Measuring Supply & Return Flows - HVAC - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum > HVAC Measuring supply & Return flows
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

11-19-2011, 02:30 PM   #1
Member

Join Date: Nov 2011
Posts: 145
Rewards Points: 239

## 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)
Attached Thumbnails

11-20-2011, 10:48 PM   #2
Member

Join Date: Oct 2007
Posts: 400
Rewards Points: 380

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.

11-21-2011, 10:15 AM   #3
Member

Join Date: Nov 2011
Posts: 145
Rewards Points: 239

Quote:
 Originally Posted by HVACDave 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.

 11-21-2011, 02:47 PM #4 Member   Join Date: Oct 2007 Location: Manitoba, Canada Posts: 400 Rewards Points: 380 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.
11-21-2011, 07:12 PM   #5
Member

Join Date: Nov 2011
Posts: 145
Rewards Points: 239

Quote:
 Originally Posted by HVACDave 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!

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Kurt123 Plumbing 2 09-05-2010 10:48 PM Rowhousefix HVAC 0 08-11-2010 03:39 PM windowman3 HVAC 4 09-13-2009 09:23 AM honola HVAC 11 08-13-2009 08:41 AM chrisd HVAC 6 02-17-2009 04:25 PM

Top of Page | View New Posts