My Hydro Air Heat started spraying hot water from some kind of vent or valve or nipple
It's a Bell &amp; Gossett Air Separator.  If you run your system with the nipple closed then it does not do its job and air bubbles are trapped making your system less efficient and shorter lifespan.  But tightening down the nipple does not render the system unusable or in danger of immediate break-down.

It's a Bell &amp; Gossett Model 11211x.  The reason for the 'x' at the end it that it seems like it has been discontinued from the Bell &amp; Gossett website here.  At the site you see a picture of it in a group of other items, but if you click the picture you see the other items but not this one.

One suppler seems to still have some in stock here.  And maybe this one as well.  They seem to think it is SKU 112113 or 112112, but others seem to have a slightly different picture of the 112113 SKU.