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Manuel6 10-25-2006 10:25 AM

Plenum/Duct sizing
I am about to fix a duct job that was all screwed up by a contractor, and I have a few questions about sizing the duct coming out of the supply plenum.

My 3 ton air handler is located in a garage, single story home, and is at the beginning of the supply duct run. My understanding is that a 3 ton system will flow at least 1200 cfm. The supply trunk come out of the supply plenum now is 12", which in my opinion is too small for the flow of the air handler. The supply trunk runs about 10 feet before reaching the first distribution box. The second distribution box is another 12 feet away to the right of the first and supplies the last three rooms of the home. Can anyone give me any info on what the supply trunk should approximately be. I understand that air speed is important, but it just seems to me that 12" supply trunk would have the air moving too fast, greatly increases the static pressure, and would ultimately be an energy waster. Any help is appreciated.

pjpjpjpj 10-25-2006 11:00 AM

When you say the supply trunk is 12", I assume you mean 12" diameter round duct.... At the unit outlet, assuming you are pushing 1200 cfm, you should have at least a 14" round, preferably a 16" round. There is no easy straght-line calculation you can do, it is a function of pressure drop within the duct (if you can get your hands on a ductulator, you can figure it easily).

In order to size the duct farther down the run, you would have to know the cfm of air leaving in each branch (and therefore the cfm left in the "main").

Manuel6 10-25-2006 11:52 AM

Thanks PJ for your reply. I did get a duct calculator which is why I was so perplexed when I so the 12" round duct. And by the way, all of the duct work I reference is fiberglass flex duct. Sorry, I live in Florida, and around here everything is flex. I just stupidly assumed people would somehow assume that. Anyway, I figured that I would like to have 1200 cfm flowing out of the supply plenum at about 900 fpm, requiring a 14" duct. Then, coming out of the first distribution box I will need another 475 cfm flowing to the master bedroom, the kitchen, and the master bath. For this run I figured I would use a 10-12" duct. The 12" duct seems a little big; however, I though oversize it a bit, since the duct has to make 4 90 degree bends before it gets to the other distribution box (it has to go up and down two A framed trusses), and I wanted to reduce the friction loss as much as possible. What do you think?

Also, I noticed on the first distribution box that the duct taking off to the other side of the house is coming out of the top of the distribution box. This, it would seem to me, would not be a good design, since the air is going to lose velocity/pressure because of friction loss and will now lose even more velocity/pressure because of an additional change of direction upwards agains gravity. Is this a concern??? Thanks again

#CARRIERMAN 10-26-2006 04:56 PM

Hi Manuel6

I was just thumbing through your response, was hoping to maybe keep you from having to due changes twice. When using your duct calculator you want to use 700fpm @.08 for a quiet system. This is going to put your trunkline @ 16".

Good luck.

#CARRIERMAN 10-26-2006 09:15 PM

Hi again Manuel6

When I put my two cents in earlier I forgot to include an important part of this equation. If you decide to go with what I recommended. When you take a run or two off of a section of the trunk, you will need to drop the next pipe size in your trunk by two inches. If you do this every time you take two taps out, the velocity will be the same throughout the system. This will eliminate the end rooms not getting any air, you will be able to go anywhere in the house and be comfortable. Sorry about this little bit of information.

Good luck.

Manuel6 10-27-2006 10:53 AM

Thanks for your help Rusty. I do plan on only doing it once and doing it right, so I really do appreciate your help, and the quietness of the system is important to me, so I will definitely take your advice. Essentially, I would just measure cfm's left in the trunk after each take off and step down the trunk size so as to keep the remaining cfm's traveling at 700 fpm...right? Also, what do you think about the trunk run coming out of the top of the distribution box (duct board box)? Is that an issue?

#CARRIERMAN 10-27-2006 01:47 PM

Hi Manuel6

What I was suggesting to you has to do with your velocity. You really don't have to worry about your runs coming off of the main trunk and figuring them for air delivery other than for the room. As long as you take two runs out and then drop your trunk line size each time you do that by 2" you will be fine. But if you have a L shaped duct system you will have to take into account for the side trunk. But at this point I am assuming that you trunk is predominatley straight? Just make sure that they sized the rooms properly to make sure your air delivery is good. The rest will be water under the bridge.

Good luck.

pjpjpjpj 10-30-2006 12:10 PM

For the main, I believe you should go with a 16" round duct - which is just under 900 fpm (859.4). That's about a 0.07 pressure drop, which is fine (I think Rusty was talking about the 475 cfm branch when he said 700 fpm at 0.08, although actually 700/0.08 is more like 400 cfm). Anyway, 900 fpm for 1200 cfm is like a 0.075 drop, so that's fine.

The 475 cfm branch should be a 12" round. That's a little oversized, but with the elbows you mentioned (and flex duct has more drop, inherently), I wouldn't go any smaller.

1200-475= 725 cfm left in your main.... 0.08 for 725 cfm is about 825 fpm. Being generous (round down to 800 fpm), that drops your main down to a 14" round.

I also suggest you install a hard duct collar with a manual damper in each branch, so you can better balance the system. Register dampers at the grille usually are open/close, and don't do well for balancing any flow in between that.

jacksprat 03-28-2011 08:36 PM

I have a similar issue but due to the fact that flex duct is already installed from the trunk feeder to the room registers, I want to just change the trunk size, Now have three 8 inch taps. what size trunk should be installed?


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