bottle air vent ???

Guys, How do these bottle air vents work? Tried to search on the web and can find nothing. I have just bought one and when I shake it something solid rattles inside. I am replacing the one that is seized up and sits at the top of a piece of pipe in my airing cupboard. Does the rattling bit press against something internally when full of water?

It has 3/8 connection and a sort of twist valve at the top (like the top of a biro pen). Should that twist valve bit be screwed tight or loose?

Thanks, Steve

The rattly thing is a float, which seals the air outlet when the bottle bit is full of water. When it fills with air, the float drops, air is released.

The twisty bit is a cap, like on a tyre valve. Usually it's left open for a week or two after filling to allow all the air to escape and then screwed shut. If the system is installed correctly and has inhibitor in it, you should not get any appreciable amount of air inside the pipes. The AAVs can stick open, if they get a bit of crud under the valve, so it's not advisable to leave them permanently open.

Kaiser Sose formulated the question :

The rattle is the float inside. They vent until the float rises on the column of water, then seal against the outlet at the top. You can remove the cap and prod the float down with small probe to check they are not stuck.

They work much like a carbuterttor float valve but are used in reverse. If they contain liquid the float shuts off the vent.