Bosch DHS64 Series Instruction On Mounting And Use Manual page 15

On some models ONLY:
A Non Return Backdraft Damper is included in the supply, as
optional accessory to be installed under certain installation
conditions (i.e. if there is no protection against accidental
backdraft of air from the outside). The damper must be
installed inside the dedicated housing, inside of the Air Outlet
Fitting, before installing the flue.
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