My system looks exactly like this. I believe it's called Hydro Air Heat. See that thing with the green rectangle around it. It started spraying hot water from that nipple on top (indicated by the green arrow).
First of all what is that thing called and what is it's function? Does anyone know that make and model (in case I need to replace it)?
I turned off the heat, closed an intake valve and it eventually stopped spraying hot water. Then I tightened that nipple all the way down and turned everything back on. It's no longer leaking and I believe I have heat. However I don't know if that thing is supposed to be tightened all the way like I have it. Have I locked air in in some way I shouldn't have? Have I ruined some kind of pressure controlling apparatus? Or was I supposed to tighten that nipple all the way for normal operation?