What's this part called and should the black bit be tight?

[DIYNotIan]

Sorry for the stupid vague question! I've tried to find out what that part pictured below is called but with no joy.

Anyhow, I was doing some work in the airing cupboard today and when I finished I noticed that the the little black plastic screw had unthreaded and come off. When I go to put it back on, do I tighten it all the way or only part way? I remember ages ago a plumber explaining something about that being used for hot air to escape, but it was a long time ago and I might have remember it wrong.

Many thanks

 

[Crofton Gas]

Automatic Air Valve and black cap should be shut.

 

[DIYNotIan]

Very quick and very helpful. Thanks a million

Just for curiosity's sake, with the black cup shut where does the air escape from? Or is it not as simple as that?

 

[heatservice]

Black cap needs to be loose or the auto air vent wont work.

 

[pete50]

LOL take your choice OP. Don't you just love em.

 

[Dan Robinson]

Cap should be lose. Only needs to be tight if fitted on negative side of the pump or if leaking.

 

[heatservice]

Im working on the assumption that your was loose that why you were able to knock it off so easily.

 

[Crofton Gas]

Auto Air Valves are for filling/commissioning only and once the air is out the system the black cap is shut to prevent leaks

 

[heatservice]

Auto Air Valves are for filling/commissioning only and once the air is out the system the black cap is shut to prevent leaks

Then it NOT an auto air vent, may as well fit thumb vents as much cheaper and they call them manual vents

 

[Dan Robinson]

Auto Air Valves are for filling/commissioning only and once the air is out the system the black cap is shut to prevent leaks

Most boiler manufacturers tell you to leave it open:

Caleffi make Hydrostatic caps too for extra protection.

 

[oilboffin]

Yes aav simples peeps .Bob

 

[dilalio]

Don’t leave the Tom Thumb ones loose though

 

[shambolic]

Auto Air Valves are for filling/commissioning only and once the air is out the system the black cap is shut to prevent leaks

Eh? How can it be auto then if shut FFS

 

[oilboffin]

Excellent point.Bob.