Mechanical Code…
A device built into an appliance or made a part of the vent connector from an appliance, that is designed to:
(1) Assure the ready escape of the flue gases in the event of no draft, back draft, or stoppage beyond the draft hood.
(2) Prevent a back draft from entering the appliance.
(3) Neutralize the effect of stack action of the chimney or gas vent upon the operation of the appliance.
(Uniform Mechanical Code® 206.0)