accurate cfm charts?


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Old 09-20-07, 04:15 PM
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accurate cfm charts?

i havee links below to two charts of cfms for different duct sizes. the charts are in line for most of the information, but it seems they differ big time as the sizes go up. i'd love to know which one folks here think is the most accurate. thanks.

from warmair.net :

http://www.warmair.net/html/ducts.htm

and one i found a link to on here:
http://www.hamiltonhomeproducts.com/.../DuctChart.gif
 
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Old 09-20-07, 05:59 PM
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Wink

Id say they are both close to the same. What you do need is a good duct calculator .To work out the duct layout of a job
 
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Old 09-20-07, 06:16 PM
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Those numbers are just a rough guide.

The first chart tells you what the APPROXIMATE air velocity the sizing is based on and the second does not say anything about it.

As Ed says if you want more accurate numbers you need to do more detailed calculations.
 
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Old 09-21-07, 08:47 AM
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this is a bit long but...

after reading your replies, i actually just grabbed a ductulator off ebay. should have it in a few days. its one of those basic ones like you get from trane or carrier. i'm wondering how detailed i should expect it to be?

i'm replacing two separate furnaces from a two family house with one central furnace for use as the house is now a single family. the previous systems and their ducts were all functioning well, with the only problem being old inefficient furnaces ( 40 + 30 years old each, VERY PRICEY ). i was fortunate to have been given for free a nice carrier two stage 95% furnace from a friend, and now i just have to put in the ductwork.

i'm really curious about using large diameter round takeoffs from the plenum feeding smaller runs to each of the registers, as opposed to the more common ( at least around here ) square extended plenum feeding all the individual runs. the main reason is that i have a lot of 10" round metal pipe left from one of the old systems return layout. the same friend who gave me the furnace also has a few different sizes of flex i can take, 14" up to 20". i'm wondering if this thing will help me determine a layout using a series of round supplies, with a layout similar to a typical extended plenum. will i be using one large round duct the whole length with a series of wyes coming off, will it invlove feeing into dist. boxes an then sizing down after each...ect.
 
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Old 09-22-07, 05:23 AM
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duct sizing

Round ducts are better than rectangular ducts from an air leakage standpoint. Duct air leakage has been cited as one of the most significant reasons for low efficiency of HVAC systems; if the air doesn't get where you want it, you are wasting money! Round ducts have a single longitudinal seam, whereas commercially available ductwork comes in 2 "L" shapes to be field assembled. Hence you have twice the amount of longitudinal seam for which you have twice the opportunity to lose air through that joint. Smooth round duct offers less resistance to airflow than flexible duct of the same length, approximately 3 times less. So a 10' length of flexible duct has the pressure loss equivalent to approximately 30 ft. of smooth metal duct of the same diameter.
 
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Old 09-26-07, 11:31 AM
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so i got my little ductulator in the mail today. so my question is, since i'll be using some ( and i mean some ) short runs of flex, what should i use as a friction per 100ft for that vs. rigid? i've looked around a bit for info, and it seems ".1" is good for rigid metal, but what for the flex? if i drop to ".5" it seems like i would just size up the flex from what i'd use if it were rigid ( ie instead of a 7" metal, i''d go up to an 8" flex ).

also, is there a place i can find what effect elbows, dist boxes, or other transitions have on the equivalent length of the run ( ie a 90deg bend adds xxft to the overall run )?
 
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Old 09-26-07, 11:41 AM
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duct sizing

Charleydutch, I normally size my commercial ducts for 0.10 in./110' on supply, 0.08 in./100' for returns, but 0.10 for both is close enough for residential. This is for smooth metal duct. There are ductulators for flex duct also, but I just use 3x the smooth metal duct loss for the flex duct portions, e.g. if you have a 10 ft. run of flex, figure this is 3x the loss of smooth metal, so use the smooth metal duct factor for a 30 ft. run instead of the 10 ft. of flex. Most residential and light commercial fans will work fine for 0.5" static pressure total [supply & return ducts], and it is safe to assume that if you are under that total and you have selected a fan that delivers, say 1200 CFM at 0.5" s.p., you are getting close to that amount of air.
 
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Old 09-30-07, 06:22 PM
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yuda, thanks for all the info. as it stands i'm trying to figure out the actual and equivalent lengths to determine the longest run overall, and to see if a set-up i'd prefer would work. i found a book at the library that had a listing of tons of different fittings and transitions and their eq. length, but i'm at a loss for how to factor in a distribution box?

i'm thinking of sending my second floor heat via two 10" round metal ducts, from the basement up to the attic to distribution boxes, with duct runs going from there to the registers. it would enable me to reuse the exitsting ceiling vents from the old setup and some of the ducts, as well as minimize the amount of demolition and rebuilding of walls and soffits. and so far the way the numbers are looking the overall length of the runs will be much less thanks to a limited number of bends ( only 1 90 on one, 2 on the other before the attic boxes ). i've hit a snag as far as how to factor in a distribution box. the ducts would enter it at the bottom, with round ducts running straight out of them to the registers. to i just count the takeoffs? does the size of the box matter ( i'm thinking 12x square )? does this make sense?
 
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Old 10-01-07, 02:51 PM
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duct heating loss

Charley, I am currently out of town and w/o my ductulator to help with this question until Wednesday. But you are on the right track to take the longest run and calculate the loss on it to get the total external static pressure that the fan will work against. As I said earlier, using 0.1 in./100 ft. loss for both supply & return ducts, and since most fans are rated at 0.5 in. external static pressure for nominal CFM ratings, a system could tolerate an equivalent duct length of 500 ft.; quite a lot. This external static pressure is not entirely duct loss though. You must allow for entrance losses to the filter, filter loss, exit losses at the supply register, etc. to do it exactly. But these are normally very small losses for residential systems. So if you lumped them together and estimated that they my total to 20% of the total loss, or 0.1 in. [0.20 x 0.50= 0.1], then you would have available 0.4 in. of static pressure max. loss allowable for ducts alone; still a lot. Using 0.1 in./100 ft factor, this would be an equivalent length of 400 ft.
For the distribution box [called a plenum], look at your ductulator for 12x12 duct and set the fan CFM arrow, you should see that the est. loss for the 12x12 duct is a value, "x" in./100 ft. Multiply that value by the approximate length of the box to get the loss for that length of plenum and add it to your running total. Then continue with the supply ducting downstream of the box. Unless you have some very small duct sections trying to handle a lot of air, you will likely find that your total estimated duct equivalent length is well below 400 ft. and that the nominal rating of your fan will work with your system.
I will be glad to assist you further once I return home if you need any assistance.
The best thing that you can do to improve performance of your system is to seal it tightly, as this is one of the leading causes of inefficiency. You pay $ to move the air in fan HP, and you pay to heat or cool that air, so you want it to stay inside the duct until it gets where it is needed, and not exit into your attic or crawlspace instead. It would be nice to cut all the duct insulation open to expose all seams and to tightly seal them all [then reseal the insulation], but this may not be practical. You may find the real big leaks by just feeling or listening closely as you inspect your duct system while the fan is running on high speed, and sealing just the big ones. If you intend on doing this, let me know and I will provide the name of a quality ducy sealing tape to use, or a paint-on sealant that is ok, but not as good. Do NOT use "duct tape".
 
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Old 10-01-07, 04:54 PM
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again, yuda big help. and i only keep referring to them as "distribution boxes" since thats what i seem to find on the different hvac supply sites. i kind of figured maybe plenum was only used when it was attached sdirectly to the furnace/handler, and that anything down the line connecting a series of ducts to one larger supply ( like in a spider set up ) was called a distribution box.

i guess i never thought to just use the ductulator for that...i was too focused on eq. lenghths of fitting i guess. never the less i'm going to be way, way under 400. hell, i've been figuring on that 100' mark from the ductulator abd trying to have my longest run hit that. i don't think i'll come close to 150, let alone 400!
 
 

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