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Ventilation - Round Air Vent 4in

CG Series Adjustable Plastic Supply/Exhaust Contour Grille w/ Metal Mounting Collar for 4" Duct

HRV vents for inside 

http://www.supplyhouse.com/Fantech-CG4-CG-Series-Adjustable-Plastic-Supply-Exhaust-Contour-Grille-w-Metal-Mounting-Collar-for-4-Duct        $12.49 - $11.99

These are adjustable so you can control how much air goes to a particular room.  This means that if a room is not being used you can close up the vent for that room so the fresh air mainly goes to the rooms you are using.