I understand that HVAC vents should be aimed at the windows (ceiling and floor vents) to maintain the target ambient temperature and avoid extremes in a room.
In the case of a residence in a humid climate where some rooms are unused, I was discussing the closing of vents in unused rooms with a homeowner and the following uncertainty came up: When in the "closed" position, should the output of a vent still blow on/towards a window, or is it permissible for it to blow inward towards the room?
The intent behind directing outflow from a vent towards a window is to maintain the ambient temperature more consistently by targeting the most extreme location in the room (aka the energy "sink"). As I understand it, this reduces the energy used in maintaining the environment.
However, when considering a closed-off unoccupied room with vents in the "closed" position, these vents would be blowing outflow directly on to the window, not across its face. These intermittent "shocks" of temperature change seem like they have the potential to introduce local condensation.
Example under consideration: Residence with "adequate" insulation, but not excellent insulation in a humid area of the Southeastern United States.
During a summer day, a room with a closed door on the Southwestern corner of the residence receives direct sun, heating the room, and likely inducing some humidity through the two exterior walls/windows. The central air conditioning system feeding this room would provide some cooling, but since the door is closed, air exchange with the larger structure is significantly limited. (No returns in this room.)
With the vents in the "open" position, air blows across the face of the window, without being aimed directly on the glass itself. This satisfies the principle of normal operation.
With the vents in the "closed" or partially-closed position, and facing outward towards the window, the outflow moves directly on to the windows, producing a noticeably cold spot on the windows. Because the room is closed off, the ambient temperature in the room is likely a little bit higher, and the air more humid. Could these cold spots encourage condensation, and would it become a problem? With the vents in the "closed" or partially-closed position, an facing inwards towards the center of the room, the outflow still circulates air in the room, but does not direct it on to the windows. There should be no cold "shock" locations, except perhaps the vents themselves, which I believe it why the vent in my related post looked so grungy.
How does all this factor out in real life? Are these theories of an armchair quarterback, or could it be better in this (or certain) situations to orient the vents such that "closed" blows inward?