CEMP-E
TI 811-12
18 August 1998
(2)
Supply-Fan and Return-Fan Control.
(a)
Occupied Mode. Supply fan and return fans will start in sequence and will operate
continuously.
(b) Unoccupied Mode. Supply and return fans will cycle on and off according to the night
setback schedule setpoint.
(3) Supply-Duct Pressure Control. When the supply fan starts, the UMCS will modulate the fan
inlet vanes from the signal of a static pressure sensor to maintain the setpoint. When the fan is off the
inlet vanes will be closed.
(4) Return Fan Volume Control. When the return fan starts, the UMCS will modulate the fan
inlet vanes from the signals of airflow measurement stations and transmitters in the return and supply
ducts in order to maintain a constant flow differential setpoint between supply and return airflows.
(5) Freeze Protection - All Modes. A low temperature device, located as shown, will stop the
supply and return fans, 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's setpoint. Return to the normal mode of operation will require manual reset at the low
temperature device. The UMCS will indicate an alarm condition when the low temperature device trips.
(6)
Cooling Coil and Preheat Coil Control.
(a) Occupied Mode. The control valves will be modulated in sequence by the UMCS from
the signal of a temperature-sensor located in the supply fan discharge air duct to maintain the setpoint.
(b) Unoccupied Mode. The UMCS will close the cooling-coil control valve to the coil and
will open the preheat-coil control valve to the coil.
(7) Mixed-Air Temperature Control. When the UMCS places the system in the economizer
operation, it will modulate the dampers from the signal of a temperature sensor in the mixed-air stream to
maintain the setpoint based on the conditions shown in Table 8-21.
Table 8-21. Mixed-Air Damper Modulation.
Condition No.
Description
Control
1
OA temperature
Modulate OA, RA, and relief
< SA temperature
dampers to maintain mixed air
< Changeover temperature
temperature at cold deck supply
temperature setpoint minus 2
degrees F.
2
SA temperature
Set OA and relief dampers at
< OA temperature
100% open; RA dampers closed.
< Changeover temperature
3
SA temperature
Set OA and relief dampers at their
< Changeover temperature
minimum positions, and set RA
< OA temperature
damper at its corresponding open
position.
(8) Pressure-Independent VAV Terminal Box Control. The control damper in the VAV box will
modulate in response to the signal from a flow-sensing element at the inlet (or discharge) of the VAV
terminal box to the UMCS Unitary Controller. The UMCS will control the VAV box damper from its
8-10
Previous Page
