BASIC DUCTWORK GUIDE
The first step in designing your duct system is going to depend a lot on your situation, so for the purposes of this guide we will assume you are changing both the unit and the duct.
Measure the square footage of the home and using the chart below, size an appropriate unit. 1 ton of air conditioning capacity for each 600 square feet of floor area, rounded up. For example: An 1300sqft home would need a 2.5 ton unit. Take care when sizing the your unit as too small a unit will not be able to keep up in extreme weather, whereas too large a unit will suffer from short run times. While short run times may sound ideal, the unit needs at least 15 - 20min average run times to have a meaningful impact on the humidity levels in a home.
Now that you have sized your unit you are ready to begin laying out your duct system. Start by selecting an appropriately sized supply and return line from the above table. Notice that the CFM values in the above table are listed as a range. Whether your system flows at the minimum or the maximum of this range is dependent on the system's "friction loss". In order to minimize friction loss you can: use a larger return, avoid sharp or unnecessary bends in the duct, take the most direct rout, use metal duct.
The simplified duct system pictured above will help guide you through the basics. Sizing for a 2.5 Ton unit, we start with a 16" supply at 875 CFM -1125 CFM. We then take a 7" and 5" run (-200 CFM) off of this section bringing the remaining total to 675 CFM - 925 CFM. A drop from 16" to 14" in now required to keep pressure and velocity up. With the 14" supply installed, we take three 6" runs from it (-300 CFM). This brings the total down to 375 CFM - 625 CFM and we are ready to reduce from 14" to 12". With the 12" supply installed we take two 7" runs (-300 CFM) and two 6" runs (-200 CFM) off the remaining trunk for a final total of -125 CFM - 125 CFM. It is ideal to leave between ~100 CFM and ~250 CFM in a duct system as "backpressure" on most systems. At the end of the trunk 1' - 2' receives no taps, this helps the system pressurize. To ensure proper airflow, this system is then equipped with a 16" return capable of flowing at 1300 CFM - 1500 CFM.
Using the chart above you can now begin designing a system to meet your application. When determining what size run is appropriate for a given room 1sqft per 1 CFM is the rule of thumb, however this assumes a 8ft ceiling height, normal insulation, just a few windows, and no more than one external wall. Whenever possible vents should be placed near the windows and doors on the exterior wall. This practice is commonly called "washing the windows". Conversely, the return should go as close to the center of the system as possible, in an open area not exposed to excessive humidity or grease.