18 August 1998
outside-air damper to admit outside-air beyond the minimum quantity and after the outside air damper is
fully open the UMCS will then operate the DX stages of cooling in sequence.
Unoccupied Mode. Cooling will be off and the heating-coil control valve will open to the
(5) Mixed-Air Temperature Control. When the UMCS places the system in the economizer
operation, it will modulate the dampers from the signal of the space temperature sensor to maintain the
setpoint based on the conditions shown in Table 8-23.
Table 8-23. Mixed-Air Damper Modulation.
Modulate OA and relief dampers
< Changeover temperature
open, and the RA dampers closed
to maintain the space temperature
Set OA and relief dampers at their
< OA temperature
minimum positions, and set RA
damper at its corresponding open
(6) Smoke Control. Smoke detectors in the supply-air and return-air ductwork will stop the
supply fan and initiate a smoke alarm if smoke is detected at either location. Restarting the supply fan
will require manual reset at the smoke detectors.
p. Heating and Ventilating System Sequence of Operation (Figure 8-16 and Table 8-16).
Outside-Air, Return-Air, and Relief-Air Dampers.
(a) Occupied Mode. The outside air will open to its minimum position at start-up. The
outside-air, return-air, and relief-air dampers will be modulated under mixed-air temperature control.
Unoccupied Mode. The dampers will return to their normal positions as shown.
(2) Ventilation Delay Mode of Operation. During the ventilation delay mode, the dampers
remain positioned in the unoccupied mode while the supply fan runs continuously. Until the ventilation
delay mode ends, the HVAC system circulates return air to bring the building to comfort conditions, using
a minimum of energy.
Occupied Mode. The supply fan will start and will operate continuously.
(b) Unoccupied Mode. The supply fan will cycle on and off according to the night setback
(4) Freeze Protection - All Modes. A low temperature device, located as shown, will stop the
supply fan, cause the outside-air, return-air, and relief-air dampers to return to their normal position, and
will initiate a low-temperature alarm if the temperature drops below the low temperature device' setpoint
as shown. Return to the normal mode of operation will require manual reset at the low temperature
device. The UMCS panel will indicate an alarm condition when the low temperature device trips.