The throw value means just how well air moves across a room from a vent, or diffuser. A major factor in the throw value is the terminal velocity of the air coming from the diffuser. It is influenced by the velocity the fan blows air in the HVAC unit, the duct work's size and the distance from the fan. Manufacturers provide the throw value for their HVAC unit based on standard installations.
Ceiling and Diffuser
Another throw value factor is the relationship of the diffuser and the ceiling. Standard throw value charts are based on 8- to 10-foot tall ceilings, with the diffuser set about 1 foot below that height. Moving the diffuser up or down, or adjusting the ceiling height, affects the throw value. Placing the diffuser more than 3 feet from the ceiling, for example, reduces the rated throw value by 20 percent.
Ideally, the throw value from an HVAC unit allows the air to flow over the heads of the room's occupants, expanding sideways and pulling the rest of the room's air into a mixture that reduces hot or cold spots. The moving air that bounces off walls opposite the diffuser helps to circulate the heated or cooled air, but other solid objects such as support beams may interfere with the air flow. Hard-to-reach areas may require a diffuser pushing the air downward instead of horizontally.
If rising hot air or sinking cool air interferes with the throw value at any point in the room, then the incomplete mixing of the air may cause noticeable drafting. A professional HVAC installer takes all the variables into consideration when designing the proper system for a home. The room's size, activities such as cooking, the location of hanging lamps or other obstacles and the sunlight coming in through a window and heating the room are all considerations that must be thought through when determining the proper throw value for a room.