The textbook definition of Free Area is “The minimum total area of the openings in the register or grill through which are can pass.”** I think of it as out of the total faceplate, what is the area “free roam” for the air- ie how much of the faceplate allows for air to escape. There is really no formula that applies to free area and almost every return/registers out there will have a different set of input for calculating free area. Effective area is in square feet while Free Area is in square inches.
The other main difference between Free Area and Effective area is unit of measure. This results in Free Area being a greater number/measurement that Effective Area. They are similar in the fact that they are both measuring the sum of areas where air exits, aka the spaces between fins “see through area.” The biggest difference between the two terms is that Free Area does not factor in the friction for the air through the bars or louvers on the register/grille. Getting free area and effective area confused is very easily done. The 515 would be a better choice given your face velocity with a higher Effective Area of. 956, while the 500 – Return Air Grille has an AK of.
From the charts below, we can see that the 515 – Return Air Grille for the size 20×10 has an AK of. We can refer back to the engineering data for the return air grilles (manufacturers will usually have multiple styles of grilles). Let’s say you were looking for a 20×10 return air grille with the highest Effective Area. We can determine the CFM’s by looking at the chart (it usually is listed) but we can use our formula from above to double-check…. 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).įor example… if your HVAC system has a velocity of 500 and the grille size is 20×6, using the chart from the manufacturer we can see that the AK (Effective Area) will be. 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. The Effective Area (noted as Ak) is used in the formula to find the Cubic Feet per Minute (CFM) of the register/grille.Ĭubic 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. 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 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. ****Įffective Area is noted in engineering data of products as “Ak” and is usually measured in square feet. 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. Most decorative residential registers/grilles will NOT have engineering data. ****Normally you do not need to know all of the following information when purchasing a grille or register. If you have specific requirements for your home, please consult with your HVAC specialists before making any decisions. 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.
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. Once in a blue moon we get a customer request for the effective area of a register or grille.
If you find any error in the explanation, please let us know and we will fix/add immediately.** **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.