What is an air vent (bleed valve)?
An air vent is a small valve or fitting that allows trapped air to escape from an installation. Air inside a water-filled pipe disrupts the flow and the heat transfer; the vent gives that air a way out.
Air almost always finds its way into a system: when filling, when topping up with fresh water, or because dissolved gases come out of the water over time. Since air is lighter than water, it collects at the highest points of the installation — which is exactly where an air vent belongs.
Where you find them
- Radiator — the familiar bleed valve at the top. A radiator that stays cold at the top while it is warm at the bottom is full of air.
- Underfloor heating manifold — automatic vents on the manifold, usually at the highest point of the circuits.
- Boiler and pipework — quick-release automatic vents at high points and on the boiler itself.
- Drainage — here venting means letting sewer gases out through the stack and the roof terminal. Its counterpart is admitting air, so that water traps are not siphoned dry.
How to bleed a radiator
- Switch the heating off and wait about half an hour so the water settles and the air rises.
- Start on the lowest floor and work upwards.
- Hold a cloth or a small container under the valve and open it carefully with a bleed key.
- Close the valve as soon as water rather than air comes out.
- Afterwards check the system pressure on the boiler and top up to the pressure specified by the manufacturer.
Common mistakes
- Bleeding while the pump is running — the air simply circulates and cannot be removed.
- Forgetting to restore the pressure, so the boiler goes into fault mode.
- Having to bleed again and again: that points to a leak or a failed automatic vent, not to normal maintenance.
Related terms
- Radiator
- Roof penetration
- Vent pipe
- Balanced ventilation
- System pressure
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