Miele DA 2668 Operating And Installation Instructions page 9

Warning and Safety instructions
In order to ensure safe operation and to prevent gases given off by
the heating appliance from being drawn back into the room when
the cooker hood 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
cross-section 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, seek the advice of a competent building
regulations inspector or qualified gas fitter (GasSafe in the UK).
If the cooker hood 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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