Duct design is a very popular topic in our industry that does not get covered often, a lot of people are confused on what ductwork to use on what area. They do not understand that if the static pressure is too low you will hear it blowing too hard, or sucking in too hard, this could mean that the system is in pain and could shorten the life of the equipment and can cause high head pressure on the compressor, over heated coils and frosted copper lines if the system is undersized. Other issues that could happen are lower efficiency, high energy bills and shortened life of the equipment. The different flex diameters include flexible ductwork from a range of 4” up to 20” and there are many manufures, Hart and Cooley, Thermaflex, Goodman Flex, Thermafoil, Flex Master, and Ultra Duct just to name a few. We recommend using the aluminum foil flex because of its durability and longevity. No matter how well you design a duct system the return and supply grills have to be able to deliver the air to the conditioned space, if you have supply or return grills that are 3/8” or less they need to be replaced to a minimum of ½” width in order for the system to breathe properly. The bottom line is the ductwork should not be too small and the grills shouldnt be either to achieve the maximum efficiency the system is able of providing.