reducting home


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Old 07-11-06, 09:01 PM
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reducting home

I have a 28 X 56 mobile home that the duct board is in need of repair and replacement. I am curious if I could rework the ducts with flex.

I currently have a 3.5 ton unit that has 1-16" X 20" return opening and has 9-4" X 10" floor registers. The unit is installed on the front half of the home and has a plenum that has trunk for front half and 12" flex exits below the plenum for the cross-over to rear half.

I am having trouble determining how I should size ducts to ensure that proper static pressure is maintained.

Basic arrangement of existing duct is as follows:

Front Half has 1 register in end bedroom and is 5' from plenum and next register is about 2' from plenum in opposite direction with additional registers continuing with the next being 18' from previous and the next being 18' from the last.
Rear half has 1 register in line with end room on front half, then has next register in line with cross-over from front half, then has register 14' from cross-over (Living room both registers), then one register 2'(bedroom)from previous register then one register in bath being 15' from the bedroom.

please advise of my options.

thanks for your time.
 
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Old 07-12-06, 05:28 PM
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I don't know that I can tell you how to resize the ducts. But, one piece of advice I would give you is when you do resize them, take into consideration if you have one room that is maybe not quite cool or warm enough as the case may be. Whatever duct sizes you do choose consider making the duct to that room one size larger.

There are several duct conversion programs out there (square to round). If you don't find one, you could always sign up for HVAC-Calc program for a one month permit ($49 I think) and I know they have a good duct sizing program. Find the program via Google. I used it to design the hvac and ducts for the house I am building. My one month permit expired months ago (or I could do it for you).
 
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Old 07-13-06, 09:59 AM
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thanks for the info. I really appreciate it.
 
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Old 07-13-06, 11:31 AM
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If you are satisfied by your current system and only wish to replace the existing ductwork with flexible because of the deterioration of the existing ductwork then the calculation is quite simple.

Simply find the cross-sectional area of the existing duct, main or branch, and substitute a flexible round duct with an equivalent cross-sectional area.

To find the cross-sectional area of a rectangular duct simply multiply the inside width by the inside height in inches. To find the cross-sectional area of a round duct multiply the internal diameter by itself (square the diameter) and then multiply this figure by 0.7854 (area = 1/4 PI times diameter squared).

Example: A 9" x 12" rectangular duct would be 108 square inches in cross-sectional area. A 12" round duct would be 12 x 12 = 144 x 0.7854 = 113.1 square inches cross-sectional area. The sizing does not need to be exact but the closer the better. Larger ducts will result in a lower static pressure at the furnace outlet, lower duct velocities, lower noise levels and higher air flows. Smaller ducts will be just the opposite.
 
 

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