CEMP-E
TI 810-11
30 November 1998
(4) A freezestat, located as shown, shall stop the supply fan, 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
initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan shall require
manual reset at the smoke detector.
(6) 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 outside air temperature rises above the setpoint, the DDC system shall hold the system in the
minimum outside air mode. When the outside 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, the dampers shall be modulated to
admit additional outside air to maintain the space temperature setpoint as shown.
(7) Space temperature control shall be maintained as follows.
(a) During unoccupied mode, the dampers shall remain in their normal positions as shown
and the DX condensing unit shall remain off. The supply fan shall be cycled by the system's night
thermostat and the heating coil valve shall be modulated to maintain the space temperature low limit
setpoint.
(b) During occupied mode, the supply fan shall run continuously and the heating coil and the
DX cooling coil shall be under space temperature control. While the system is in ventilation delay mode,
the dampers shall remain in their normal position unless the system is also calling for economizer
operation. When the DDC system is in economizer mode, on a rise in space temperature, the DDC
system shall first gradually close the heating coil valve. After passing through a deadband, the outside
air damper shall be modulated to admit outside air beyond the minimum quantity and after the outside air
damper is fully open, the DDC system shall operate the stages of cooling in sequence to maintain the
space temperature setpoint. When the system is in the minimum outside air mode, the dampers shall
remain in the minimum outside air position. On a rise in space temperature, the DDC system shall first
gradually close the heating coil valve. After passing through a deadband, the DDC system shall operate
the stages of cooling in sequence.
4-69
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