Main Vent Hissing

Two Pipe System

I've got a two pipe low pressure steam system with one main vent in the basement. My understanding is that this vent will allow the cold air to escape at the beginning of a cycle as the steam rises into the radiators. Once the traps close and the boiler reaches operating pressure is it normal to here loud hissing from the main vent? I've also seen a very small wet spot on the ground under the vent. Would this be something to expect under normal operations?

thanks

Big Ed_4

No

Seems like you have a bad main vent and one or more traps are not closing.

Two Pipe System

clarification

Thanks Big Ed - just so I understand correctly. If I replace the main vent and still hear hissing after the boiler has reached operating pressure the first logical thing to assume would be a bad trap somewhere causing the returns to be pressurized?

Steamhead (in transit)

Fix the traps first

they're letting steam into the dry (overhead) returns, which is a no-no. Don't forget to check to see if there are traps connecting the ends of the steam mains with the dry returns. These serve as steam main vents.

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Two Pipe System

Main Vent Replaced

Thanks Steamhead - i'll discuss this with my guy. He did replace the main vent. Is it normal that at the end of the cycle there would be a vacuum? This is to say, at the end of the cycle the main vent is a slight hissing a bit but nothing is coming out, it's actually sucking air in.

Steamhead (in transit)

Yes

a little of that is normal. But it also might mean you have some uninsulated piping which allows steam to condense faster than it should. The fix for this is obvious.

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