Where is the main induction valve?

To quote the guide:

The ventilation system is controlled from the Misc. Systems Control Panel in the control room.

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The ventilation system can be purged and filled with fresh air by opening the main induction valve while the submarine is surfaced. DO NOT open the main induction valve while the submarine is submerged! This will flood the ventilation system and rapidly fill the submarine with water.

I can't find this valve. How do I open it, or is it taken care of automatically?