Miele DA 3668 Operating And Installation Instructions page 9

Warning and Safety instructions
In order to ensure safe operation and to prevent gases given off by
a heating appliance from being drawn back into the room, when
the rangehood and the heater are both operated simultaneously,
an underpressure in the room of 0.04 mbar (4 Pa) is the maximum
permissible.
Sufficient ventilation can be maintained by air inlets which cannot
be blocked, e.g. in windows, doors and outside wall vents. The
diameter of the inlet openings must enable sufficient ventilation. A
ventilation brick alone is not generally sufficient to ensure safe
ventilation.
The overall ventilation condition of the dwelling must be taken into
account. If in any doubt, the advice of a competent builder, or for
gas, a qualified gas fitter should be sought.
If the rangehood is being operated in recirculation mode, whereby
the air is redirected into the room in which it is installed, the above
restrictions do not apply.
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