CEMP-E
TI 810-11
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 control dampers of the dual-duct box to heat and cool its respective zone by mixing cold duct air and
hot duct air to maintain the setpoint. On a rise in space temperature, the hot duct damper shall gradually
close, and the cold duct damper shall gradually open.
20. BYPASS 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 bypass deck, individual zone mixing dampers,
and individual zone heating controls. Figures 4-16a through 4-16f show the control system design for
this type of air handling system using a SLDC control panel. Figures 4-16g through 4-16j show the
control system design for this type of air handling system using DDC controls. Since this system
provides for a deadband between heating and cooling, the use of an economizer cycle does not have the
problems associated with the previous two systems. A more detailed discussion is presented in chapter
three.
Figure 4-16a. Control system schematic for bypass multizone HVAC system.
Figure 4-16b. Control system ladder diagram for bypass multizone HVAC system.
Figure 4-16c. Control system equipment for bypass multizone HVAC system.
Figure 4-16d. Control panel interior door layout for bypass multizone HVAC system.
Figure 4-16e. Control panel back panel layout for bypass multizone HVAC system.
Figure 4-16f. Control panel terminal block layout for bypass multizone HVAC system.
Figure 4-16g. DDC control system schematic for bypass multizone HVAC system.
Figure 4-16h. DDC control system ladder diagram for bypass multizone HVAC system.
Figure 4-16i. DDC control system equipment for bypass multizone HVAC system.
Figure 4-16j. DDC control system I/O table and data terminal strip layout for bypass multizone HVAC
system.
b. General sequence of operation.
(1) Supply air and return air fans off. When the fans are off, the cooling coil valve, the outside
air and relief air dampers are closed. The return air damper is open. Zone heating coil valves and
dampers are under the control of their zone thermostats.
4-36
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