Since I got started on venting… I haven’t seen these before, found on every vent stack on the house i inspected today. They look really nice, but they seriously reduce the size of the vent pipes.
They keep the unwanted critters out of the vent stack (like frogs).
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As I understand, 2 inch is allowed as the main vent. It used to be 3 inch. That reduction isn’t an issue.