There are a number of factors that go into answering that question…
First, the ducts must be sized properly. Too often we find that the duct system was not properly designed for the size of the home and/or the system. It is important that you have properly sized ductwork to allow the airflow needed. If you do not have enough ductwork (or too small of ducts) you will add restriction to your air flow and limit the capacity of the system. It will also cause unnecessary stress on the system. However, you can actually have too much ductwork (or too large of ducts) also. This may cause too little restriction causing the blower to move too much air across the coil. The unit will not be able to cool or dehumidify the air well. The blowers are designed to have a certain level of restriction or static pressure.
Secondly, how leaky are your ducts? Studies show that a typical home can have between 30% – 40% of the air leaking out of the ducts before it is delivered into the home. Not only is this a waste of energy, it actually causes hot, dirty, outside air to be brought into your home. When air leaks out of the ducts into the attic, it puts the inside of the home into a negative pressure to make up for that lost air. This causes hot, dirty, outside air to be pulled in through the cracks around doors, windows, vents and other penetrations. Having a well-sealed duct system is key to energy savings and clean air.
Thirdly, consider the insulation level of your ducts. Historically, ducts were made with 1” of insulation giving them approximately an R-4 level of insulation. As your cooled air travels through your hot attic it will drop it temperature before it reached the end of the run. A higher insulated duct will help prevent that heat gain. (The opposite is also true during the winter.) Building codes now requires all new homes have at least R-6 insulated ducts. However, we typically use R-8 since it provides the best level of protection available on the market.
Lastly, the design of the duct system will have a large impact. Flexible ducts are only designed to be about 20-25 feet long. Anything longer than that will greatly increase the restriction in those ducts and cause poor air flow to those rooms. Consider a trunk system. A trunk system uses a hard metal pipe with smooth interior walls to deliver the air the majority of the way to the rooms. Small branches are then run to each outlet using flex ducts. This is especially helpful in long or large homes where the air handler or furnace is located near one end of the home.
