30 November 1998
When the DDC system places the system in the economizer mode, it shall modulate the dampers from
the signal of a temperature sensing element and transmitter in the mixed air stream to maintain the
setpoint as shown.
(9) A space temperature sensor for each zone shall signal the DDC system to gradually operate
the zone mixing damper to heat and cool its respective zone by mixing cold deck air and hot deck air to
maintain the setpoint. On a rise in space temperature, the hot deck damper shall gradually close, and
the cold deck damper shall gradually open.
19. DUAL-DUCT HVAC CONTROL SYSTEM WITH RETURN FAN.
a. Description of the HVAC system. This is an air handling system with supply and return fans,
economizer dampers, a cold duct with a cooling coil, and a hot duct with a heating coil. In addition, each
dual-duct zone has a thermostat and a dual-duct box with damper and damper actuator. Figures 4-15a
through 4-15f show the control system design for this type of air handling system using a SLDC control
panel. Figures 4-15g through 4-15j show the control system design for this type of air handling system
using DDC controls. Note that because a dual-duct multizone system has no deadband between heating
and cooling, the decision to use an economizer cycle with this type system needs to be evaluated in
accordance with the guidance presented in chapter three.
Figure 4-15a. Control system schematic for dual-duct HVAC system.
Figure 4-15b. Control system ladder diagram for dual-duct HVAC system.
Figure 4-15c. Control system equipment for dual-duct HVAC system.
Figure 4-15d. Control panel interior door layout for dual-duct HVAC system.
Figure 4-15e. Control panel back panel layout for dual-duct HVAC system.
Figure 4-15f. Control panel terminal block layout for dual-duct HVAC system.
Figure 4-15g. DDC control system schematic for dual-duct HVAC system.
Figure 4-15h. DDC control system ladder diagram for dual-duct HVAC system.
Figure 4-15i. DDC control system equipment for dual-duct HVAC system.
Figure 4-15j. DDC control system I/O table and data terminal strip layout for dual-duct HVAC system.
b. General sequence of operation.
(1) Supply and return fans off. When the fans are off, the cooling coil valve and the outside air
and relief air dampers are closed. The return air damper is open. The heating coil valve is modulated
under the control of the hot duct temperature controller. Dual-duct boxes are under the control of their