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Loud hiss from vent in return line?
I've got a two-line steam heat system in my house (built ~ 1928); have replaced the boiler and recently insulated all of the steam lines (not the returns). Believe it to be a "Baker" vacuum steam system.
When the thermostat calls for heat, we initially get hammering noises which subside after a little bit, but a vent (replaced about four years ago) on one of the return lines hisses very loudly (no water coming from it that I've noticed) until the thermostat stops calling for heat. Some radiators provide a lot of heat, others to a lesser extent, and a few are always cold.
The system does not, as far as I know, have an F&T trap; each radiator has its own trap (bi-metallic?), and I haven't noticed any hissing at the radiators. The system does have a large elevated cast-iron cylinder (about 18" high and 12" in diameter with a float glass near the boiler, which I think creates the vacuum for the steam mains.
I'm not sure if this is due to a clogged return, or poor return pitching due to house settling, or a poorly-sized vent (or if the vent is making it too easy for steam to escape without going through the radiators) or something else. Also not sure if this is causing the uneven heating across the radiators.
Thank you in advance for your thoughts and ideas.