HVAC Procedure and Performance Guide

Balance duct system for correct CFM at each register

Training for this procedure can be found in these courses:

                         131 – Oil Heat l  and  161 – Boilers l


Air distribution system must deliver specified CFM at each register.  Dampers must be adjusted individually to direct specified flow from each register. Applicable safety procedures must be followed.


  1. Review register manufacturer’s specifications and job specifications.
  2. Collect tools, equipment and materials.
  3. Put on safety and protective equipment and apparel.
  4. Determine total air-flow for system, branch air-flow and register air-flow.
  5. Collect register data.
  6. Open all dampers in duct and registers and ensure filters are clean.
  7. Measure fan output and adjust to specified CFM.
  8. Review duct layout and number of branches and registers.
  9. Adjust registers for proportionate balance.
  10. Measure air-flow at Register 1 (farthest register on farthest branch).
  11. Calculate ratio of actual air-flow to specified air-flow.
  12. Measure air-flow at Register 2 (next register on branch) and calculate ratio.
  13. Compare ratios (Register 1 and Register 2) and adjust Register 2 to bring ratios within 10% of each other. (Note: Do not adjust Register 1).
  14. For Register 3, determine ratio, compare to Registers 1 and 2, and adjust to within 10%. (Note: Do not adjust Registers 1 and 2).
  15. Repeat procedure for remaining registers on branch, then move to Register 1 on next branch (second farthest from fan) and repeat procedure for that branch (Steps 9a through 9e).
  16. Repeat in order for all branches until all register ratios are within 10%.
  17. Adjust branches for proportionate balance.
  18. Select typical outlets in Branch 1 and Branch 2, measure air-flow and calculate ratio of measured air-flow to specified air-flow for each of these outlets.
  19. Adjust damper in Branch 2 to bring ratios within 10% of each other.
  20. Repeat procedure (Steps 10a and 10b) for typical registers in Branch 2 and Branch 3 and adjust Branch 3 damper.
  21. Proceed until all branches are complete.