Original message
| TKTJ2000
 |  "Furnace Ductwork"
, posted Sun 4 Nov 21:36  
I am installing a Trane TUD100R9V3K high efficience furnace in a garage area. I have the furnace sitting up on 4 cinder blocks, and the plenum box that sits on the furnace is 20X20 by 3.5 feet high. This plenum box takes me to the bottom of floor rafters to the second floor. I then reduced the duct down to 12X8 duct to get me through the floor joice and into a crawl space. The reduced duct go about 3 feet high and then has a long elbow and then goes for about 30 feet and then gets capped off. Their will be about 5 supplys and one main return. I was goung to use 6 inch round ductwork for the supplies. If I have to to make sure that my static is not to high across the furnace i could also cut in a supply in the main duct coming off the furnace. Do you think that I will have a problem with temperature rise being that I reduced the duct form 20 X 20 to a 12 X 8? Any help would be great. Thanks.
slimshaddy
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Replies:
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| smythers1968
| "Re(1):Furnace Ductwork" , posted Sun 4 Nov 21:48
I would make another 8x20 and put on the other side of the the joices. In other words make two 8x20 going to the main run. You might use one 8" and one 10" flex duct instead of metal duct.
I don't think you have enough supply feeding the main trunk.
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(2):Furnace Ductwork" , posted Sun 4 Nov 21:54
I do not understand your answer. I have reduced the main plenum duct from a 20X20 by 3.5 feet high duct to a 12 X 8 going through the floor joice. I then have to go about 36 feet for two take offs. One for a bedroom and one for a bathroom. The other 3 take offs will be right off the main plenum duct off the furnace. Please advise.
slimshaddy
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| | smythers1968
| "Re(3):Furnace Ductwork" , posted Sun 4 Nov 22:05
OK I understand you just stubed through enough to make two runs off it. You are not running a 8X12 duct 30+ ft. Right?
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(4):Furnace Ductwork" , posted Sun 4 Nov 22:12
Yes I am running a 8 X 12 forabout 30 feet once I reduce off the vertical plenum off the furnace. Then taking 2 take offs off of that. The other 3 will be off the main truck vertical plenum beforte the duct reduces. What size take offs should I use?
slimshaddy
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| | TKTJ2000
| "Re(4):Furnace Ductwork" , posted Sun 4 Nov 22:09
No.. I had to reduce off the main vertical duct down to about a 8 X 12 duct which runs for about 30 feet or so and then is capped at the end. This is the only duct off the furnace. I will then cut in two 6 or 8 inch take offs one for a bedroom which is 10 X 12 and then one for a bathroom 8 X 12. The other 3 take offs will be off the main vertical duct on the side of the main plenum for 3 more supplies. I hope I am making this clear. Appreciate your quick responses.
slimshaddy
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| | smythers1968
| "Re(5):Furnace Ductwork" , posted Sun 4 Nov 22:19
Is there A/C too?
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(6):Furnace Ductwork" , posted Sun 4 Nov 22:23
No Sir. This is just heating. Do you think this sizing of the ductwork will work? Please give me some suggestions. Thanks so much.
slimshaddy
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| | smythers1968
| "Re(7):Furnace Ductwork" , posted Sun 4 Nov 22:33:
I typically don't make runs directly off the pleum. You really need to do the take offs off the main trunk. However with five 8" round you can move approx 1100-1200 cfm which should work.
With 6" it's only allitle over 600 cfm which will not work
I would really like to see a main trunk of 12"x16"x10' and the five runs off the main trunk
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
[this message was edited by smythers1968 on Sun 4 Nov 22:37] |
| | TKTJ2000
| "Re(8):Furnace Ductwork" , posted Sun 4 Nov 22:44
I do not understand what you mean by 8 inch will deliver 1100 to 1200 cfm. This is what the furnace will deliver all together on high speed? I am just looking to heat some bedrooms. I am looking for a simple way that will work for a furnace that will be used as a secondary heat source. Thanks.
slimshaddy
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| | TKTJ2000
| "Re(8):Furnace Ductwork" , posted Sun 4 Nov 22:41
Dear Friend
I do not have the option to take all the take offs off the main truck especially when the main truck will be above the furnace abouve a floor. Will the reduced truck deliver enough air to the end of the 30-36 feet truck using the 12 X * ductwork? If so should I use the 3 take offs off the the main plenum that go to the bedrooms as 6 inch duct runs or 8 inch duct runs? And what about the 2 runs off the main plenum on the second floor. 6 inch or 8 inch runs. Thanks so much.
slimshaddy
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| | smythers1968
| "Re(9):Furnace Ductwork" , posted Sun 4 Nov 22:49
One 8" round duct will move 220 cfm.
five 8" round will move 1100 cfm.
Too little cfm high temp rise
too much cfm low temp rise.
Propper sizing has to be done to achieve proper heating.
Use 8" for all of the take offs
Also High is mostly used for cooling.
Med or Med/High is used for heating.
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(10):Furnace Ductwork" , posted Sun 4 Nov 22:57
Will the reduction from 20 X 20 Ductwork vertical off the furnace going to a 12 X 8 section of ductwork work ok? Remember I am only reducing the duct for about 1.5 feet just to get up through thye floor. Should I go bigger than a 12 X 8 once I get through thye floor into the crawl space and still go with 8 inch round take offs? What do you think?
slimshaddy
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| | TKTJ2000
| "Re(2):Re(10):Furnace Ductwork" , posted Sun 4 Nov 23:01
Once I get through the floor I will run about 30-36 feet to get close to the bathroom area and bedroom for the take offs for each room. Thanks so much. I can really run any size once I get through the floor. Thanks so much for your expertise.
slimshaddy
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| | smythers1968
| "Re(3):Re(10):Furnace Ductwork" , posted Sun 4 Nov 23:07
I think it will work just use 8" for the take offs
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTj2000
| "Re(4):Re(10):Furnace Ductwork" , posted Sun 4 Nov 23:10
What do you think about the total static pressure across the furnace for the duct sizing and take off sizes? I will be using dampers in all supplies. Do you think the reduction in the smaller duct going through the floor will hurt the airflow even though I am using 8 inch take offs? How much will the smaller duct going through the floor hurt my airflow and static? What about the main truck that I am going to use the 12 X 8. Is this ok? I would like to buy you a beer, you have been a great help.
slimshaddy
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| | TKTj2000
| "Re(5):Re(10):Furnace Ductwork" , posted Sun 4 Nov 23:17
I guess my real concern is allowing the furnace to be able to flow enough air through the ductwork. What would happen if your plenum was to small, but you had your takeoffs sized properly to handle the total cfm that your furnace would deliver?
slimshaddy
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| | smythers1968
| "Re(6):Re(10):Furnace Ductwork" , posted Sun 4 Nov 23:28
The 12x8 is limited to about 500 cfm it should be fine.
I would forget the dampers.
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(7):Re(10):Furnace Ductwork" , posted Mon 5 Nov 20:09
Can you tell me what size a 6 inch round duct will deliver for airflow? What about a 10 X 6 duct. What airflow will carry through this duct. Thanks much.
slimshaddy
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| | smythers1968
| "Re(8):Re(10):Furnace Ductwork" , posted Mon 5 Nov 21:03
6" = approx 100cfm
6x10 = 250 cfm
Life lesson #12 Know when not to do it yourself.
WOW That Can Hurt!!
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| | TKTJ2000
| "Re(9):Re(10):Furnace Ductwork" , posted Sun 11 Nov 20:52
Sir
What if I kept all the 5 out of 6 runs 6 inch and then the one left over run go to 8 inch? Then I would add a 8 X 12 register in the face of the main plenum for additional air movement to get me closer to the 1100 to 1200 cfm that the furnace requires? The reason I ask is because all the 6 inch elows that are connected to the registers above the celing are very difficult to reah without having to remove the whole ceiling. I actually have all 5 out of the 6 connected with 6 inch duct, but was going to use the last takeoff and go 8 inch. All these takeoffs will heat approximatley 10 X 12 bedrooms. What do you think> I really appreciate all your help. Will the bedrooms heat ok, and will adding the 8 X 12 register on the face of the main plenum to help flow more air help? This is a two stage furnace with a variable spped blower. If I increase my static pressure won't I deliver more air through these takeoffs?
slimshaddy
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