[. . . ] [. . . ] Can be installed in a section of metal duct and spliced into a section of flex duct. Installs in round metal ducts from 5" - 8" diameter or rectangular ducts as shallow as 3 1/4". ACCESSORIES: DB-2 can be activated with the optional model PS1503 Duct Airstat. CAUTION: This device is not designed to vent bi-products of combustion or combustible fumes. [. . . ] [. . . ]