**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.
You are viewing: What Is The Free Area Of A Register Or Grille
****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.****
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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 and the grille size is 20×6, using the chart from the manufacturer we can see that the AK (Effective Area) will be .58. 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…. 500 x .58= 290. This matches the CFM on the chart. 
Let’s say you were looking for a 20×10 return air grille with the highest Effective Area. We can refer back to the engineering data for the return air grilles (manufacturers will usually have multiple styles of grilles). From the charts below, we can see that the 515 – Return Air Grille for the size 20×10 has an AK of .956, while the 500 – Return Air Grille has an AK of .93. The 515 would be a better choice given your face velocity with a higher Effective Area of .956.
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Free Area –
Getting free area and effective area confused is very easily done. 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. This results in Free Area being a greater number/measurement that Effective Area. The other main difference between Free Area and Effective area is unit of measure. Effective area is in square feet while Free Area is in square inches. 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.
I find myself asking many times, that if Effective Area is listed on engineering data for grilles/registers and not Free Area, than what exactly do you need free area numbers for? Some Velometer (measures speed of air in HVAC systems) manufacturers require the use of free area. If you are using such meter, be sure to convert the Free Area that is in square inches to square feet. What info to have when wanting to find specific engineering data on registers/grilles: 1)Duct size… Make sure you have the actual opening/duct size of your register. Wall registers or grilles are listed by width (horizontal) x height (vertical). 2)Whether you have a register or return. The register is a supply (air comes out), and comes with a damper to control air flow. A return is an exhaust air inlet to duct, and will not have a damper. Need more info on the difference, see our earlier blog entry Registers vs. Grilles. 3) The face velocity of your HVAC system. This can be calculated by your HVAC specialist using the open area of your duct/fume hood and measuring the airflow with a thermal anemometer. **FPM= Feet per minute. Typical/recommended face velocities for residential applications: 500 to 750 FPM apartment applications: 500 to 750 FPM 4)If you are looking for the highest Effective Area, let the sales person know, or let them know what type of engineering data you are looking for. Just a note again that most decorative registers we carry do not have engineering data. If you need something that has a higher Effective Area, choose a design with a larger design/pattern, such as the Victorian Design. We can provide you with any engineering data we have available per request. Please make sure to have your duct opening and face velocity.
**Definition and some information taken off Hart & Cooley TECH TALK
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