mobile home ductwork

Old 06-10-05, 12:30 PM
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mobile home ductwork

Hello. I have a 1,200 square foot single-wide mobile home with a rectangular metal duct. The duct runs under the floor in a single line and the register boxes and plenum attach from above. The metal ductwork is rusted and full of holes. I'd like to replace the entire system with flex duct and new registers. Does anyone have the formula for determing the correct duct size? THanks.
Old 06-10-05, 10:37 PM
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Duct sizes

Post the sizes and I can get you equivelents (sp). The only thing I'd worry about is animals getting inside flexible ductwork by chewing thru it.
Old 06-11-05, 03:37 PM
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those are usually high static pressure units, duct under floor is generally 4X14.

if you drop a new box from under the unit, put a 6'round to each outlet. use wyes, 8X6X6, etc to ease the install

just my 2 cents
Old 06-12-05, 08:41 AM
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Thank you both for the reply. At first I thought I could just replace the duct with one that has approximately the same area but after looking through 20 pages of posts, I see that this is not as easy a job as I previously thought. I have removed the vapor barrier and all insulation so the entire duct system is exposed and easy to work on.

Here are some of the unit specs; it's a Nordyne split system. The AC is 3.5 tons and the furnace 41,000 btu (12.0 kw). The fan is a 1/3 hp, 4-speed, 4 ton, 2.9 F.L.A.

The original duct is 5 by 12" rectangular. I figured a 9" round duct would have approximately the same area but then you have connection issues. So then I think maybe I should go with either 8" or 10" but I'm not sure which size would work best with the unit. I also see that perhaps I should use rigid instead of flex duct. Love to hear your opinons.

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