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Old 11-28-2012, 09:57 PM   #1
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Reduction of ducts per foot


I want to run a duct 55 feet from the furnace with a register at the end and four ducts going off it along the way. How much do I need to reduce the duct each time that it splits. What size ducts should go off of it to the registers 15 feet away.

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Old 11-28-2012, 10:04 PM   #2
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Reduction of ducts per foot


10 x 8 no reducing

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Old 11-29-2012, 06:18 AM   #3
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Reduction of ducts per foot


Quote:
Originally Posted by HVACTECH96 View Post
10 x 8 no reducing
Wouldn't how much air he wants to move be need to be known to answer the question somewhat accurately.
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Old 11-29-2012, 11:45 AM   #4
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Reduction of ducts per foot


yep agree with bt ..what is the total sq foot of space needing to be heated or cooled ...then size duct to carry the load... ben sr...he may not have enough to do what he want to do....

Last edited by ben's plumbing; 11-29-2012 at 11:47 AM. Reason: another thougth
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Old 12-01-2012, 01:14 AM   #5
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Reduction of ducts per foot


There are 1280 sq ft. I'm working on a floor plan that I can post.
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Old 12-01-2012, 10:08 AM   #6
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Reduction of ducts per foot


I had to split the file into three smaller files that were less than 100 kb. I put little green marks on it to help put it back together.
Under the Livingroom, there is a cold crawl space. Under the Kitchen, there is a heated crawl space. The rest is an unfinished basement. The floor joist runs North and South. The furnace is electric. Heil style AME053A40, Model NMEA053DG01 86758732, Serial H647 33944 MFRNO NMEA053DG01.
240 volts or it can run at 208 volts.
82.50 min circuit ampacity 72.09
90 max rating overcurrent device 80
52695 BTU/hr 39580
15.44 KW / rating 11.60
62.50 rated amp element 54.17
3.5 rated amp moter 3.5
44 moter hp 44
Thank you for your help.
Attached Files
File Type: pdf SCAN0001.PDF (65.5 KB, 30 views)
File Type: pdf SCAN0002.PDF (72.5 KB, 27 views)
File Type: pdf SCAN0003.PDF (59.1 KB, 31 views)
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Old 12-01-2012, 10:28 AM   #7
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Reduction of ducts per foot


Livingroom 449 5/16 sq ft
Kitchen 215 5/8 sq ft
Sittingroom 182 67/72 sq ft
Bathroom 47 2/3 sq ft
Laundry room 118 41/48 sq ft
Bedroom 94 1/2 sq ft
Bedroom 171 71/72 sq ft
Total 1280 7/8 sq ft
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Old 12-01-2012, 01:38 PM   #8
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Reduction of ducts per foot


Starting size should be a min of 22X8. Since a 15KW heat package needs roughly 1,000 CFM for a 50°F temp rise.

Livingroom 449 5/16 sq ft 351CFM
Kitchen 215 5/8 sq ft 168CFM
Sittingroom 182 67/72 sq ft 142CFM
Bathroom 47 2/3 sq ft 36CFM
Laundry room 118 41/48 sq ft 92CFM
Bedroom 94 1/2 sq ft 73CFM
Bedroom 171 71/72 sq ft 134CFM
Total 1280 7/8 sq ft

The above CFMs are just rough guesses base on square footage. And are not real accurate. 15KW is probably alot more heat then you need for a small place.
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Old 12-01-2012, 08:28 PM   #9
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Reduction of ducts per foot


We decided to leave the furnace where it is and run the trunk line 45’ W, and 6’ E from it. If we go two directions, should the line to the E be smaller because it is shorter?
What size should the legs be?
How much should we reduce the trunk line each time that a leg is added?

We were just going to add 4 vents to the Living room and 1 vent to the other bedroom, but while we are at it, there may as well be vents in the other rooms.

Trunk line going W 22x8:
Living room
45’ from furnace – leg going N 4’ to a register
42’ from furnace – leg going S 8’ to a register
32’ from furnace – leg going S 8’ to a register (if 4 registers are needed)
20’ from furnace – leg going S 8’ to a register
Kitchen
10’ from furnace – leg going N 8’ to a register
Bedroom
9’ from furnace – leg going S 27’ to a register
Laundry room
5’ from furnace – leg going S 8’ to a register

Trunk line at furnace 22x8 (45’ from W end of house; 6’ from E end of house)

Trunk line going E 22x8 ? (if we go two directions, should the E one be smaller):
Bathroom
Straight out from furnace – leg going S 3’ to a register
Sitting room
6’ from furnace – leg going N to two registers – 2’ and 10’
Bedroom
6’ from furnace – leg going S to two registers – 12’ and 24’

15 KW is when it is running on 240 volts. The furnace can also run on 208 volts which would be 11.6 KW. If you think that will be enough, we can put the upstairs on the same furnace. We were going to buy another furnace to connect the 3 bedrooms and 2 bathrooms on the second floor. Maybe the upstairs can be added to the trunk line that goes to the E.
Right now it is running on 208 volts with 2 vents in the large bedroom and 1 vent in the bathroom. There is a fireplace in the living room (we are just using fans to spread the heat).
Total distance E to W is 53’ 6”. It’s an old farm house that has been added to several times (inside walls use to be outside walls). I had a 2800 kg file that showed more detail, but I couldn’t upload it.
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Old 12-01-2012, 09:10 PM   #10
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Reduction of ducts per foot


I added the CFM #s that you gave me and they equal 996. It looks like you divided the 1000 CFM by the rooms proportion to the total sq ft. If 1000 CFM needs a duck 22x8, what formula should I use to find the duck size needed for 351 CFM?
Can I set it up as a proportion and use a duct about 8x8? Do I need to consider the distance and find the volume of air in the duct before you turn anything on?
351 CFM is for all of the living room; I need to divide it by the number of vents in the room?
It's ok if the formula is a bit complicated. I just learned to do compound interest; I can ask my instructor if I need help with it.
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Old 12-01-2012, 09:36 PM   #11
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Reduction of ducts per foot


I did a proportional since you didn't provide heat loss of any of the rooms. Seems like you have an over sized unit.

Determine how much air you are going to put into each room. then use a duculator at a .08" friction rate max. it will be close.

Reduce trunk line after 400 CFM of supply line.
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Old 12-01-2012, 10:42 PM   #12
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Reduction of ducts per foot


Thank you so much for your help. This is starting to look possible.
What is the max square feet that 15 KW can heat?
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Old 12-02-2012, 05:07 AM   #13
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Reduction of ducts per foot


Its not sq ft that counts. Its the insulation in the walls, the windows, and the air infiltration that determine it.

For my place at 1650 sq ft, 15 KW is still more then needed, but 10 would be borderline, and heat it fine on all but the coldest of nights. I have done homes with 1800 sq ft where 10 KW is all they needed.

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