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Old 01-03-2010, 01:11 AM   #1
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Cfm fpm


Is there a way that I can check the CFM and FPM of my furnace and ductwork? By check I mean find out what they are.

I probably need a meter for this, right?

I could get one on e-bay for under $200.00. Are they worth it???

Thanks

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Old 01-03-2010, 02:05 AM   #2
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Cfm fpm


For that price you will be measuring air velocity.
If you are measuring round ducts with no grills then Pie R squared divided by 144 times fpm = cfm.

If you are measuring grills with this meter your results may vary greatly.

A balnor hood is much more accurate but 10 times the price.

I would get a manometer and measure static pressure across the furnace and compare the reading with your owners manual to determine cfm.

A manometer is a much more useful meter for the same price as an air velocity meter.

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Old 01-03-2010, 08:51 AM   #3
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Cfm fpm


Good digital manometers go for around 200 bucks. Which means he may just be measuring static pressure, and not velocity pressure.


OP. Do you have the install manual for your unit. If so. Then you can determine CFM and FPM from the static pressure.

Can also check register pressure drop to determine CFM from each register, to determine duct leakage.

Takes a long piece of duct with no ells, tees or take offs to get an accurate velocity pressure.

Digital manometers are worth their money. Because you can use them to also diagnose infiltration problems in your house. If your willing to do the leg work.
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Old 01-03-2010, 09:07 AM   #4
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Cfm fpm


BeenThere: Can you cite any sources (links) for testing with a manometer, other than SP?

Particularly, your comment: Digital manometers are worth their money. Because you can use them to also diagnose infiltration problems in your house. If your willing to do the leg work.

Thanks.
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Old 01-03-2010, 09:14 AM   #5
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Cfm fpm


Off the top of my head, no. Think someone on another board posted one awhile back though. Just don't remember the thread to find the link right now.

But you can check pressure differential between your home and the outdoors with them(need to be able to measure in pascals to do it).

If your house has rooms that are neg or pos 8 or more pascals then the outside(I believe ASHRAE says 4 pascals). You have a problem with that room.

When a blower door test is done. Its done using a manometer to read the pressure differential. The house is taken to 50 pascals above outdoor pressure.
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Old 01-03-2010, 10:12 AM   #6
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Cfm fpm


http://www.ueitest.com/product-em201.html

I have this one.
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Old 01-03-2010, 06:53 PM   #7
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Cfm fpm


Thanks for all your replies.

BeenThere- I have the owners manual and the install manual and neither one gives any info about static pressure or fpm or cfm.

Houston204- Your formula is great but, to use it I need the fpm which is one of the things I'm trying to find out.

SO, is there a formula to find the fpm & cfm out if I know the static pressure? Am I looking for the supply static pressure before or after the A coil? Speaking of which, what is a permissible pressure drop across the A coil if any?
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Old 01-03-2010, 07:38 PM   #8
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Cfm fpm


I found some blower performance data on line for my unit. It gives cfm at CFM @ external static pressure-in.w.c with filters.
I understand the in. in w.c. but what is external static pressure? Is that the house static pressure with the furnace running?
Now all I need is the formula to find the FPM and I'm set, I think, for now.

thanks
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Old 01-03-2010, 08:59 PM   #9
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Cfm fpm


If you have a gas or oil furnace.
The external static pressure is the pressure between the evap coil and the furnace supply. Added the the return pressure measured between the air filter and blower(ignore the negative sign for teh return pressure).

To find FPM in a duct when CFM is known. Divide the CFM by the sq ft area of the duct.

EG:

600CFM
8X20 duct.

8X20=160, 160/144=1.11sq ft.
600/1.11=540.5FPM
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Old 01-04-2010, 12:15 AM   #10
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Cfm fpm


Isn't air velocity and fpm same same??
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Old 01-04-2010, 01:46 AM   #11
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Cfm fpm


If you don't have a manometer, flow hood, etc, here is a ball park method using a watch, a thermometer and the sensible heat formula. (1.08 is a special constant you use for this formula)

turn your furnace on (high heat). go out to your gas meter. clock the meter.
i.e.-- how many seconds does it take for 1 cubic foot of gas goes though the meter. example below is 50 seconds
[CF standard gas] (1000btu) X 60 sec. X 60 min. / #seconds= btu per hour

Example... (1000 btu X 3600) / 50 seconds= 72,000 btuh (how much gas you are burning per hour)

Multiply 72,000 by furnace efficiency example 90%= 64,800 btuh output.
measure return air temp at return grills (example 70F).
measure the air temp at the closest register to the furnace.(example 125F)
125- 70= 55F degrees. (this is the delta T or temperature rise) [ballpark]
DO NOT EXCEED MANUFACTURERS SPECS. (inside furnace burner side)

now this formula
BTU / (1.08 X delta T) = CFM

sensible heat formula would be 64800 / (1.08 X 55)= 1091 CFM.

take off every register and measure the duct size.
calculate the area of each duct add all the areas together. (example- 10- 6 inch ducts would total pie R squared X 10--- (3.14 X 9)(10)= 282.6 CI
divide by 144= 1.9625 square feet.
Average system velocity at the boots is [1091 cfm / 1.925 sf = 556 FPM]
(if the ducts are sealed good)
measure the temperature at each register and feel for good air flow and room temp.
velocity is how many cubic feet you shoving through how many square feet.
after this you can decide if you want to buy some tools or call a tech.
be safe.

happy 2010

Last edited by okrite; 01-04-2010 at 02:48 AM.
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Old 01-04-2010, 06:28 AM   #12
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Cfm fpm


I know you stated ballpark.

That formula is ok. If your really getting 1000BTUs per cubic foot of gas.
And if the furnace is really at its rated efficiency.

But don't use the AFUE efficiency. That is NOT combustion efficiency.
So you have to look up combustion efficiency. Or test combustion efficiency.

Its also flawed because you don't know how much temp the return lost from the register to the furnace. Nor how much it lost from the furnace to the supply register.
Can easily be a 10 to 20 degree total inaccuracy in temp measurement that way. Which really throws off the calc.

If that supply and return temp were accurate. It would also only be good for that supply register.

Next, 1.08 is only good at sea level(generally close enough up to 2,000 foot elevation).

While usable for a wild ballpark. A 20% inaccuracy, isn't good for much.

Seldom will you find the first supply and the last supply on the average install close to each other in CFM or velocity.
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Old 01-04-2010, 06:32 AM   #13
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Cfm fpm


Quote:
Originally Posted by thumbkins View Post
Isn't air velocity and fpm same same??
In this case. FPM is air velocity stated in FPM.

To make it easy for use to work with. Instead of saying 800 CFM at a velocity pressure of .002 in a duct of .9 ft cross section.

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