Thread: How to measure air flow

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We just had A/C and forced air with new duct work installed in an older house. The air flow out of the each duct varies considerably. On duct will not blow off a piece a paper and another feels like a wind tunnel. So my question is there a device to measure the air flow at the registers. What airflow should we be looking for at each register and how much difference between registers is acceptable. All the registers are the same size 3.5" x10"

Thanks for your help.

Brian

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go to WWGRANGER.com & ck \$ for air flow hood = \$\$\$

forget the amount of air velocity --
are the rooms comfortable?
are the rooms within 3F of each other just after blower shutoff?

if you think any one supply has too much velocity, then just have another added, or get bigger pipe & outlet.

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Originally posted by cem-bsee
go to WWGRANGER.com & ck \$ for air flow hood = \$\$\$

forget the amount of air velocity --
are the rooms comfortable?
are the rooms within 3F of each other just after blower shutoff?

No the room (35'x18' west facing)where there is a lack of air velocity isroughly in the afternoon about 15 to 20 degrees hotter than the rest.

if you think any one supply has too much velocity, then just have another added, or get bigger pipe & outlet.
There is just one main supply trunk line in the basement, the duct with the most velcity is ay the end of it. Is there a rough formula I could plug in to see how big the ducts should be. (i.e. one 3x10 register needs a 6" round duct, 2 6" round duct need a 4x10 duct an so on.

Thanks
Brian

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Install dampers at each take off of the main duct then you can tweak it.

5. Manual D

>>Is there a rough formula I could plug in to see how big the ducts should be.

There is ACCA MANUAL D, which calculates precisely how big each duct should be. It is rather more complicated than a formula but the pros who use it tell us it is a lot more correct. You can buy the book around \$40, but I think it would serve you better to hunt down an AC technician who knows how to do Manual D calculations. That guy will be more knowledgeable than 90% of the techs out there.

Best of luck -- P.Student

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If you have a heat pump or aux electric elements you can use this method.

http://www.bacharach-training.com/norm/cfm.htm

7. Cheap method

Just remembered something... I have heard someone on this board tell about holding a big garbage bag up to the vent and measure with a stopwatch how many seconds it takes to fill.

You might have to do some math to get actual CFM but if you compare one vent with another you certainly would identify which is flowing more, and by how much.

I would really appreciate hearing the criticism of a pro for this method. Has this been documented before?

Best of luck -- P.Student

8. Re: Cheap method

Originally posted by perpetual_student
Just remembered something... I have heard someone on this board tell about holding a big garbage bag up to the vent and measure with a stopwatch how many seconds it takes to fill.

You might have to do some math to get actual CFM but if you compare one vent with another you certainly would identify which is flowing more, and by how much.

I would really appreciate hearing the criticism of a pro for this method. Has this been documented before?

Best of luck -- P.Student
This is a little trick the scientist at Lawrence Berkley National Laboratory cooked up.
Personally I view this method to be at the least very inaccurate.
When placing a trash bag over any diffuser you immediately change the airflow ditribution of the duct system due to an unidentifiable back pressure.
Even with the use of a flow hood there will be some back pressure but usually not enough to effect the readings on residential type diffusers.
IMHO the only reliable way to test airflow through ducts is by use of a flow hood or a traverse of the duct feeding the diffuser while the system is in actual operation.
Plus when you add the fact that there is no trash bag that has NIST calibration certification you can see my reasoning for questioning it as being valid.

9. Originally posted by b-astill
We just had A/C and forced air with new duct work installed in an older house. The air flow out of the each duct varies considerably. On duct will not blow off a piece a paper and another feels like a wind tunnel. So my question is there a device to measure the air flow at the registers. What airflow should we be looking for at each register and how much difference between registers is acceptable. All the registers are the same size 3.5" x10"

Thanks for your help.

Brian
Brian check out http://www.nationalcomfortinstitute.com for contractors who have been trained to deal with the problems you are expieriencing.

10. size of the regester is not much of a factor. what size pipe is ran to the regester. size of the trunk line and how many runs off it between each reducing transition in the main trunk. how many cfm does the room need and btu
just feeling the register is not going to tell you any thing. are the rooms reaching temp. if not how much differance is there in temp. balanceing is not hard to do.
since you just had the system installed call the contractor and ask them to balance it for you.
if the system was sized and installed properly this should not be a problem and they should know how to balance it out. may take two or three trips but it is thier job

11. Are you sure the reg with no air coming out, isn't a return.

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Originally posted by tinknocker service tech
if the system was sized and installed properly this should not be a problem and they should know how to balance it out. may take two or three trips but it is thier job
If the system was installed properly, it would not need to be balanced out like it does at the present time. If they could not install it correctly, I guarantee you they do not have the equipment to balance it out correctly either.

It only takes one trip to balance a system if you have the proper tools and knowledge.

13. Originally posted by tinknocker service tech
size of the regester is not much of a factor.
The selection of proper registers is very important & often overlooked.
This selection will determine the throw & spread of the conditioned air at a given CFM.
Take a look at ACCA Manual T for more info on this.

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