TV Show Measuring Duct CFM Airflow or Velocity


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Old 05-26-08, 02:46 PM
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TV Show Measuring Duct CFM Airflow or Velocity

I occasionally watch Discovery Home & Leisure TV, "Holmes on Homes"

Holmes is a General Contractor that does it Right!
His TV program is EXCELLENT!

However, he had an AC company in to check residential duct airflow & they supposedly had a Cubic Feet per Minute (CFM) meter. It looked like a velocity meter, but it read that it was measuring CFM.

They said they were checking a a first floor 4" duct that showed 389-cfm!

For a 4" duct to deliver 389-cfm the velocity would be in the 4,000-fpm category, or over 4 times acceptable velocity.

IMO, there is no way that over 4,000-fpm velocity could be forced through that 4" duct on a blower's low speed! The larger ducts to the other areas would bleed off the forces on that small duct.

I believe they had to be measuring 389-fpm velocity from that 4" duct. Duct area squared times velocity equals CFM or, just under 34-cfm. They said the blower was set on low speed which would explain the low 389-fpm velocity.

Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the Squared footage of the duct area)
I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM

A 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm

Let us hear your opinion on what you think was illustrated on that TV program.

http://www.udarrell.com/proper_cfm_b...g_systems.html
- HVAC RETIRED
 
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Old 05-26-08, 05:39 PM
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I've caught the show here and there.. Didn't see that show on the the subject you're talking about.

You made a good point, and I agree with you've said.. There's no way a 4" would move that much!

And I never seen 4" used on HVAC other than a dryer exhaust.
 
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Old 05-26-08, 06:05 PM
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my guess would be they mis-spoke, I can't see 4" used for hvac unless space was an issue. even then it wouldn't flow much air unless you put adjustable diverters on the larger ducts to make it flow air.
murphy was an optimist
 
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Old 05-27-08, 10:19 AM
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A THERMO ANEMOMETER "DAFM2" was used by the HVAC contractor on Mr. Holmes’ TV program, on DHOME, Holmes on Homes.
Do a Google search for - CFM instruments DAFM2

When Holmes’ was explaining the duct tests he (misspoke) stated the duct by the entry was a 4” line with 389-CFM, that of course is literally impossible because the velocity would have to be well over 4,000-FPM.

My conclusion is that the HVAC person was taking the DEFAULT FPM Velocity readings while the air handler was on the Low Speed blower setting, which he said he was going to switch to Med/High.

I video taped the program & with replays I did NOT see any of the required actions taken to acquire a CFM reading. Some of those steps should have been evident also with explanations of how he acquired the CFM readings.

Yes, the HVAC man's name & company are in the video. If you eventually see this video I would NOT reveal the HVAC company or his name.

(Repeat),When Holmes’ was explaining the duct tests he (misspoke) stated the duct by the entry was a 4” line with 389-CFM, that of course is literally impossible because the velocity would have to be well over 4,000-FPM.

Do any of you have this test instrument?
Your take on this TV program.

Instruction Manual for the DAFM2
Direct measuring of air flow (Single Point) CFM
Air velocity measurement is calculated by multiplying the air velocity
readings times the free area dimension. Free area is publishing by the
grill and register manufacturer you are servicing.
Determine the free area of the air source you are measuring and enter
it into the meter.
1. Turn the power ON.
2. Press the mode select once (you will hear one beep). The meter
will display AREA in upper case letters at the top of the display,
and 1.111, with a flashing initial digit (Fig 3). This is the free area
default setting.


3. To increase numbers, the flashing digit can be changed by
pressing the HOLD/ADVANCE key.
4. To change the value of other digits, press the NEXT DIGIT key.

5. Press the RECORD key. The flashing will stop. Then press
the HOLD key to save the changed value. The meter returns to
FLOW mode automatically.

6. Press the MODE select button once to return to the "velocity" mode.

The meter is now ready to measure air flow (CFM).
MIN/MAX/AVG CFM READING
Repeat steps 1-6 from earlier instructions to obtain MIN/MAX/AVG CFM
readings from a single point.
http://www.udarrell.com/proper_cfm_b...g_systems.html
- HVAC RETIRED
 
 

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