Installing new duct work in a Mobile Home

I bought a Mobile Home to fix up and rent out but its in pretty bad condition. The existing duct was removed by the last owners. I made 3d model for what I'm thinking of doing for the duct work. I'll be putting in a 3 ton ac unit. The mobile home is about 1500 sf with the add on. The original mobile home is about 55' x 24'. The original size of the duct work were two 15"x4" duct on both sides. I want to replace that with 1 12" rd duct and branch off that too all the rooms. Does it mater that I have the 12" Flex tube connecting to the end and not T into the main pipe?

 

Welcome to the forums.

My specialty is the electrical/electronic servicing part of the system. I don't get involved in ductwork at all but I can tell you that one 12" supply duct will not cut it.

Were those both supply ducts originally ?

 

On both sides there would of been a 15"x4" duct. They would of ran parallel to the 12" duct but more center to each half.
This is my reasoning behind the 12" rd duct. The sq area 15"x4" is 60 and double that so it covers both ducts. The area for the 12" rd duct is 113. The area for the 14" rd duct is 153. The reason I was thinking the 12" duct would work because there 12 flex tube coming from the 3 ton ac.
When I read up on this in different forums theirs allot i do not understand about static air pressure.

 

Supply should be 16in metal coming off. Return should be 17in 17 hard to find so use 18. The two branch return lines will need to be 16in each for 1200 cfm

 

Can you add a few more supply's? or make a couple 8in?

 

I'm a little confused why I would jump up from a 14" return to 17/18" return if at the 3 ton air conditioner has only 14" return at the box. Below is a link to a type that I used in the past for Mobile Homes. Please let me know


http://www.nationalairwarehouse.com/specs/P7RD.pdf