30 November 1998
temperature drops below the freezestat's setpoint. Return to the normal mode of operation shall require
manual reset at the freezestat. The DDC panel shall monitor the freezestat through auxiliary contacts
and shall indicate an alarm condition when the freezestat trips.
(5) A space temperature sensing element and transmitter operating through the DDC system
shall first gradually shut off the heating coil valve. After the heating coil valve is fully closed, the DDC
system shall then gradually operate the outside air damper to admit outside air beyond the minimum
quantity to maintain the setpoint as shown.
(6) Smoke Detectors in the supply air and return air ductwork shall stop the supply fan and
initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan shall require
manual reset at the smoke detector.
18. MULTIZONE 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 deck with a cooling coil, a hot deck with a heating coil, individual zone
mixing dampers, and individual zone heating controls. Figures 4-14a through 4-14f show the control
system design for this type of air handling system using a SLDC control panel. Figures 4-14g through
4-14j show the control system design for this type of air handling system using DDC controls. Note that
because a two-deck 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 4-14a. Control system schematic for multizone HVAC system.
Figure 4-14b. Control system ladder diagram for multizone HVAC system.
Figure 4-14c. Control system equipment for multizone HVAC system.
Figure 4-14d. Control panel interior door layout for multizone HVAC system.
Figure 4-14e. Control panel back panel layout for multizone HVAC system.
Figure 4-14f. Control panel terminal block layout for multizone HVAC system.
Figure 4-14g. DDC control system schematic for multizone HVAC system.
Figure 4-14h. DDC control system ladder diagram for multizone HVAC system.
Figure 4-14i. DDC control system equipment for multizone HVAC system.
Figure 4-14j. DDC control system I/O table and data terminal strip layout for multizone 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