Heating System Device -What is it?

Known as an air bottle. It's an automatic vent to remove air from the system.

It's an automatic bleeder. You undo the knurled nob on the top slightly and it will let air out of the system. The things eventually start leaking if left permanently open, so the nob is screwed shut a couple of weeks after filling the system. You can open it occasionally afterwards to see if any more air comes out.

The staining would suggest your one leaks, although slight moisture around the top when open is not uncommon.

It's an automatic air vent.

The purpose is to allow air to automatically vent from a high point in the system.

Normally, it should not need attention. If you have refilled the system, or if there is some other reason for air or gases to be in there (the latter perhaps being hydrogen due to corrosion), then these should automatically be released through this valve. Inside the brass chamber there is a little float which when it drops because there is collected air/hydrogen, opens a port in the top releasing it. The float then rises and closes the port.

The red cap is there to allow the exhaust port to be closed off. You can untwist it and the valve should then release any collected gases. These can be left loose after refilling etc. for a few days, but then closed.

If the system is working OK and the water is being heated properly, there is no reason to touch it.

Thank you all for your very helpful replies.

David

There is a small red button under the screw cap. Does this have any function?

David

Adagio was thinking very hard :

It is an automatic vent or bleed valve...

A small float in the chamber drops when there is no water in the chamber, permitting the air to escape. As the water rises it then closes again. It is only able to operate whilst the red cap is loose and the cap should be tightened down once the air has all escaped, as they can sometimes tend to leak a little.

...very likely it will cause the vent to open irrespective of whether there is air or water waiting to come out.