**Disclaimer – This blog is meant to help customers understand the engineering data of common floor registers and grilles from an “outside the HVAC profession” view. We are not trained HVAC specialists. If you find any error in the explanation, please let us know and we will fix/add immediately.**
Once in a blue moon we get a customer request for the effective area of a register or grille. This isn’t commonly asked because the technical aspect of the answer, and most asking don’t really know what effective area or free area is, but know they need a grille that has a specific effective area for their HVAC system. I, nor any sales staff member of Installerstore are experts in HVAC installation or specifications, but this blog serves as a tool to understanding just what the effective area and free area mean. If you have specific requirements for your home, please consult with your HVAC specialists before making any decisions.

****Normally you do not need to know all of the following information when purchasing a grille or register. Most decorative residential registers/grilles will NOT have engineering data. Most people who come to us who need a certain Effective area is because the standard grilles or registers make a noise when air passes through.****
Effective Area-
Effective Area is noted in engineering data of products as “Ak” and is usually measured in square feet. The textbook definition of Effective Area is “The calculated area of an outlet based on the average measured velocity between the fins.”** To break this down, it is a measurement of the grille or register in square feet, that is utilized by air flowing through it – how much of the grille allows for air to flow through. The Effective Area keeps in mind the friction caused when the air makes connection with the bars as it flows through. The Effective Area is calculated by the manufacturer of the grille or register. The Effective Area (noted as Ak) is used in the formula to find the Cubic Feet per Minute (CFM) of the register/grille.
CFM= Face Velocity x Ak
Cubic Feet per Minute has to do with the air flow rate in cubic feet that passes through the grille per minute. If you want to find the CFM for a certain grille/register, you would find the particular grille you are looking for in the manufacturers engineering data. Once you find the chart, use your average face velocity (measured in feet per minute– FPM) and multiply it by the Ak listed for that particular grille/register. You can see that the entire equation is in basis of feet and minutes (this will come in handy later for the Free Area Explanation below).
For example… if your HVAC system has a velocity of 500 Continue reading →