THREE STEPS TO SIZING AIR DUCTS
1. Calculate the CFM needed for each room
HOW: Perform a whole house and room-by-room load calculation (Manual J). Then apply the following formula:
Room CFM = (Room Load / Whole House Load) X Equipment CFM
*A room-by-room load calculation must be performed in order to determine the CFM to be supplied to each room otherwise the temperatures will be uneven throughout the house
2. Calculate the friction loss rate
HOW: Determine the external static pressure of the blower using the manufacturer's specs. Then, deduct the pressure drops created by any components added to the air distribution system (a/c coils, filters, grills, registers, dampers, etc). This is the available static pressure for the duct system. Next, determine the total effective length. The total effective length equals the measured length from the farthest supply outlet through the equipment and to the farthest return outlet plus the equivalent lengths of all turns and fittings. To get the friction rate apply the following formula:
Friction Rate = (Available Static Pressure X 100) / Total Effective Length
NOTE: Although it is not an engineering acceptable practice, for most typical duct systems, a default friction rate of .05-.08 may be used in lieu of performing the above calculation.
3. Select duct size using friction chart or duct calculator
HOW: Using friction chart. For sizing the supply and return trunks select the duct size at the intersecting lines of the friction rate and the equipment CFM. As the trunks are reduced use the total CFM of the remaining downstream branches. To size the branch ducts, select the duct size at the intersecting lines of the friction rate and the room CFM.
Using a duct calculator. For sizing either the trunks or branches simply align the friction rate with the CFM and the duct size will be displayed
NOTE: Whether using a friction chart or duct calculator, always check the velocity on the friction chart or duct calculator. Generally, the velocity should be no more than 700 FPM. If the velocity is too high, select one pipe size larger until the velocity falls below 700 FPM.
Before proceeding with your ductwork project please take time to read DUCTWORK MYTH BUSTER
TO FIND ECONOMICAL ALL-IN-ONE LOAD CALCULATION AND DUCT SIZING SOFTWARE CLICK HERE
Below is an example of duct sizing
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