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10-04-2010, 11:49 PM   #1
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## Duct size/cfm

Hi, i'm a journeyman electrician and am new to the DIY site. Now I understand that cfm and duct sizing goes way beyond my HVAC knowledge, but I have a general question. I've been looking at cfm charts and noticed that a 4" round smooth duct will handle about 50 cfm. A 6" duct will handle about 95 cfm. For example, why would a OTR microwave be rated at 350 cfm, if all that it's capable of being connected to is a 3x10 duct? I also told my father in law about this topic, and he said my figuring is bogus. He says if he put a 10 hp blower on a 4" duct, he would get well over 50 cfm. How do these charts measure up? Any kind of enlightenment on the subject would be great!

Jason.

10-07-2010, 08:44 PM   #2
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Come on guys!

10-07-2010, 09:28 PM   #3
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hey, it's only been 3 days

a lot of the HVAC guys I normally see don't seem to have been around lately.

I'm not an HVAC guy but here is a little bit of what you might be looking for.

saying a duct flows xxx cfm is like saying a hose flows xx gpm. It is only part of the equation. If you increase the pressure, you can make any duct flow more cfm.

here is a statement from another site that should help:

Quote:
 air distribution design takes into account the volume of air to be moved, the desired velocity of the air, and the static pressures developed at the chosen conditions of volume and velocity. without working with all paramaters involved, volume (cfm), speed (velocity), and static pressures (resistance), poor designed system is the final outcome.
so, basically there is no one number that a duct is rated, such as the 4"@50cfm. That is only going to be at a specific pressure. Increase the pressure and you have more cfm, less pressure, lower cfm.

and with your fathers statement; yes, you will get more than the 50cfm with a big blower. You will also have a higher static pressure. Using that pressure, the size of the duct, and the coefficient of friction determined by the material used, you can calculate the actual cfm.

 10-07-2010, 09:40 PM #4 HVAC / Plumbing   Join Date: Feb 2009 Posts: 1,801 Rewards Points: 1,000 see if you can find what your looking for here http://www.engineeringtoolbox.com/eq...ter-d_443.html
10-08-2010, 12:28 AM   #5
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Quote:
 Originally Posted by nap hey, it's only been 3 days a lot of the HVAC guys I normally see don't seem to have been around lately. I'm not an HVAC guy but here is a little bit of what you might be looking for. saying a duct flows xxx cfm is like saying a hose flows xx gpm. It is only part of the equation. If you increase the pressure, you can make any duct flow more cfm. here is a statement from another site that should help: so, basically there is no one number that a duct is rated, such as the 4"@50cfm. That is only going to be at a specific pressure. Increase the pressure and you have more cfm, less pressure, lower cfm. and with your fathers statement; yes, you will get more than the 50cfm with a big blower. You will also have a higher static pressure. Using that pressure, the size of the duct, and the coefficient of friction determined by the material used, you can calculate the actual cfm.

There is a such thing as too much HP.

In a closed duct system such as a home there is a definite limit to the size of the HP that can be used. Going over that limit a motor can't move any more air despite the HP employed. The blower wheel just spins with out an increase in air flow.

Now granted there are high pressure system that move a lot of air thru small diameter ducts. These are specially designed to do a specific industrial application and so on.

A good example is if a duct system was designed to move 1200 cfm with a 1/3 HP motor, putting in a 3/4 HP motor is not going to three times the air flow .

Motor theory is not something easily explained in everyday language.

 The Following User Says Thank You to hvaclover For This Useful Post: jasonj (10-09-2010)
 10-08-2010, 12:54 AM #6 You talking to me?     Join Date: Dec 2007 Location: sw mi Posts: 7,551 Rewards Points: 6,290 In theory, I can always increase output. It is just that increased HP is not all that it takes to increase that output. Regardless, I was simply trying to give the OP something to chew on until you HVAC guys showed up. This isn't my thread and I don't want to hijack it. What can you do for the OP?
 10-08-2010, 03:27 AM #7 Member   Join Date: Oct 2008 Location: Suburbs of Detroit Mi Posts: 3,704 Rewards Points: 2,000 On a motor chart the cfm plot line and amp draw plot lines meet a point an bisect one another showing that the extra power in HP does not move any addtional air.
 10-08-2010, 06:17 AM #8 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,584 Rewards Points: 6,132 The duct chart you are looking at is a FR/pressure loss chart. Not a static pressure chart. I can put 2,000 CFM through a 6" pipe. It would be at a pressure measured in PSIG instead of inches of wc. But I can still do it(got one at 5 PSIG, and use a 100 HP motor on it ). Its a special application though. A 3X10 duct can generally handle 60 CFM without excessive noise. As a range, or microwave vent line. Your not going to get 350 CFM through it unless the blower/fan is capable of working against a 3" static pressure.
 The Following User Says Thank You to beenthere For This Useful Post: jasonj (10-09-2010)
 10-08-2010, 10:57 AM #9 Member   Join Date: Jan 2008 Location: N.C. Posts: 425 Rewards Points: 264 Hi Jason, This site helped me with my dyi HVAC project. http://efficientcomfort.net/jsp/ResDuct_Web.jsp
 The Following User Says Thank You to Earnie For This Useful Post: jasonj (10-09-2010)
 10-09-2010, 12:31 AM #10 Newbie   Join Date: Oct 2010 Location: manitoba Posts: 9 Rewards Points: 10 Thanks so much guys! It makes more sense now. I just don't quite understand what that chart would be used for if their are so many variables.
 10-09-2010, 12:37 AM #11 Newbie   Join Date: Oct 2010 Location: manitoba Posts: 9 Rewards Points: 10 the supplier sized it all for me! I'm building a new house and all the HVAC is up, it went up quite well!
 10-09-2010, 05:35 AM #12 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,584 Rewards Points: 6,132 So what size did they say for your exhaust duct.
 10-09-2010, 12:09 PM #13 Newbie   Join Date: Oct 2010 Location: manitoba Posts: 9 Rewards Points: 10 They sized the whole furnace, hrv, and air conditioning system. I did the bath fans and otr microwave. I went with a 6 inch smooth duct for the microwave and 4 inch smooth for the bath fans.
 10-09-2010, 12:11 PM #14 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,584 Rewards Points: 6,132 I'd be leary of their furnace sizing. Unless you sent them a blue print of your house. Other wise. they just went by sq ft. And guessed. And you could end up with an under or oversized furnace.
10-10-2010, 08:30 AM   #15
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I gave them the blue print and they asked all the questions like overhang size, types of insulation used. etc. I went with a 15 kw nortron ecm furnace and a van eer 90h hrv. The house is a 1600 sq ft bungalow with an icf (foam) basement, R-30 walls, R-50 attic, and foam under the basement slab. I can't wait to see the heating bills this winter

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