Sizing ducts is a science. It wasn’t always that way, however. A glance at the ductwork of homes built decades ago when energy was cheap reveals a “guesstimate” approach that relied on over-sizing the air conditioner or furnace to adjust to shortfalls in duct sizing. Sizing ducts for energy efficiency and comfort today means ensuring adequate airflow and temperature distribution throughout the ductwork, while keeping duct size cost-efficient and minimizing installation difficulties.
The industry standard for calculating duct sizes is the Air Conditioning Contractors of America’s Manual D worksheet. These days, Manual D is usually incorporated in software that allows your HVAC contractor to enter a variety of parameters about your home while the program figures the correct duct sizes.
The sizing procedure always begins with determining the heating/cooling load of your particular home and lifestyle. Utilizing the ACCA’s Manual J load calculator, the contractor will produce a cumulative load figure for each room, expressed in BTUs. The Manual J figures will be input into Manual D to customize the duct calculation for your individual living spaces.
One size doesn’t fit all
A typical residential calculation may include a mixture of duct sizes ranging from 5-inch to 10-inch, subject to these factors:
- The heating and cooling load data from Manual J
- The necessary length, type and shape of the ducts required to serve the rooms in the home
- Performance specs of the HVAC equipment, including the cubic feet of airflow per minute produced by the blower, temperature range of conditioned air and the unit’s output pressure (These figures are modified by factors like friction loss as the air passes through the ductwork and dynamic loss as the air encounters fittings that divert the air flow.)
- The number and span of branches off the central trunk, the amount and types of fittings such as elbows and Y-fittings and the overall symmetry of the layout
Serving the Ft. Worth/Arlington area since 1955, AirRite Air Conditioning is your source for interior comfort in all seasons. Contact us for more information on professional duct sizing techniques.
Our goal is to help educate our customers about energy and home comfort issues (specific to HVAC systems). For more information about sizing ducts and other HVAC topics, download our free Home Comfort Resource guide.
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