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Old 10-29-2011, 01:40 AM   #1
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Ductwork sizing help


Hi, I sure could use a hand getting my ductwork properly sized. My home had a floor furnace with no ductwork that I removed. I bought the HVAC -CALC program to do a manual j on my house . That worked great. I bought the manual d book but just can't puzzle it all out. I have all the numbers from the manual j and really need help figuring out the size for the duct. I have a diagram of how I think it should be layed out just no sizes . Can any one help or point me to the right place? Thanks in advance, Tyler

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Old 10-29-2011, 04:47 AM   #2
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Ductwork sizing help


Post diagram, and all lengths of ducts.

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Old 10-29-2011, 11:08 AM   #3
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Ductwork sizing help


first floor
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Old 10-29-2011, 11:18 AM   #4
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Ductwork sizing help


The second floor. The supply from the first floor is 10'. I have the specs for my blower motor for the furnace but am unsure at what speed it will operate.
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Old 10-29-2011, 11:19 AM   #5
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This is the second story.
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Old 10-29-2011, 02:29 PM   #6
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Ductwork sizing help


You have roughly 120' of TEL for the longest supply. I don't see any return duct work drawn.
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Old 10-29-2011, 03:18 PM   #7
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It is in the first scan. It will be appx. a 5' run. Just beside the furnace.
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Old 10-29-2011, 03:20 PM   #8
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Ductwork sizing help


Does it go straight up to a floor register?
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Old 10-29-2011, 04:04 PM   #9
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Yes. I would like to use round duct through out the system except the return.
Any thing else you need , just ask as I am unsure as to what to post for you. Tyler
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Old 10-29-2011, 04:19 PM   #10
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Ductwork sizing help


The return will add another 105 foot of TEL. So you have roughly a total of 225 foot.

Add up at the other pressure drops, air filter, supply boot(just one), return grille, dampers, A/C coil. Subtract this from the furnaces rated ESP, most likely .5". this will give you the ASP(Available Static pressure). Say it comes out to .22" ASP.

>.22/225=.000927, .000927X100=.097. So you would use .09" friction rate in that example.
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Old 10-29-2011, 04:32 PM   #11
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So how can I determine the duct size to each vent? When I did the manual j it gave me the cooling and heating cfm for each room at a given amount of cfm from the equipment. If I post that would that help?
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Old 10-29-2011, 04:35 PM   #12
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Need a friction rate slide chart, or use the charts in manual D.

Use the info I posted above to determine the ASP you have and the friction rate to use. then select the duct size that will carry that CFM to the room.
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Old 10-29-2011, 05:12 PM   #13
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I appreciate the help but I guess I am just not getting it. The math is overwhelming and honestly I am unable to perform it. I guess I will have to figure out a different way.
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Old 10-29-2011, 07:11 PM   #14
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Look at how many CFM a room needs, then look in the manual D book at the amount of air a X size pipe will flow at say .07" friction rate, that is the size pipe you use.

EG: A room needs 80 CFM, using .07" FR, a 6" round will fl;ow that amount.
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Old 10-29-2011, 09:47 PM   #15
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That makes a little more sense , I'll give that a try. Thanks , Tyler

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