Main vent hissing non stop.,.,steam heat

Ed_9

I noticed just last night when my steam heat comes on the main vent hisses the entire time. I'm a novice at this and am learning as I go along but shouldn't the hissing stop after a while like my radiators? Also I noticed some of my radiators hiss a bit to release the air then stop for a while then hiss again and the process goes on all night. Any ideas on my 2 ??'s?

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Steamhead

Main Vent could be bad

or undersized. Either way it needs replacing. How long is that main, and what pipe size? What vent is on it now?

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Ben_3

How about the pressure

The presuure could be set to high as well which will cause those vents to hiss or whistle also the main vent could be undersized, normal operating pressure should be 2 psig or less.