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modulating vent and trap

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I have a very old house with a low pressure steam heating system.Above the boiler there is a large cast looking box that has some return lines into it, but not all of them.Printed on it has modulating trap and vent ?. How or what does it do. Thanks

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  • Steamhead (in transit)
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    Is this a \"Webster Modulation Vent Trap\"?

    if so, it means you have a Webster Modulation Vapor system- one of the better heating systems of its day. The unit in question is the only air vent in the system, unlike other types of steam systems that have vents on the radiators.

    This type of system is covered in Dan's book "The Lost Art of Steam Heating, chapter 15.
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