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Old 09-07-2011, 06:44 PM   #1
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CFM measurements for Return and Supply grids


A/C 2,5T
Return Grid 31x14 (total area)
29x11 (open area) = 2,5 feet x 0.9 feet = 2,250 ft2
1332 CFM
592 FPM

How do I test on the supply? Do I have to measure the area from round duct or the grid?
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Old 09-08-2011, 03:34 PM   #2
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CFM measurements for Return and Supply grids


You can use a surface measurement (or slide chart reference available by almost every manufacturer) on how much air can go through the supply registers. That's the same for ducting, usually that is in a wheel form for flex.

Is that a vane ammeter? You can do a quick check that will "ball park" your cfm to a zone. This is typically what inspectors will do.

To get a true reading you will also need to dig deeper into this with a tube reader in each branch of the supply ductwork.

Ultimately it's two things that really matter;

1. Cfm at the supply
2. Overall static pressure

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Old 09-08-2011, 04:37 PM   #3
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CFM measurements for Return and Supply grids


Quote:
Originally Posted by JJboy View Post
A/C 2,5T
Return Grid 31x14 (total area)
29x11 (open area) = 2,5 feet x 0.9 feet = 2,250 ft2
1332 CFM
592 FPM

How do I test on the supply? Do I have to measure the area from round duct or the grid?
JJ--

29x11 = 319 in2
319/144(in2 per 1 ft2) = 2.215 ft2

You would then have to apply a "K" factor for the grill say .8

So

2.215 x .8 (K Factor) = 1.77 Ft2

1.77 Ft2 x Velocity = Volume

You can apply the exact same principle to the supply air registers and then total the supplys up.

You will also need a slack tube to measure static pressures in the system.

Keep in mind that the accuracy of a vane anometer is less than 90%.

Mark
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Last edited by Jackofall1; 09-08-2011 at 04:41 PM.
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Old 09-08-2011, 05:51 PM   #4
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CFM measurements for Return and Supply grids


Quote:
Originally Posted by Dr.HVAC View Post
You can use a surface measurement (or slide chart reference available by almost every manufacturer) on how much air can go through the supply registers. That's the same for ducting, usually that is in a wheel form for flex.

Is that a vane ammeter? You can do a quick check that will "ball park" your cfm to a zone. This is typically what inspectors will do.

To get a true reading you will also need to dig deeper into this with a tube reader in each branch of the supply ductwork.

Ultimately it's two things that really matter;

1. Cfm at the supply
2. Overall static pressure
CFM at the supply = Velocity x area of the supply grid - How to I know if duct is correct?

Overall static pressure:

Can I use digital manometer at return and supply ?
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Old 09-08-2011, 06:06 PM   #5
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CFM measurements for Return and Supply grids


Quote:
Originally Posted by Jackofall1 View Post
JJ--

29x11 = 319 in2
319/144(in2 per 1 ft2) = 2.215 ft2

You would then have to apply a "K" factor for the grill say .8

So

2.215 x .8 (K Factor) = 1.77 Ft2

1.77 Ft2 x Velocity = Volume

You can apply the exact same principle to the supply air registers and then total the supplys up.

You will also need a slack tube to measure static pressures in the system.

Keep in mind that the accuracy of a vane anometer is less than 90%.

Mark
So If I don't have the K factor the area will be:

(29x11)/144 right?

If I have the K factor the area will be:

{(39x14) x K factor }/144 right?


Can I use my digital manometer to get static pressure?
Attached Thumbnails
CFM measurements for Return and Supply grids-grid.jpg  
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Old 09-08-2011, 06:25 PM   #6
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CFM measurements for Return and Supply grids


Sounds ok to me, again, check for the resistance levels from the mfg. Assumptions make the whole process a time consuming guess.

I use a u tube. I have had a few digital manometers but still feel a more accurate constant on the liquid tube (the internals of an old diaphragm just sketches me out)

Technically, static pressure is made of averaging several measurements on the supply/return side....I do mostly residential and am fairly happy to get three that are decently spaced from the equipment.
Testo has some good videos on how to use their products on trunks for accurate readings (I haven't looked, I bet they're on YouTube)

Hope that helps

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Last edited by Dr.HVAC; 09-08-2011 at 06:29 PM.
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