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Old 08-12-2012, 07:30 PM   #1
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Ive asked alot of good and stupid questions. Most stupid, some good. I think i have everything i want planned out for a week during the fall / winter time.

Last thing i need to know; I recently googled for Plenum Distribution and got a few learning tools that i could apply to my house.
Upstairs i have 4 rooms (11x10, 12x10, 15x13, 9x10)
The duct work for each pair of rooms is pretty close to each other.
I have 5 ducts upstairs, 4 for each room and 1 for a 2 way split to the bathrooms all 8 inch ducts.

If i decided to pair up the rooms and take the 15x13 pair it with the 12x10 by using a reducer of 10inch duct split off to an 8inch for the 15x13 then reduced to an 8inch to go to the 11x10 would it be sufficient?

If my calculations are correct 10 inch duct supplies just over 400 CFM. If i split that 3 or 4 feet from the reducer on an 8 inch then reduce the 10 inch to an 8 inch for the 2nd room would that be just as efficient as 2 8 inch ducts going separately to the rooms?

between the other 2 rooms i would do a 9inch, split an 8 inch before the ruducer to a 6inch to the 9x10 room.

Trying to save duct costs, space and the amount of air in ducts upstairs that will be hot when the AC starts up again to spit out.

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Old 08-12-2012, 07:43 PM   #2
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I guess you are taping of the Fau from downstairs so(and using flex duct)here's what you need IMO.

twye's....10x8x8....8x7x5...8x6x6

pipe(comes in 25' boxes)
box of 5",6",7",8" and 10"
duct ties(big zip ties)duct tape,1" hanger strap,#8x1/2" zip screw or tek screw.roll of 9" or 6" fiberglass wrap.2-10x6x6 floor box,12x6x7 fb,10x4x5 fb.

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Old 08-12-2012, 08:20 PM   #3
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Quote:
Originally Posted by Marbledust View Post
I guess you are taping of the Fau from downstairs so(and using flex duct)here's what you need IMO.

twye's....10x8x8....8x7x5...8x6x6

pipe(comes in 25' boxes)
box of 5",6",7",8" and 10"
duct ties(big zip ties)duct tape,1" hanger strap,#8x1/2" zip screw or tek screw.roll of 9" or 6" fiberglass wrap.2-10x6x6 floor box,12x6x7 fb,10x4x5 fb.
Fau from downstairs?
Twye's?

Sorry not sure what you mean. Do have a downstairs but im working one level at a time trying to make my ducts as tight as possible so no leaks and as little ducts as possible sounds like a clean job then an octopus box unless its necessary id LOVE to go 10inch > 8 inch > 8 Inch and 9inch > 8 inch > 6 inch then just a 3rd 6inch to split between the 2 bathroom 4 inch ducts.

As for supplies. I know how to seal them with the tape, zip ties and hanging them with the straps from the rafters.

And yes its all flex duct.

Mind explaining a little more in dumb peoples terms?
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Old 08-13-2012, 02:42 PM   #4
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Sorry to bump but does any one have any opinions about this?
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Old 08-13-2012, 03:01 PM   #5
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Originally Posted by ang1dust View Post
Sorry to bump but does any one have any opinions about this?
Well I gave a typical layout to distribute just under 400 cfm.Each room will have a volume to it(use a CFM Chart)

Small bedrooms I use a 6" supply,medium use a 7" suppply.etc.,etc.

twyes are metal duct fittings used to step down the air flow,without sending to much air supply up all at once.

the main branch supply seems' to be 10" so we know the amount of air we have to work with.10x8x8 will balance the flow,than 8x7x5 and 8x6x6 will also deliver a perfect air balance..

It's a good idea to install duct dampers in the flow in case you need to direct air else where.

hope this is clearer than mud now!
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Old 08-13-2012, 07:31 PM   #6
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Getting a better understanding of this now. I see what your saying. What do you consider Small / med / large sized rooms? What would you consider a size range for a 6 / 7 or 8 inch?

In my 3 largest rooms i would like to keep 8 inch ducts since thats what they have now and it keeps them cool. My smallest i want to shrink down to 6 inch.

Since you understand what im trying to do how efficient is using reducers over simply running a differernt supply from the Plenum?

Example 4 rooms 3 big 1 small = 4 runs from the plenum 3 8's and 1 6..
or
2 big rooms get 8 inch ducts, initially from the plenum as 10 inch, branch off to 1 room, then reduce down to 8 inch then connected to my 2nd room... Sam for the 1 large 1 small except i would use a 9 inch split off an 8 and then reduce it down to a 6 for the small room.

It takes more words to explain it but it will not only save me money by buying the ducts like this but it will also look better by taking up less space hence less air sitting in the ducts in the attic warming up while the ac is off then pushed out when its turned on.

I just want to make sure the REDUCER idea is not LESS efficient than just plain 4 direct supply lines. Thats the 2nd part of this question
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Old 08-14-2012, 11:45 AM   #7
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Not an expert but, I believe you may experience excessive pressure drop the way you want to keep extending a branch line. The main duct is designed to maintain volume and static pressure. That's why the branches come off the main duct. See if you can calculate the volume and velocity at the farthes point by plotting all the extensions from it. AND don't forget the supply duct, in this way, is trying to distribute to multiple rooms therefore multiple pressure drop points. Some how I don't think your proposed plan will work unless you can keep the branch duct at the largest size all the way to the last take off. AND Never take off from ythe very end of a duct.
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Old 08-17-2012, 06:51 AM   #8
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Quote:
Originally Posted by ang1dust View Post
Getting a better understanding of this now. I see what your saying. What do you consider Small / med / large sized rooms? What would you consider a size range for a 6 / 7 or 8 inch?

In my 3 largest rooms i would like to keep 8 inch ducts since thats what they have now and it keeps them cool. My smallest i want to shrink down to 6 inch.

Since you understand what im trying to do how efficient is using reducers over simply running a differernt supply from the Plenum?

Example 4 rooms 3 big 1 small = 4 runs from the plenum 3 8's and 1 6..
or
2 big rooms get 8 inch ducts, initially from the plenum as 10 inch, branch off to 1 room, then reduce down to 8 inch then connected to my 2nd room... Sam for the 1 large 1 small except i would use a 9 inch split off an 8 and then reduce it down to a 6 for the small room.

It takes more words to explain it but it will not only save me money by buying the ducts like this but it will also look better by taking up less space hence less air sitting in the ducts in the attic warming up while the ac is off then pushed out when its turned on.

I just want to make sure the REDUCER idea is not LESS efficient than just plain 4 direct supply lines. Thats the 2nd part of this question
If you want to use a plenum box instead of twye's it will work.
consider the size of your attic access before you have the plenum made up at a tin knocker supply.A 20'x20"x16" would fit 4 outlet's.cut in the 10" supply on 1 end,when you cut-in the other outlets be sure to leave about 2" of can at the edge of the cut out(on the cap end of the plenum)this will allow equal flow via proper back pressure.there will be room on the plenum for 6 outlets.
I sugest you also install duct dampers just near the supply vent in the floor box,this way you can adjust the air volume(go from room to room holding a thermometer and record the temp.,adjust the room temp to your liking.
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Old 08-17-2012, 11:44 AM   #9
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I will defiantly use twyes (those are the split branches?) and reducers. I will only use plenum for the HVAC and bathrooms and possibly some areas where my round flex duct will not fit and a thinner wider plenum duct will - just as a connector; i have this issue on one of my return ducts, dropping down between the rafters is very tight.

When using twye's and reducers can anyone give me any hardcore comparisons between using twyes and running direct supplies? Someone stated their non professional opinion that there might be pressure drops. Anyone confirm or deny and offer opinions with this type of setup?

Here is what ive decided on. Reminder, i will have professionals do all the work i just simply want to tell them what i want before they accept the job.
*********proposed*************************
UPSTAIRS:
mstr(200sqr ft) 8inch 225 Upstairs = 705
2nd(130sqr ft) 7inch 150 Downstairs = 830
3rd(130 sqr ft) 7inch 150 total = 1535
4th(100 sqr ft) 6inch 100
Bthrm 5inch 80(80/2 = 40 per bthrm)

DOWNSTAIRS:
Dng(100 sqr ft) 6inch 100
Lvng(300 sqr ft) 8inch x2 450
Ktch(100 sqr ft) 6inch 100
Bkfst(80 sqr ft) 6inch 100
bthrm 5inch 80(80/2 = 40 per bthrm)

******Current********************************
EDIT: (thought i would add what my current set up is)
UPSTAIRS:
mstr(200sqr ft) 8inch 225 Upstairs = 1000
2nd(130sqr ft) 8inch 225 Downstairs = 1125
3rd(130 sqr ft) 8inch 225 total = 2125
4th(100 sqr ft) 8inch 225
Bthrm 8inch 225(225/2 = 112.5 per bthrm with 4inch)

DOWNSTAIRS: (downstairs is a plenum extention from the main HVAC of 13inch X 16inch)
Dng(100 sqr ft) 8inch 225
Lvng(300 sqr ft) 11x7inch 350
Ktch(100 sqr ft) 8inch 225
Bkfst(80 sqr ft) 8inch 225
bthrm 4inch x2 100 (50x2)

*******Return*****************************
Main uninsulated duct to the HVAC 16inch (1400cfm)
Filtered Return Plenum box 25x25 25x25 filter (1300cfm)
Filtered return to plenum box 8inch 12x12 filter (100cfm)

And yes, i have a 4 ton i realize the ducts need to be increased.
Anyone see any problems with my numbers? Too much? too little? I want to over do it by 10-30% in most of the rooms upstairs simply because, its upstairs and as we all know its much hotter up there. I will be installing dampers. Can dampers be installed at the HVAC plenum? Are there motor drivin dampers that i can connect to a switch board?

Im glad this thread is making progress, i was having doubts and started to look to other site forums.

Again thanks in advance for any advice you guys can offer this newbie. Remember, if you dont know something, google it, you become an instant professional :P

Last edited by ang1dust; 08-17-2012 at 12:01 PM.
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Old 08-17-2012, 06:59 PM   #10
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Quote:
*********proposed*************************
UPSTAIRS:
mstr(200sqr ft) 8inch 225 Upstairs = 705
2nd(130sqr ft) 7inch 150 Downstairs = 830
3rd(130 sqr ft) 7inch 150 total = 1535
4th(100 sqr ft) 6inch 100
Bthrm 5inch 80(80/2 = 40 per bthrm)

DOWNSTAIRS:
Dng(100 sqr ft) 6inch 100
Lvng(300 sqr ft) 8inch x2 450
Ktch(100 sqr ft) 6inch 100
Bkfst(80 sqr ft) 6inch 100
bthrm 5inch 80(80/2 = 40 per bthrm)

Hope your using hard pipe, cause you won't get that CFM through flex duct.
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Old 08-17-2012, 07:09 PM   #11
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really? all the sites im pulling up say 200-225 at 700fpm for an 8 inch and so forth? What would you say is closer to reality?
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Old 08-17-2012, 07:23 PM   #12
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Maybe 80% of what those sites posted.

Most sites that give a set CFM for a duct size are using a rule of thumb that doesn't hold true.

ACCA has recently lowered the CFM listing they gave for flex duct.

The blower has to over come the resistance to air flow that the duct work creates. While with a strong enough blower, you can move 2,000 CFM through a 6" pipe, the noise would be unbearable, and the motor would be drinking electric to do it.

Size your duct by friction rate. To do that you need to know the total equivalent length on the longest supply and return combined.
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Old 08-17-2012, 08:19 PM   #13
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really? all the sites im pulling up say 200-225 at 700fpm for an 8 inch and so forth? What would you say is closer to reality?
You will get about %66 of that with a flex duct.
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Old 08-17-2012, 08:19 PM   #14
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i did over do it for most of the areas. I do have some ability to make some bigger. I could care less about the bathrooms.
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Old 08-17-2012, 08:23 PM   #15
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Quote:
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You will get about %66 of that with a flex duct.
66% is based on straight 25 foot or bent?

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