30 November 1998
(2) During occupied and ventilation delay modes the supply fan and return fan shall operate
continuously. During unoccupied mode the supply fan and the return fan shall cycle according to the
night setback schedule. The fans shall start and stop at the setpoints as shown.
(3) A differential pressure switch across the filter shall initiate a filter alarm when the pressure
drop across the filter exceeds the setpoint as shown.
(4) A freezestat, located as shown, shall stop the supply and return fans, cause the outside air,
return air, and relief air dampers to return to their normal position, and shall initiate a low temperature
alarm if the 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) Smoke Detectors in the supply air and return air ductwork shall stop the supply fan and the
return fan and initiate a smoke alarm if smoke is detected at either location. Restarting the fans shall
require manual reset at the smoke detector.
(6) During occupied and ventilation delay modes, the cooling coil control valve shall be
modulated by the DDC system from the signal of a temperature sensing element and transmitter located
in the coil discharge air to maintain the cold duct temperature setpoint as shown. During the unoccupied
mode, the cooling coil control valve shall remain closed.
(7) The DDC system shall accept the signal of an outside air temperature sensing element and
transmitter and the signal of a return air temperature sensing element and transmitter. The DDC system
shall perform switch over between outside air economizer control mode and minimum outside air mode.
Until the return air temperature rises above the setpoint, the DDC system shall hold the system in the
minimum outside air mode. When the return air temperature rises above the setpoint, the DDC system
shall place the AHU in the economizer mode or in the minimum outside air mode as determined by a
comparison of the outside air and return air temperatures in accordance with the differential temperature
setpoints as shown. When the outside air temperature is low with respect to the return air temperature,
the AHU shall be in the economizer mode. When the DDC system places the control system in the
minimum outside air mode, the outside air damper shall be open to the minimum outdoor air setting.
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.
(8) A space temperature sensor for each zone shall signal the DDC system to gradually operate
the zone mixing damper and heating coil to heat and cool its respective zone by mixing cold deck air and
bypass deck air to maintain the setpoint. On a rise in space temperature, the heating coil valve shall
gradually close, and after a deadband as shown, the bypass deck damper shall gradually close and the
cold deck damper shall gradually open.
21. VARIABLE AIR VOLUME (VAV) HVAC CONTROL SYSTEM WITHOUT RETURN FAN.
a. Description of the HVAC system. This is an air handling system with an inlet vane equipped
supply fan, economizer dampers, filters, and cooling coil. Variable air volume (VAV) boxes, some of
which have heating coils, are located downstream in the ductwork near the areas served by the system.
Figures 4-17a through 4-17f show the control system design for this type of air handling system using a
SLDC control panel. Figures 4-17g through 4-17j show the control system design for this type of air
handling system using DDC controls.