Miele DA 2660 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 appliances from being drawn back into the room
when the rangehood and the heater are in operation
simultaneously, an underpressure in the room of 0.04 mbar (4 pa)
is the maximum permissible.
Ventilation can be maintained by air inlets which cannot be
blocked, in windows, doors or outside wall vents, or by other
technical measures, such as ensuring that the rangehood can only
be switched on when the heating appliance is switched off or vice
versa. 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 (registered with an official gas safety body
in accordance with national safety regulations), must be sought.
If the rangehood is used in recirculation mode, where the air is
directed back into the room in which it is located, operating a
heating appliance which depends on the room air at the same time
is not hazardous.
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