Drainage, Waste & VentQuestion 162 of 360
As a vent extends upward toward its terminal, its required diameter…
a.may be reduced by half at the roof
b.must not be reduced (no decrease in size in the direction of the terminal)
c.may be reduced to 1/2 inch above the roof
d.must double at every branch
Explanation
A vent may not be reduced in size in the direction of flow toward the outlet; the required diameter is maintained or increased, never decreased, up to the terminal. Reducing the vent would restrict airflow and could allow frost or debris to close it. This is separate from the frost-closure rule that enlarges the terminal in cold climates.
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