Why Do I Hear Hissing from Main Vent/Quick Vent After a Boiler Stops Running

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HotChocolate1947
HotChocolate1947 Posts: 30
in Strictly Steam

Please excuse my ignorance. As I understand things, the hissing sound from an air valve on the radiator is cold air leaving the system followed by a click to indicate it closed. The other day I was in the basement doing laundry. The boiler finished running after completing a heating cycle. Around 5-10 minutes later, I heard a click from the main vent (I've seen it called quick vent too) followed by a long, hissing sound. Is that steam coming out? Is that normal? Thank you for any replies.

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  • Jamie Hall
    Jamie Hall Posts: 27,975

    No, it's air going in as the steam in the system collapses. And it is more or less normal… the vent doesn't open to let air in until it cools off a bit.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    ethicalpaulHotChocolate1947
  • HotChocolate1947
    HotChocolate1947 Posts: 30

    OK… Thank you so much for the clarification.

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