Static Pressure, Throw, NC Level, and Other GRD Terms (Get to Know the Language of GRD)
by Greenheck |
Anyone who works in the HVAC industry knows it has its own language. Heating, ventilation and air conditioning (HVAC) language contain words and acronyms that, outside of the industry mean little to the average person. Subsets of the HVAC industry also use specialized terms to describe the function and operation of those products. Some match with the rest of the industry, while others are a little different. Grilles, registers, and diffusers (GRD) are no exception. The following are a few important terms (terminology taken from several sources, including ASHRAE) to know when working with GRD.


A grille generally resembles the look of a louver, although some grilles use an egg crate pattern. Grilles allow air to pass through into a space or from a space.


Registers are grilles that have a damper used to adjust the airflow.


The diffuser, unlike grilles, spread the airflow into a space in an adjustable or specific flow pattern. Most diffusers are located in ceilings.


Air being sent through a grille or ceiling diffuser flows out horizontally (from the grill or diffuser along the ceiling) and vertically (from the ceiling into the room area). Drop measures the vertical distance between the grille/diffuser and the lowest edge of the airstream at the end of its throw into the room.

Effective Area

Sometimes called effective free area, is the area of a GRD product used for airflow, measured in square feet.

Free Area

The total of all open spaces between bars or blades of a grille. Free area measurement is in square inches.

Jet Velocity

The average velocity measured passing through the blades. Also called face velocity. Measured in feet per minute (FPM).

NC Level

Noise criterion (NC) level is a rating system for indoor noise levels in different types of room settings that range from very quiet to noisy. The NC level helps establish acceptable levels for continuous sounds, such as those from HVAC systems (including GRD products).

Outlet Velocity

The average velocity of airflow emerging from an outlet measured in the plane of the outlet.

Pressure Loss

Pressure loss is the decrease in total pressure due to friction and turbulence. It is a measure of the mechanical energy lost by the flow per unit volume of the airflow.

Primary Air

Treated supply air that enters the space through any supply air device, such as an air outlet (grille, register, diffuser) and mixes with room air in the 150-fpm envelope.


The spread of an air jet in mixing air diffusion is the maximum distance (dv) between two vertical planes tangent to a specified isovel and perpendicular to a plane through the center of the core of an air jet.

Static Pressure

A uniform exertion in every direction of the airflow in a duct. Static pressure is measured in in. wg

Temperature Differential

The difference between the average zone temperature and the average supply air temperature.

Terminal Velocity

The speed at which total air drops to the fpm amount (50 or 75) depending on the application.


Throw is the horizontal or vertical axis distance an airstream travels after leaving an air outlet before the stream velocity is reduced to a specific terminal value. The rated throw will be a function of a predetermined terminal velocity.

Total Pressure

The sum of the velocity pressure and static pressure, measure in in. wg

Now that you know basic GRD terminology, it’s time to investigate the grilles, registers, and diffusers that make up a portion of Greenheck’s new air distribution product line.
Grilles, Registers & Diffusers

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