What is this valve and how do I use it?

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Banjo Bud

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I’m at my dads house and he has hot water heat. The hot water tank needed a new pressure relief valve so I had to drain a few gallons from the tank to install a new valve. Then of course I had to get air out of the system as i refilled it. I did that by opening the pump at one end. All went well but this small valve atop the expansion valve has me stumped. Am I supposed to open the thumb nut and leave it open or do I keep it tight? Sometimes when I open it a little air comes out but other times nothing comes out. And water never comes out.
 

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Weekend Handyman

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Banjo Bud

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Thanks. I just read all of it. So does that mean that I leave the thumb nut tight and somehow it’s spring loaded to release air? There is a rubber gasket in the bottom of the nut and the nut top has slits on the sides. I’m thinking that the gasket gives way at a certain psi to expel air. Is that how it works? Right now I have the nut snug but not tight.
 
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John Gayewski

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You leave it loose for a while. Then when all of the air is gone you can tighten it up.
 

Fitter30

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Auto air bleed has a float with a needle valve. Full of water valve is closed, air float drops valve opens till water displaces the air.
 
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