens silently, hundreds of times per heating season. A hygroscopic air vent (such as the Caleffi 1/4", € 9.95) works differently: discs that swell on contact with water close the valve; when dry it is open. It is smaller and clogs less quickly, but vents slightly more slowly.

Air vents sit at the highest point of a circuit: on top of the heat exchanger or the pump in the boiler, on the flow bar of the underfloor heating manifold and sometimes on the highest radiator or in the loft. Air rises and collects there.

Cap open or closed?

Open, half a turn to a full turn. With the cap screwed shut, the air vent works like an ordinary plug and all the air stays in the system. Manufacturers sometimes supply it closed to prevent leaks during transport; it should be opened when the system is commissioned. If a boiler suddenly needs venting after a service and the radiators start gurgling, the cap is the first thing to check. The exception: if the air vent is leaking and you cannot replace it straight away, temporarily close the cap to stop the dripping.

When it is faulty

An air vent that drips almost always does so because of limescale. The system has then been topped up too often with fresh tap water, which brings new limescale and oxygen with every refill. Fix the leak that makes you top up, not just the air vent.

Which one fits

The connection is the size that counts: 3/8" male thread on most boilers, 1/4" on small air vents and 1/2" on manifolds and pipes. Branded boilers sometimes have their own version with a clip or bayonet on the pump; only the branded part fits there.

Looking for another brand or type? Select in the spare parts catalogue your appliance and then the type of air vent; the rating plate shows the way (how to read it).

Replacing it yourself

You can replace an air vent on the manifold, a pipe or on top of a boiler connection that is accessible from the outside yourself: switch the boiler off and let it cool, release the pressure via the drain cock to 0 bar (or, with a manifold, just isolate the manifold), unscrew the old air vent, screw in the new one with PTFE tape or the supplied seal, open the cap one turn, top up the system to 1.5 to 2 bar and keep an eye on the pressure during the first day. If the air vent sits on the pump inside the boiler behind the casing, the job is for the installer: on many models the casing has to come off and it sits close to the burner and wiring. What you can and cannot do yourself is explained in Which boiler part can you replace yourself?.

Frequently asked questions

Do I still need to bleed the radiators if there is an automatic air vent?

Yes. The air vent only removes the air that reaches it; air trapped in a radiator does not get out that way. Bleed the radiators at the start of the heating season and after topping up, see Bleeding a boiler.

How long does an automatic air vent last?

Five to fifteen years, depending on water quality. It lasts longest in a system that is rarely topped up.

Can I repair a leaking air vent?

Sometimes, by dismantling it and descaling the float and valve in vinegar. At a price of € 9 to € 26, replacing it is usually the better choice.

Where does the air keep coming from?

From fresh filling water (which contains dissolved air), from a leak where air is drawn in as the system cools, or from an empty expansion vessel that lets the pressure drop below zero at night so that air enters via air vents and seals. Persistent air is therefore a symptom, not the illness.