If you want to get a heated discussion started among HVAC professionals, start asking about the practices used in duct sizing. There is a passion among those in the HVAC industry in regards to the methods they use to determine what size ducts to install.
These methods are often traditions passed down over decades, similar to what was passed down to me when I started installing many years ago. If you question the duct-sizing method, you question something that dad or even granddad passed down to you as the truth.