Damper Size Selection

Typically in an existing install, the duct size will determine the damper sizes needed.  But if starting with a new duct system 2 variables must be known to determine damper sizes.

• Total CFM of System

Calculate using the “whole house load” with maximum diversity.

• Zone CMF (needed for each Zone)

 Total the rooms associated with each zone separately to get the “Zone CFM” (No diversity)

Round Damper Sizes

Round damper sizes range from 4” to 18” typically in 1” increments up to 10” and in 2” increments up to 18”.  There are two numbers that must be used when designing a zoning system with regard to all dampers.  

1. Design CFM – Used to properly size the dampers based on a maximum FPM air velocity at the register to avoid air noise and still provide enough throw to heat or cool the space.  (Typical acceptable FPM 600 to 800)

2. Max CFM – The maximum amount of air that might move through a damper if it is the only zone open.  This assumes the pressure relief is also sized properly (see Pressure Relief below).  This will result in more air noise but must be kept at an acceptable level (no more than 900 FPM).

The chart shows the recommended round damper sizes to be uses for Design and Max CFM. 

NOTE: These are rough estimates & other duct restriction variables could affect the size damper needed.  Consult an HVAC professional if you are unsure.