08/25/2010 13:45


10 CRITICAL STEPS to Correct Duct Sizing

  1. Size the load with the Manual J approach. For duct sizing purposes, you must calculate the Btuh for each room.
  2. Size the equipment using Manual S techniques. You must determine the total airflow based on your equipment selection. Use the nominal medium speed cfm for your duct design.
  3. Calculate the cfm per room based on the cooling sensible Btuh room load.
  4. Determine the design type (trunk and branch, radial, etc.) and lay out the ductwork in the most effective manner.
  5. Compute the total effective length of the duct system. Add the longest effective length of the supply and return to determine the longest critical path.
  6. Determine available static pressure by deducting air side devices in the critical path from the design static pressure.
  7. Calculate friction rate per 100 ft. by applying the available static pressure times 100 and dividing it by the total effective length.
  8. Apply friction rate with the calculated room cfm to determine duct size (Use the appropriate duct calculator or friction chart for the chosen duct material).
  9. Check chosen duct size for appropriate velocity.
  10. Reduce friction rate to 0.04-in. wc or 0.05-in. wc for return sizing calculation to ensure adequate sizing and permissible velocity.


HVAC Comfortech

This article is based on the presentation, Duct Design 101, which Paul Wiebolt gave at HVAC Comfortech 2006, held in Baltimore, MD, Sept. 13-16, 2006.

For more information about HVAC Comfortech 2007, which will be held September 26-29 2007 in St. Louis, MO, call 216/931-9550 or watch for updates on the show website: www.hvaccomfortech.com.

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