You're at a cocktail party when, as it so often does, the discussion turns to HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. design. What do people talk about? Load calculations. Manual J. Oversizing. Maybe a little about duct sizing or location. But how many times have you been in that conversation and heard someone talk about what happens at the end of the ducts? Yes, I'm talking about the often overlooked part of HVAC design in which the designer selects the proper terminations for the duct runs.
from Building Science http://www.greenbuildingadvisor.com/articles/dept/building-science/designing-duct-system-vents-good-air-flow