Hissing air vent after boiler shuts down

chuck_6

During the winter, in one of the upstairs radiators, about 5 minutes after the boiler shuts down there would be a hissing type sound coming out of one of the air vents. It's a free-standing radiator, is pitched properly and there is a vent on the main, and the air vent itself works properly. Any ideas why there is a hissing type noise. I'm just a homeowner, so please explain in simple terms.

Alan(CaliforniaRadiant)Forbes

Sounds as though

you have a steam system (vs. hot water).

When your boiler turns on, the steam that's made in the boiler pushes air out of your main and radiator vents.

When your boiler turns off, the steam collapses and sucks air back through the vents. That's what you are hearing and don't worry, that's how your system is supposed to be working.

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Steamhead

Alan's right

but you might want to check for uninsulated steam piping, which will make the steam condense and collapse faster and make the whistling louder.

Insulating steam piping will also make the system heat faster and use less fuel. You can't lose by insulating.

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