I am in Houston and AC is a necessity. Most residential AC guys here are awful- they can make a killing in the hot seasons for simple calls to replace a $30 run capacitors on compressors (very common failure, often encouraged by power blips). Most of them do not fully understand duct design and many will pass on jobs to re-do ducts if they can since it is more profitable to focus on break-fix type calls. Because of this, I had to learn about AC systems and duct design myself. Here are some things I have learned which are not often mentioned:
- Intake filter flow rates are important, 1" filters have varying pressure drops across them and can account for a significant loss of available static pressure in your system. Most companies do not list this figure on their product, or even provide it.
- Return air is far too often neglected in duct design. I have a section of my house which requires %25 of the available air flow to cool but it has no return air path. So if the interior door to that area is closed, the air is instead forced outdoors and the loss is reclaimed on the other side of the interior door from (you guessed it) outdoors. So if a door to an area is barely open and then closes when your system kicks on, you probably don't have adequate return air.
- Most newer homes use flexible duct. It is much cheaper and lighter than metal duct work but how it is installed makes a massive difference in how efficient your system is. Flexible ducts lengths which have slight compression (15%) or sag might double the loss for that section.
Overall the duct work design and installation is one of the most important aspects of your HVAC system which is the LEAST profitable (earnings/time) to your residential AC guy and therefore, is often neglected.
If you want to go deeper I recommend the ASHRAE Fundamentals handbook which covers heat load calculations, fenestration, systems, and duct design. You can pickup a older used copy for cheap. Avoid ACCA guides, they obscure all the underlying math and engineering behind a more simplistic design method based on total 'effective' length calculations which makes duct design difficult to understand intuitively.