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Confused On Air Flow And Cfm

 
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Old 12-13-2016, 09:16 PM   #1
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Confused On Air Flow And Cfm


Hoping you guys can explain ductwork cfm to me. Seems simple but don't understand this case.
In the past I have used a couple types of exterior dryer vents dampers that seal pretty good for energy efficiency.
Never ever found a good sealing one for a kitchen or bath exhaust fan. And most of the dryer ones are junk also.
Of course for a kitchen it has to have a light flimsy flapper for when the fan is used on low speed.
Now from the info I have.
An average dryer puts out 120 cfm
Average broan range hood fan puts out 160 cfm. duct reduced down to 4" for this test.
The panasonic inline bath fans we like to use are 120 cfm .
Now in the past I have tried a good dryer 4" vent on the bath and kitchen fans above and they did not have enough air flow to open the damper.
At the time I did a test to try it out. I assumed a Clothes Dryer put out much more cfm's and that explained the problem. Also just placing my hand by a dryer vent also seems to feel like more air flow.
I just recently I found out that dryers are only rated for 120 cfm's.
So why would the dryer have the airflow to push open my preferred type of dryer vent but the kitchen and Panasonic fan would not.
Thanks
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Old 12-14-2016, 05:46 AM   #2
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Re: Confused On Air Flow And Cfm


The fans are rated to move X CFM against X amount of resistance.

A clothes dryer may be rated to move 120 CFM against a resistance of .7". While the range hood fan is only rated to move 160 CFM against a resistance of .3".

Look at the rating of the Broan hood. See what static its rated to work against. 4" duct may be the main problem.

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Old 12-14-2016, 08:20 AM   #3
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Re: Confused On Air Flow And Cfm


CFM = (fpm X area)

The dryer and the range hood motors likely have a different feet/min air velocity outputs. Whirlpool's website says their dryers require between 105-230 CFM for effective operation.

In this case for example, if you find the fpm listed in the dryer specs, as well as the dryer CFM requirements, you can solve to find the correct duct diameter.

For Example: The dryer requires 250CFM to operate effectively. And the air velocity that the motor can put out is 1000fpm. After you do the math (and realize everything in feet) you come up with 0.25' for a duct diameter. Which is 3.0"

Should you go bigger than the 3.0" the dryer/range hood/furnace/whatever fan you have doesn't have the ability to push enough air through the bigger duct. Thus, not being able to open a vent.

Bear in mind, this doesn't mean that 3.0" pipe is the way to go, this example simply shows how the formula works using made up numbers from my head. YMMV regarding the correct ducting to use.

Last edited by CharlieDelta; 12-14-2016 at 09:01 AM.
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Old 12-14-2016, 08:28 AM   #4
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Re: Confused On Air Flow And Cfm


Quote:
Originally Posted by CharlieDelta View Post
CFM = (fpm X area)

The dryer and the range hood motors likely have a different feet/min air velocity outputs. Whirlpool's website says their dryers require between 105-230 CFM for effective operation.

In this case for example, if you find the fpm listed in the dryer specs, as well as the dryer CFM requirements, you can solve to find the correct duct diameter.

Example: The dryer requires 250CFM to operate effectively. And the air velocity that the motor can put out is 1000fpm. After you do the math (and realize everything in feet) you come up with 0.25' for a duct diameter. Which is 3.0"

Should you go bigger than the 3.0" the dryer/range hood/furnace/whatever fan you have doesn't have the ability to push enough air through the bigger duct. Thus, not being able to open a vent.

Have you ever seen a dryer vented on 3" pipe? Choking down a dryer vent to 3" will create back pressure and over heat the dryer, cycling the heat by a high temp limit and cause delayed drying times.
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Old 12-14-2016, 08:50 AM   #5
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Re: Confused On Air Flow And Cfm


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Originally Posted by Mech Diver View Post
Have you ever seen a dryer vented on 3" pipe? Choking down a dryer vent to 3" will create back pressure and over heat the dryer, cycling the heat by a high temp limit and cause delayed drying times.
My numbers were arbitrary for example purposes only. Just pulled from my head to exercise the CFM formula. I'm not saying use a 3" pipe for his or anyone's dryer by any means.

Last edited by CharlieDelta; 12-14-2016 at 08:57 AM.
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Old 12-14-2016, 08:52 AM   #6
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Re: Confused On Air Flow And Cfm


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Originally Posted by CharlieDelta View Post
My numbers were arbitrary for example purposes only. I'm not saying use a 3" pipe for his dryer.
Somebody will read that and think it is the way to go because 3" is cheaper than 4":

"Should you go bigger than the 3.0" the dryer/range hood/furnace/whatever fan you have doesn't have the ability to push enough air through the bigger duct. Thus, not being able to open a vent."
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Old 12-14-2016, 08:53 AM   #7
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Re: Confused On Air Flow And Cfm


Quote:
Originally Posted by Mech Diver View Post
Somebody will read that and think it is the way to go because 3" is cheaper than 4":

"Should you go bigger than the 3.0" the dryer/range hood/furnace/whatever fan you have doesn't have the ability to push enough air through the bigger duct. Thus, not being able to open a vent."
If they have a reading comprehensive above a 2nd grade level, they will understand "For example purposes only"

Last edited by CharlieDelta; 12-14-2016 at 08:59 AM.
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Old 12-14-2016, 09:02 AM   #8
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Re: Confused On Air Flow And Cfm


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Originally Posted by CharlieDelta View Post
If they have a reading comprehensive above a 2nd grade level, they will understand "For example purposes only"
LOL, you have seen some of the questions that people ask on forums?
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Old 12-14-2016, 09:05 AM   #9
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Re: Confused On Air Flow And Cfm


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Originally Posted by Mech Diver View Post
LOL, you have seen some of the questions that people ask on forums?
Many times on many forums over yes. Lol just like ole George Carlin said “Think of how stupid the average person is, and realize half of them are stupider than that.”
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Old 12-14-2016, 09:22 AM   #10
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Re: Confused On Air Flow And Cfm


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Originally Posted by CharlieDelta View Post
Many times on many forums over yes. Lol just like ole George Carlin said “Think of how stupid the average person is, and realize half of them are stupider than that.”
George was ahead of his time.
Imagine if he was around for 'snowflakes' and the rest of the new 'kinder-gentler' millennial?
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Old 12-14-2016, 09:29 AM   #11
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Re: Confused On Air Flow And Cfm


Quote:
Originally Posted by Mech Diver View Post
George was ahead of his time.
Imagine if he was around for 'snowflakes' and the rest of the new 'kinder-gentler' millennial?
We wouldn't have enough safe spaces in the country for those idiots.

I loathe people my age (18-30). Cannot stand the attitude of my generation.
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Old 12-14-2016, 09:38 AM   #12
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Re: Confused On Air Flow And Cfm


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Originally Posted by CharlieDelta View Post
We wouldn't have enough safe spaces in the country for those idiots.

I loathe people my age (18-30). Cannot stand the attitude of my generation.
I'm an old grumpy guy that thinks they need to suck it up, and move on.
My kids are 40, 38, 30, and 25, I thank God none of them are whiners.


Just the fact you are working in a trade speaks volumes towards values.
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Old 12-14-2016, 09:52 AM   #13
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Re: Confused On Air Flow And Cfm


Quote:
Originally Posted by Mech Diver View Post
I'm an old grumpy guy that thinks they need to suck it up, and move on.
My kids are 40, 38, 30, and 25, I thank God none of them are whiners.


Just the fact you are working in a trade speaks volumes towards values.
I spent 5 years on campus, and graduated with an engineering degree. I see people graduate with degrees that make me cringe. Those are the same ones who can't find work because no one wants to hire a women's studies major or a liberal arts major.

I'm in the trades because I want to be in the trades. Corporate life isn't for me. Sitting at a desk 9 hours a day isn't for me either lol.
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Old 12-14-2016, 10:06 AM   #14
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Re: Confused On Air Flow And Cfm


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Originally Posted by CharlieDelta View Post
I spent 5 years on campus, and graduated with an engineering degree. I see people graduate with degrees that make me cringe. Those are the same ones who can't find work because no one wants to hire a women's studies major or a liberal arts major.

I'm in the trades because I want to be in the trades. Corporate life isn't for me. Sitting at a desk 9 hours a day isn't for me either lol.
Ancient American Civilizations and Art History are also handy degrees to have, lol. I have a degree in Telecommunications Engineering, worked 4 years for a large corporation and if two of those years weren't out in the field I'd have left sooner. I was raised in the trades and always went back to it when I got bored with other things. Mechanical and electrical work have been very good to my entire family.
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Old 12-14-2016, 11:31 AM   #15
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Re: Confused On Air Flow And Cfm


Thanks all, makes more sense now.

Just have always looked for a way to have a bath fan or range hood energy efficient.
Any damper is so light weight that there is no way to get a good seal.
And with the gap around the damper its probably like always having a 1/2" hole in your wall.

A motorized damper is usually not an option.
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Old 12-15-2016, 05:26 PM   #16
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Re: Confused On Air Flow And Cfm


The type of fan will have a huge effect on what range of back pressure they can perform under, in general a squirrel cage's performance will drop off slower then the flower bladed fan...

Placement of the exhaust vents where they can be observed, cleaned, or replaced is strongly recommended. shorter the better for grease and dryer vents. The grease vents having the added downside of being a source of food(grease) for various varmints, feathered and Rodent.

I don't understand why the EPA doesn't require direct vent style double walled piping to increase dryer/vent efficiency, other than increased maintenance/ fire hazard
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Old 12-15-2016, 05:59 PM   #17
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Re: Confused On Air Flow And Cfm


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I don't understand why the EPA doesn't require direct vent style double walled piping to increase dryer/vent efficiency, other than increased maintenance/ fire hazard
I don't understand how double walls would increase the efficiency of a vent. Educate me?
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Old 12-15-2016, 07:21 PM   #18
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Re: Confused On Air Flow And Cfm


A direct vent fireplace heats the incoming air with "waste" heat removed by the counter flowing air layer being plumbed to the combustion chamber intake via double or triple walled pipes... and not using 'conditioned' interior air.


The dryer air exhaust would be much cooler and a higher volume then waste from a fire place with much differ air fuel ratio...

Condensation and icing would be a greater issue, all other things being equal, but a trapped clothes washer drain is usually next to most dryer installs.
Another downside would fewer house air exchanges in closed windows seasons.

Last edited by Fouthgeneration; 12-15-2016 at 07:24 PM.
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Old 12-15-2016, 09:23 PM   #19
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Re: Confused On Air Flow And Cfm


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I don't understand how double walls would increase the efficiency of a vent. Educate me?
He's right, only problem is it would be one big ass pipe.
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Old 12-15-2016, 09:51 PM   #20
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Re: Confused On Air Flow And Cfm


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Originally Posted by Fouthgeneration View Post
A direct vent fireplace heats the incoming air with "waste" heat removed by the counter flowing air layer being plumbed to the combustion chamber intake via double or triple walled pipes... and not using 'conditioned' interior air.
Point taken, but that really has nothing to do with venting efficiency.

Quote:
The dryer air exhaust would be much cooler and a higher volume then waste from a fire place with much differ air fuel ratio...
Don't see how that follows. Most dryers I'm familiar with simply crank the heating coils for a fixed amount of time to heat the air.

Warmer incoming air would be a benefit in terms of shorter drying times for a unit with a humidity sensor, but again, that has nothing to do with how efficient a vent is.

Inner has a point; a big enough double wall would provide a good source of make-up air--which would indirectly increase the efficiency of venting.

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