How To Calculate Return Air Duct Size

There are many different sizes available for a render air grille. Using the correct return air grille size is important to ensure that the HVAC system has sufficient airflow as well as depression noise. And then, I created several CFM charts and a guide on how to calculate render air grille size.

To calculate return air grille size, take the CFM of the HVAC unit and divide information technology by 350 to become the grille area in square feet. Side by side, multiply the grille area by 144 to convert it into foursquare inches. Finally, choose your preferred grille size that matches the required grille area.

Alternatively, you tin use the following CFM charts to quickly notice the grille size based on the CFM or airflow of the HVAC unit. Parameters used for the following charts are as below:

Confront Velocity = 500 feet per minute (fpm) or two.5 meter per second (m/southward)
Free Area = 70%

Face up velocity is the amount of air volume passing through a grille. Typically, 500 fpm is optimal and balanced between air volume and noise.

Free area is the amount of space on a grille available for the air to pass through. It's depending on the blazon of grille and the manufacturer of the grille. However, many grilles accept near 80% free area while some external grilles take l% free expanse. Hence, I employ 70% complimentary area.

Design and sizing skills are essential to HVAC engineers. However, transmission calculations are inefficient. Hence, I arranged 9 excel calculators, 5 diagrams and 3 charts in my Pattern Engineer Starter Pack which I encourage yous to check information technology out every bit it tin take your design skills to the side by side level.

CFM Chart for HORIZONTAL Return Air Grilles

Grille Expanse (sq.in)	Grille Size (in)	Airflow (cfm)
24	four x vi	58
48	4 x 12	117
lx	half dozen x ten	146
180	vi x 30	438
128	viii x 16	311
144	8 ten 18	350
192	viii 10 24	467
240	8 x thirty	583
256	8 x 32	622
200	ten x twenty	486
220	ten x 22	535
240	10 x 24	583
300	10 x 30	729
216	12 10 18	525
240	12 10 20	583
288	12 x 24	700
360	12 x 30	875
238	14 x 17	578
280	14 x twenty	681
350	14 x 25	851
364	14 x 26	885
288	xvi x eighteen	700
320	sixteen x 20	778
400	16 x 25	972
416	sixteen 10 26	1011
432	eighteen x 24	1050
468	xviii x 26	1138
540	eighteen 10 xxx	1313
576	eighteen x 32	1400
480	20 x 24	1167
500	20 x 25	1215
600	20 x xxx	1458

Horizontal Grille Selection Table

CFM Chart for SQUARE Render Air Grilles

Grille Area (sq.in)	Grille Size (in)	Airflow (cfm)
36	6 x half dozen	88
64	viii x viii	156
100	ten ten 10	243
144	12 10 12	350
196	14 10 14	476
256	xvi x 16	622
324	18 x eighteen	788
400	20 10 20	972
484	22 ten 22	1176
576	24 10 24	1400
676	26 x 26	1643
900	30 x thirty	2188

Square Grille Pick Table

CFM Chart for VERTICAL Return Air Grilles

Grille Expanse (sq.in)	Grille Size (in)	Airflow (cfm)
48	12 x 4	117
72	12 x half dozen	175
120	12 ten 10	292
84	14 x 6	204
112	14 x eight	272
140	14 x 10	340
96	16 10 half dozen	233
128	16 10 8	311
192	xvi x 12	467
180	xviii x 10	438
216	18 ten 12	525
252	18 10 14	613
160	twenty ten 8	389
200	20 x 10	486
240	xx x 12	583
280	xx ten xiv	681
320	twenty ten 16	778
360	xx 10 18	875
176	22 x eight	428
220	22 10 ten	535
96	24 x 4	233
144	24 x half-dozen	350
192	24 ten 8	467
288	24 10 12	700
336	24 x fourteen	817
384	24 x 16	922
432	24 x 18	1050
364	26 x 14	885
416	26 x 16	1011
468	26 x eighteen	1138
224	28 x viii	544
644	28 x 23	1565
180	thirty 10 6	438
240	thirty ten 8	583
300	30 ten 10	729
360	30 x 12	875
420	30 10 14	1021
480	30 x sixteen	1167
128	32 x four	311
256	32 x 8	622
384	32 10 12	933
640	32 ten 20	1556
272	34 x 8	661
360	36 x 10	875
432	36 ten 12	1050
504	36 x 14	1225
608	38 x xvi	1478
800	xl x 20	1944

Vertical Grille Choice Tabular array

Honestly, I did spent a lot of unproductive time searching for the best grille size when I was working as a junior project engineer. Why I never idea of creating a whole table for myself? Anyway, let'due south see how to calculate return air grille size in the manual way.

How to Calculate Return Air Grille Size?

Render air grille size is calculated using the post-obit formula:

Grille Surface area (sq.in) = Airflow (cfm) ÷ Face up Velocity (fpm) ÷ Grille Free Area (%) 10 144

From the above formula, using 500 fpm with 70% free area, we go:

Grille Area (sq.in) = Airflow (cfm) ÷ 500 ÷ 0.7 x 144

Let's say we have an HVAC unit of measurement with 1050 CFM of airflow, and then:

Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.7 x 144
Grille Area = 432 sq.in

Using the CFM chart for horizontal grilles, the 18 inches by 24 inches grille has a grille area of 432 which is suitable for the HVAC unit with 1050 CFM of airflow.

If yous don't adopt to use a horizontal grille, you can use the CFM chart for vertical grilles to find the 24 inches by eighteen inches grille or the 36 inches by 12 inches grille. Both grilles are suitable for the HVAC unit with 1050 CFM of airflow.

Alternatively, you lot can simply match the grille size to the airflow (cfm) of your HVAC unit.

If yous are sizing external grilles, I propose yous use 50% gratuitous area because external grilles usually have narrow blades contour to forbid rainwater from entering.

If you find the sizing process is time consuming, you must check out my Design Engineer Starter Pack. I included nine excel calculators and one of it is for this one. It tin save you time while provide you with more insights almost HVAC sizing and blueprint.

What Happen If You lot Use a Smaller Return Air Grille?

Using a smaller return air grille will result in a college face velocity with the aforementioned type of grille (thus, same costless area). With a college face velocity, the grille may create audible noise. The smaller the grille, the greater the racket.

If you actually need to utilise a smaller return air grille, I recommend you don't exceed 800 fpm of face velocity. Alternatively, you tin seek return air grilles with a larger free area. Otherwise, use more than one grille if possible.

Dissimilar grilles accept different applications. If y'all desire to acquire near the different types of grille used in HVAC, feel complimentary to check out my blog mail 4 Types of Grilles in HVAC (Common Applications).

If you lot detect this post is helpful, I must inquire y'all to check out my Design Engineer Starter Pack for one final fourth dimension. It is the production of my years of practical working experience. No other products are comparable to what I accept to offer. I recollect information technology'due south worth your time.

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