8
ESTIMATING FREE AREA (EFFECTIVE AREA) OF A GRILL
To obtain air volume on a grill with unknown effective or free area you
must estimate the free area. This is difficult, but can be used to provide
an estimate of the volume for comparison testing. If multiple grills of the
same design are compared using this method, you will have an accurate
relative comparison, although the actual value may not be exact.
Method 1
• Measure length and width in inches.
• Obtain overall area in square inches by multiplying length
times width.
• Divide square inches by 144 to convert to square feet.
• Multiply the result by 0.70 (70%) to estimate the actual
open area.
Note: For return registers that are more open, increase the factor to
0.9 (90% open). For louvered registers with much more covered area
decrease to 0.5 or 0.6 (50~60% open).
Example – 4”x10” residential floor register
• Step 1 – 4” and 10”
• Step 2 – 4 x 10 = 40 square inches
• Step 3 – 40 ÷ 144 = 0.2777 square feet
• Step 4 – 0.277 x 0.70 = 0.194 square feet
(use this when entering free area “A” on the DAFM3B)
Method 2
Measure the grill area inside the frame of the register, calculate area in
square inches and convert to square feet using the method 1 steps. You
do not need to perform step 4.
Example – 4”x10” floor register
• Step 1 – actual louver area dimensions are 9” x 3.5”
• Step 2 – 9 x 3.5 = 31.5 square inches
• Step 3 – 31.5 ÷ 144 = .219 square feet
Note: This method may result in a slightly elevated result as it does not
fully remove the louvers that are in the air path.
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