AEG HL 7275 Operating And Installation Instructions page 5

If the hood is used in its recirculation mode, it will operate simply
and safely in the above conditions without the need for any of the
aforementioned measures.
When the hood is used in its extraction mode, the following rules
must be followed to obtain optimal operation:
- short and straight outlet hose
- keep bends in outlet hose to a minimum
- never install the hoses with an acute angle, they must always
follow a gentle curve.
- keep the hose as large as possible (preferably the same diam¬
eter as the outlet hole).
- the length should be no more than:
3 metres with one 90° bend
2 metres with two 90° bends
Bends of more than 90° will reduce the efficiency of the hood and
reduce the airflow.
Failure to observe these basic instructions will drastically reduce the
performance and increase the noise levels of the extractor hood.