why so many bubbles from preheater vent?

[markct]

so i made a copper parallel flow preheater for my 2x8 evap, test boiled on water last night and it works great, 180 degree sap going into my pan sure beats 40 degree! its all pitched uphill towards the outlet, and has a 5/16 dia vent tube as was recomended by folks and seems pretty common on most preheaters. i am just curious why the vent seems to constantly be letting bubbles out the tube, just slowly upward bubbles you can see going up the vent. where does this air come in is it released from the water? its not hot enough to be boiling? doesnt affect the operation just made me curious

[KenWP]

When you heat water it releases the oxygen stored in the water. That is why boiled water can be sort of flat. If you watch the bottom of a pot just as it gets hot little air bubbles form all over the bottom and sides.

[maple flats]

My first pre heater was a continuous incline then dropped straight to the float valve, no vent and never an air lock. If it does not drop straight down or almost so you will likely risk vapor lock. Try closing the vent and see if it works. If not you'll need to use the vent.

[markct]

thanks guys, i kinda figured it was the releasing oxygen or simular as it was heated but figured i would ask rather than wonder forever! as for closing off the vent i plan to leave it that way, works fine so why mess with it, and also it allows the whole thing to drain out better when im done.