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Bert

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Reply with quote  #1 
Here's an article on several ways to measure airflow.

http://www.greenbuildingadvisor.com/blogs/dept/musings/simple-methods-measuring-air-flow#ixzz4NxAUb8bf

Home-made flow hood 2.jpg 

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Bert K.
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gbwillson

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Reply with quote  #2 
I'm not sure how accurate this hood would be for our use. Even if you used this for all the vents in a home, it's not going to give you an accurate measurement of the air in the duct itself. Since behind the wall you could have anything from a 5" round duct to a large rectangular return duct. But, if you we're an HVAC pro you would likely have higher-end anemometer that will read the results from the hood and convert it to the correct CFM readings, regardless of duct size. 

Greg in MN
Bert

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Reply with quote  #3 
I guess the theory is since it's one foot square the FPM would be the same as the CFM. Probably not real accurate but if it was about four feet long it may be close enough.

I like the garbage bag idea that's been around on these forums but wish there was a bigger bag available for high CFM fans for more precision. 

In my case I don't have much room. The vent in the wall is about 4 or 5 feet from the opposite wall so it's hard to even put much of a length of duct up there.

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Bert K.
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gbwillson

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Reply with quote  #4 
The trash bag method would be hard pressed to fill accurately(pun intended). The bags would fill so fast that accurate measurements would be difficult. The larger the bag(s) make for easier fill times. Admittedly, flow rates,even with an expensive anemometer, can be difficult if done carelessly. But by taking several readings, or an average reading over several locations within the stream you can have a fairly accurate reading. 

Bert-You have enough space to insert a straight duct or use a large bag. Using both would be ideal at least for the initial testing. Once you have an idea of your flow characteristics, you should be fine using only the anemometer.


Greg in MN
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