Duct Size for Air Conditioners

Sizing Problems

• Many air conditioning contractors tend to follow general rules of thumb when selecting ductwork and air conditioners. However, the Air Conditioning Contractors of America manual provides design specifications and ideas that contractors can follow that will increase the effectiveness of their installation efforts, according to the Proctor Engineering Group. The organization notes that improperly sized air conditioners, improperly sized ductwork and poor installation tend to be the main culprits in air conditioning problems.

Sizing Methods

• Sizing methods used by contractors vary and often depend upon the individual preference of the contractor, using an established method. One common method is to calculate room-by-room loads to determine the size and length of the necessary ductwork. Some contractors avoid this method, though, because of the perceived discrepancy between the calculations and the amount of cool air actually delivered once the ductwork is installed. In any case, increased capacity does not always result in increased cooling. Another common method is to use the square footage for the entire house to determine the necessary capacity. As with the room-by-room approach, it is still unnecessary to overestimate the capacity needed.

Velocity

• Aside from the length and design of the ductwork, the duct cross-sectional area also factors in the equation, providing an indication of the actual size of the duct itself. The Engineering ToolBox notes that the equation used to calculate this size is obtained by dividing the airflow rate, which is measured in cubic meters per second, by the air speed, as measured in meters per second. The actual design of the ducts can be determined in relation to the velocity and will be based on the type of air system as well.

Goals

• When sizing air conditioner ducts, you may be able to deliver more air all at once by going with a bigger and more elaborate duct design system. However, other factors should take a primary role in determining what size ducts you decide to go with in your system. Choose a duct size that is ideal for the maximum efficiency of the system you are installing. The Proctor Engineering Group recommends choosing a duct size that will hold the Noise Criteria or NC level below 25 and the airflow below 700 feet per minute.