Water Valve
WARNING: This page is about a different car, the 2013 Nissan Armada. However, it is still accessible from the selected car via links, so may be relevant.
The water valve (1) cuts the flow of engine coolant to the front and rear heater cores to allow for maximum cooling during A/C operation. It is controlled by the A/C auto amp.
Courtesy of NISSAN NORTH AMERICA, INC.