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Old 11-19-2011, 01:30 PM   #1
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Measuring supply & Return flows


Our system is a bit noisy. Therefore have been trying to take some measurements.

I have an anemometer (propeller type). Because velocities vary over the face of the registers, I removed all registers. I left the single return grille and filter in place.

It was easy enough to measure the air velocity in the register boot necks by holding the probe in the boot about 1/2" from the exit. But I was not too sure how to measure the return air flow. Hart & Cooley say to hold the probe exactly 1" away from the grille and measure the velocity there.

Finally, this is my question:
Do I use Face area or Ak when calculating the return flow? (See H&C pdf data below)
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Old 11-20-2011, 09:48 PM   #2
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Measuring supply & Return flows


If the return grille is still in place then you would have to use effective area. The face area could be larger than the effective area by as much as 40% on some registers. You should be able to find some generic data on effective area of standard return registers.

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Old 11-21-2011, 09:15 AM   #3
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Measuring supply & Return flows


Quote:
Originally Posted by HVACDave View Post
If the return grille is still in place then you would have to use effective area. The face area could be larger than the effective area by as much as 40% on some registers. You should be able to find some generic data on effective area of standard return registers.
I did that at first, but flow calculated was low as compared with 1000cfm setting on air handler. I removed the grille and filter and measured velocities inside duct. Then instead of a low number I got a high number! But the high number corresponded with the total measured for the supply side registers. Probably better though to eliminate need for using grille Ak.
The ECM motor controls are supposed to maintain the set AHU flow so flow should not be affected by removing filter.

Seems flow measurement at these flows with anemometer is not too accurate! But seems OK for relative balancing flows which is all I am really trying to do.

Thanks for input.
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Old 11-21-2011, 01:47 PM   #4
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Measuring supply & Return flows


You may also be picking up return from loose duct connections and the opening around the filter rack (if external) etc. The 1000 cfm rating will be at a certain staic pressure esp. of the unit in manufacturer. That rating may change due to ducting configuration and resistance etc. Even though you have the ECM that will try and maintain a certain airflow (static pressure difference) it will only be compensating for static drop through the unit, and not necessarily the entire ducting system.
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Old 11-21-2011, 06:12 PM   #5
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Measuring supply & Return flows


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Originally Posted by HVACDave View Post
You may also be picking up return from loose duct connections and the opening around the filter rack (if external) etc.
We sealed up the filter rack on the handler. It had one of those magnetic closures - lot's of leaks. We are using ceiling return air filter grille which has new sealed duct leading to handler. Both Supply and Return ducts have been checked and sealed.

Quote:
The 1000 cfm rating will be at a certain staic pressure esp. of the unit in manufacturer. That rating may change due to ducting configuration and resistance etc. Even though you have the ECM that will try and maintain a certain airflow (static pressure difference) it will only be compensating for static drop through the unit, and not necessarily the entire ducting system.
Just how the air handler determines flow, is the mystery. This is what ADP says:
"Variable-Speed Motor Features
Maintains a selected CFM over wide range of static conditions." Elsewhere it says it will maintain flow up to 0.8". That indicates it is external or total static difference it controls on.

I don't see any pressure sensors on wiring diagrams. Only way I can think of, is that they use power draw and fan curve to determine flow. There is an LED that flashes once per 100rpm. But it is inside case where it can't be seen unless panel is removed, but the door switch closes unit down and air escapes??? Thinking of putting in one of those door peep holes!
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