Air Flow Meter for Homeowner

[collins8]

I am looking to buy a simple air flow meter to allow me to take air flow readings at my supply registers and return duct. I am not looking for the industrial grade device or the most accurate, but something that will work for me at home and is simple to use.

Will this work
http://store.yahoo.com/newadv/ke41bynknewm.html

Rob

[air2spare]

having hot and cold spots?

[davidr]

I don't think I would trust any direct CFM readings taken by it.

Most pre-programmed grille correction factors in those meters are not even close.

I would also have to ask the question are you having hot & cold spots?

What are you looking to confirm with the airflow readings?

[collins8]

I want to enure I have a total of 800 SCFM going upstairs.
Shouldn't the Kestrel be close? I can pull the diffusers and get a reading in the center of the air steam.

[davidr]

The best method for measuring airflow at the grilles would be through the use of a flow hood.

Was a load calc done to verify you need 800 CFM on the second floor.

[collins8]

NO the 800 SCFM was not verified but there are some Duct issues. I have a variable speed AEPT 30 air handler that I can adjust. The main trunck is only 16" x 8" and it goes up 20 ft to the attic. I just want something simple and not real expensive that will allow me to make my own checks.

Rob

[pstu]

Heck, I think this is an interesting proposal of a gadget. Of course it *claims* to be accurate, but I do kinda believe the pro who says there is a problem with the built-in grill correction factor. As a homeowner myself, I don't have the education or experience to say definitively how to make this measurement. But hopefully one of the pros can offer instructions -- and if he does we ought to thank him profusely.

Is there any way you could satisfy your goals with a relative measurement, i.e. comparing the output with the other registers? That would be a cheap way to cut corners, avoiding having to deal with the expense of a flow hood. Which I think is a couple thousand dollars or more.

I'm not convinced the skills to use a flow hood are all that common, either. In two cases I have had flow hood measurements done, and in one of them the sum of supply ducts was 723 CFM for a 3.5 ton system (which ought to be at least 1200 CFM). We spent some time scratching our heads and looking for a solution, the tech found nothing but recommended a larger return duct. After I invited a 2nd tech to review my situation, we found the air handler was not even on its "high" fan setting, you would think this is a no-brainer to check if there is insufficient airflow. After this one change, the 2nd tech did his own flow hood test and the sum of supplies came to almost 1500 CFM!

From 723 to 1494 CFM because of a fan speed setting? I don't really think this is plausible. I have to conclude that one or the other tech made a bonehead error in measurement. Since tech #1 had already made one bonehead error, I would like to believe #2 is correct. But objectively the only real way to know is to have a 3rd test done.

So I would say, if you have a flow hood test done, that would deliver your answer and probably cost no more than that Kestrel device (which otherwise looks attractive). Even with flow hoods people can make errors and you have to watch the results for plausibility though.

One third possibility, perhaps you can use a manometer to measure airflow. My dad who is a retired aero engineer has done this, and gave me a pitot tube with some instructions. But of course you would have to locate the instructions and acquire the know-how to do the test properly yourself, or else hire an experienced guy to work with you.

Is there any possibility of getting useful info using static pressure measurements in this situation? If so that would be a fourth method. But I certainly don't have the know-how for that.


Best of luck -- P.Student

[dan sw fl]

Originally posted by collins8
I want to enure I have a total of 800 SCFM going upstairs.
Shouldn't the Kestrel be close? I can pull the diffusers and get a reading in the center of the air steam.

Alternate simplified flow hood to
improve air flow accuracy and ease of measurement.

Make a 14" x 14" x 42" styrofoam box with weatherstrip.
Weatherstrip must be used to seal box to floor or ceiling.

Styrofoam substitutes:
Duct Board
Insulation http://www.dow.com/styrofoam/na/iso/tuff_r.htm

Anemometers:
Kestrel ___ Extech ___ Dwyer

14" square = 196 Square inch.
minus 10 sq. in of the instrument.
186 sq inch = 1.29 square foot

Enter 1.29 as area in instrument.

Measure four times with the vane anemometer placed diagonally in each corner. Measure near center.
Average the five numbers.

42" length permits for easy reading of floor or ceiling diffusers and EVEN flow distribution across the cross section.

For low flow (< 100 CFM, diffusers < ~6" x 8") a smooth transition insert to 10" x 10" area (100 -10 = 90 sq. inch = 0.63 Sq Foot) because of the instrument Minimum air flow Velocity of 80 FPM.

Readings below 50 CFM would not be assured as accurate.
However, they would be areasonable within the context of determining total AHU air flow.

[Edited by dan sw fl on 10-16-2005 at 05:35 AM]

[collins8]

Dan,

Would you leave the diffusers installed on the registers??

[pstu]

Thanks very much Dan, for that advice. That really helps me understand more, and no doubt for the original poster too. I have no doubt the Kestrel would be fast and convenient to use, that is part of its appeal.

I submit you could measure *all* the supply registers, add them up, and get some insight from that number. If it is plausibly the capacity of the air handler (at its current setting), it might not be too bad to regard that number as probably correct. If possible I would want to obtain an ESP (External Static Pressure) reading for the air handler, use that to identify where you fit on the fan tables for that air handler. Those fan tables (perhaps there is a more correct name) identify the airflow vs. ESP, information available for nearly all air handlers.

I too am interested in Dan's opinion about removing the diffusers. One might remember the diffusers are an integral part of the system, any measurement without them will give an inflated number for airflow in everyday use. Another thought is that you might have an opportunity to gain airflow by replacing stamped metal diffusers, with more expensive diffusers. If you did that only for the upstairs supplies, that would tilt the airflow in favor of upstairs at the expense of downstairs.

Best wishes -- P.Student

[dan sw fl]

Originally posted by perpetual_student
Those fan tables (perhaps there is a more correct name) identify the airflow vs. ESP, information available for nearly all air handlers.

One might remember the diffusers are an integral part of the system, any measurement without them will give an inflated number for airflow in everyday use.

FAN CURVE is the term that may have escaped you.

Right, as you stated,
Test to AS-IS (diffuser installed) conditions.

K eep I t S imple

[Knight97]

What about figuring out the volume of a large trash bag, and timing how long it takes to fill up?

[dan sw fl]

Originally posted by Knight97
What about figuring out the volume of a large trash bag, and timing how long it takes to fill up?

Bag filling is NOT true reading due to its inherent change in air flow path resistance. However, it does provide an approximate flow rate. How much inaccuracy?

[cem-bsee]

so, after you measure this, what are you going to do?

why not just do that? work on the solution, not just measuring how much problem --

eg: seal all the ducts & cracks with mastic
have dampers in the duct take-offs from the trunks
adjust dampers until all rooms are +/- 3F just after blower shuts down

it takes some patience to set dampers --
& may need different settings for summer & winter




[Edited by cem-bsee on 10-16-2005 at 10:19 PM]

[collins8]

Which one of the 2 instruments below would be the best to spend my money on?? They are close in price.

http://store.yahoo.com/newadv/ke41bynknewm.html

http://www.ambientweather.com/ex40heducfmv.html

[dan sw fl]

Originally posted by collins8
Which one of the 2 instruments below would be the best to spend my money on?? They are close in price.

http://store.yahoo.com/newadv/ke41bynknewm.html

http://www.ambientweather.com/ex40heducfmv.html

I prefer and have used the
Extech 407114 Heavy Duty CFM Vane Anemometer
which has a 2 3/4" Diameter vane.

TWO people would be needed if you used a 'home-made flow hood'.

I believe that the larger vane would provide a more accurate reading with 'inherent averaging' due to a larger 'sampling area'.

Therefore, a 'home-made flow hood' would not be a necessity in making an initial total air flow assessment.