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Automatic air vent vs bleed key

Updated: 23 October 2026

The top of the bathroom radiator is cold, the boiler is gurgling, and a neighbour says an automatic vent means you can throw the bleed key away. That is only true for the pocket of air the new valve actually sits on. An automatic air vent vs bleed key choice starts with where the air is. A float vent on a pipe, a radiator key, and a hygroscopic cartridge are three different tools. The product rows are on the [automatic air vent comparison](/hub/diy-supplies/automatic-air-vent). Products Compared used maker notes and amazon.co.uk listings from 19 October 2026. If you buy through our links, we may earn a commission at no extra cost to you.

The top of the bathroom radiator is cold, the boiler is gurgling, and a neighbour says an automatic vent means you can throw the bleed key away. That is only true for the pocket of air the new valve actually sits on.

Our top picks

Flamco Flamco Flexvent 3/8 inch

Best overall

Flamco Flamco Flexvent 3/8 inch
8.9/10about £16.79
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Altecnic Altecnic Robocal 15 mm

Best value

Altecnic Altecnic Robocal 15 mm
7.5/10about £10.69
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mizikuu mizikuu 1/2 inch bleeders (3)

Alternative

mizikuu mizikuu 1/2 inch bleeders (3)
5.4/10about £10.99
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On this page
  1. 1.Where the air actually is
  2. 2.Bleed key
  3. 3.Float vent on the pipe
  4. 4.Hygroscopic radiator bleeder

Where the air actually is

Air in a sealed heating system collects at high points. If the upstairs circuit is noisy and a loft pipe or manifold is the highest tee, the pocket is in the pipework. A float vent there can let the air out, then close when water reaches the float. If only one radiator is cold across the top, the pocket is in that panel. A valve in the loft does not reach inside the panel. Even with a new float vent, you bleed the radiators by hand on a first fill. Vaillant's glossary splits the hardware into a key point, a float valve, and a swelling element. Sizes are on the automatic air vent buying guide.

Bleed key

A bleed key opens a small screw on the radiator. Air hisses, then water follows, and you close the screw. The joint is shut when you walk away. It suits one cold radiator, a new system on its first heat-up, and any panel you can reach. It does not add a float that can stick open later. It is a poor plan when the air returns every week at the same high pipe. It is also a poor plan on a radiator you cannot reach, behind a fixed bath panel, if the only access is that hidden screw. Choose the key when you can see the bleed point and the air is occasional. Keep the key after you fit a pipe vent.
Flamco Flamco Flexvent 3/8 inch

Testsieger

Flamco Flamco Flexvent 3/8 inch
8.9/10about £16.79
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Float vent on the pipe

A float vent belongs upright on the high pipe, the manifold, or the towel-rail tee. The float lifts a needle onto a seat when water arrives. Flamco wants the cap loosened while you fill, then nipped hand-tight. Caleffi's Robocal sheet wants the plug left about one turn open. Reviewers of the Altecnic and the Wärmer found a tight cap trapped the air. One Wärmer reviewer left the red top slightly open and got water. Choose a float vent when the air is at a pipe high point you can fit vertically. In this set the top pick for a 3/8 inch port with a shut-off is the Flamco, and the best value on 15 mm is the Altecnic. Do not fit it inside a gas boiler case yourself.

Hygroscopic radiator bleeder

A hygroscopic bleeder uses discs that swell in water and shrink in air. It sits in a radiator bleed point, including where a float would be on its side. The hygroscopic cap is tightened. Aladdin's Autovent is this sort of cartridge and is not in this comparison. The mizikuu three-pack is sold as 1/2 inch radiator bleeders with a floating seal, not swelling discs. UK texts do not confirm the automatic setting. Choose a hygroscopic cartridge when the radiator top is inaccessible, the key when you can reach the screw, and the float when the problem is the pipe high point.

Common questions

If I fit one auto vent, can I throw away the bleed key?
No. The float vent clears the high point where you fitted it. A radiator that is cold across the top still needs its own bleed on the first fill and whenever that panel collects air. Keep the key in the airing cupboard. The vent does not commission the whole system by itself.
Why is only the top of one radiator cold?
The air is in that panel, not in the loft pipe. Open the bleed screw with a key until water follows the hiss, then close it. A new 15 mm vent on a different pipe will not push that pocket out. If every radiator on a run is cold, the fault may be a lockshield, a pump, or sludge, which a vent does not cure.
What is a hygroscopic bleeder?
It is a radiator cartridge whose discs swell shut when wet and open when they dry in air. It can sit where a float cannot, because a float must be vertical. The cap on that type is tightened, unlike a Caleffi float cap that stays open a turn. Aladdin makes one. None of the amazon.co.uk vents in this comparison are that cartridge.
Can a float vent lie on its side on a radiator?
No. The float has to rise onto the seat. On its side it will not close reliably, and you can get a drip or no venting at all. Use it upright on the pipe. For a horizontal radiator plug, use the key, or a hygroscopic bleeder made for that orientation, which is outside this set.
Will an auto vent drop my boiler pressure to zero?
A vent that is working should let air out and then stay dry, so the pressure should settle where you set it. A vent that sticks open, or a cap left too far open on a model that then passes water, will dump water and the gauge will fall. That is a fault, not the design. If the gauge falls and the vent is dry, look elsewhere before you blame the cap.
Is the mizikuu three-pack the same as an Aladdin?
No. The mizikuu listing is three 1/2 inch radiator bleeders that claim a floating seal, at about £10.99, with 3.8 from 52 ratings. The UK comments in the file do not show a reliable automatic seal. Aladdin's product is a hygroscopic washer. They are different mechanisms, and only the mizikuu is in this set.
When do I still call someone about air that keeps returning?
Call when the right bleed point is already in use and the air is back within days, when the gauge will not hold, or when the noise is inside the boiler. A float vent cannot fix a leaking exchanger, a failed expansion vessel, or a joint on the suction side of the pump that is pulling air in.

Related pages

How to fit an automatic air vent

Automatic air vent leaking

Automatic air vent

Best automatic air vent: top pick and best value 2026

Automatic air vent buying guide: match the joint first

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026.

  1. Altecnic, how an automatic air vent works (retrieved 19.10.2026)
  2. Altecnic 502 Robocal data sheet, October 2024 (retrieved 19.10.2026)
  3. Flamco Flexvent installation manual (retrieved 19.10.2026)
  4. Flamco catalogue chapter, order code 27750 (retrieved 19.10.2026)
  5. Caleffi Robocal 01033 (retrieved 19.10.2026)
  6. Gas Safe Register boiler appliance guide (retrieved 19.10.2026)
  7. Vaillant 104521 superseded, Heating Spare Parts (retrieved 19.10.2026)
  8. BES Robocal, WRAS 2012359 (retrieved 19.10.2026)