Automatic air vents

[MickyP10]

I want to change the two automatic air vents in my system because pressure is lost gradually and one of them, wifey reported, was wet. I have the replacements and, although externally identical, they both have black plastic knurled knobs screwed into the vent bit. I'm assuming they were removed on the old ones. Are these used to prevent the vent from 'venting' and, if so, why would you need this facility? Or, maybe if the vent's leaking water, you use these to stop the leak until you fit a working replacement?

Confused and probably missing the point here...

 

[jobloggs]

Or, maybe if the vent's leaking water, you use these to stop the leak until you fit a working replacement?

That's it. These caps should not be screwed shut in normal operation.

 

[MickyP10]

Thank you.

 

[jobloggs]

You're welcome but why replace both if only the wet one is leaking?