Miele PDW 909 Quick Start Manual page 13

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If multiple tumble dryers are to be connected to one vent ducting,
a non-return flap must be installed directly on the duct for each
tumble dryer.
If this requirement is not observed, the tumble dryers may be dam-
aged and their electrical safety could be affected.
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Follow the instructions in "Installation of the ducting".
en - Warnings and safety notes
13