On 25 Jan 2006 19:05:47 -0800, Bnchs400 wrote:
My house has a one-pipe steam heating system. Every time the heat
comes up, I hear a hissing noise comming from the steam vent. My
question is if it is normal for there to be a constant hissing noise
while the heat is comming up or should it be mostly silent? If there
should'nt be a hissing noise, I have to replace the steam vents right?
Thanks.

When water turns to steam, it expands, which blows air out of the air
vents as steam makes its way to the radiators. The air vents should close
when the steam gets to them and the steam condensing to water draws more
steam to the radiator. As the boiler cycles, more air might be pushed to
the radiators. But if the air vents are blowing steam, they are not
operating properly, or the pressure is too high. When the boiler shuts
off, the remaining steam shrinks to water and draws air back in.

Two secrets for having a quiet and efficient system are main air vents on
the pipes in the basement (to quickly and quietly fill the mains), and
having the system operate at a low enough pressure (not more than 1.5 to 2
psi max.). If the air vents are very noisy or whistle, either your main
vents are stuck closed (or you have none), or the pressure is too high.

For more info, see http://www.heatinghelp.com/ and The Wall forum there.
They have excellent books about how steam works.