Auto-Zone Control Systems Auto-Zone Plus Systems Installation & Operation (Version 03A) - Manual (Page 11)

Auto-Zone Plus Section 1
Design Guide 1-7
Bypass Damper or VFD Control
A Bypass damper or Variable Frequency Drive (VFD) is used to control the duct static
pressure. When using bypass damper control, the Zone Manager sends a signal to the
bypass damper actuator to maintain duct static pressure by opening and closing a damper
in order to bypass a portion of the discharge air stream back into the return air duct of the
HVAC unit in response to the duct static pressure.
When a VFD is used, the Zone Manager sends a 0–10 VDC signal to the VFD to
maintain duct static pressure by causing the VFD to change the Supply Fan motor speed
thereby increasing or decreasing the air flow in response to the duct static pressure.
Minimum airflow requirements of the heat exchanger must be considered when using a
VFD on units that have heating. See the design section of this manual for specific
recommendations and information.