Outdoor Air Temperature Reset Of Supply Air Temperature - Honeywell AUTOMATIC CONTROL Engineering Manual

OUTDOOR AIR TEMPERATURE RESET OF SUPPLY AIR TEMPERATURE

Functional Description
Item
No. Function
1,2 Control system energizes when fan is turned on
(See FAN SYSTEM START-STOP CONTROL).
3,4 SA temperature setpoint, as adjusted by reset
schedule, maintained by modulating hot water
valve.
5,6 OA temperature resets the SA temperature setpoint
according to a reset schedule.
7 Hot water valve modulates flow, opens upon loss
of motive force, and closes upon fan shutdown.
8 Control program coordinates SA temperature,
valve, and fan interlock control.
FEATURES
The SA temperature rises as the OA temperature falls
according to a predetermined reset schedule.
CONDITIONS FOR SUCCESSFUL OPERATION
Use multiple inline coil arrangement should be used if a high
temperature rise is required.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, heating control shall be enabled.
A SA PID control loop shall modulate the hot water valve to
maintain the SA temperature setpoint. The SA temperature
setpoint shall be reset from 60°F to 100°F as the OA temperature
varies from 60°F to 5°F.
OA
2
74
NORMAL
5
MA
00
7
RESET SCHEDULE
6
OUTDOOR SUPPLY AIR
AIR SETPOINT
60 60
5 100
AIR HANDLING SYSTEM CONTROL APPLICATIONS
SA
ON
1
75
8
CONTROL
PROGRAM
4
60
SETPOINT
M10460
PSYCHROMETRIC ASPECTS
The SA condition depends on the entering air condition and
the temperature rise needed to satisfy the space heating
requirements.
In the following chart it is assumed that:
1. The MA system is set to maintain 60°F DB MA
temperature.
2. The OA temperature reset controller increases the setpoint
of the discharge PID control loop linearly from 60°F to
100°F DB as OA temperature falls from 60°F to 5°F DB.
1
The following results are obtained:
Item
No. Explanation
1 MA temperature.
2 SA at the beginning of the reset schedule.
3 SA heated to 100°F at outdoor design
temperature.
4 Between 0°F and 60°F OA temperature, SA
temperatures are between points 2 and 3.
225 ENGINEERING CMANUAL OF AUTOMATION CONTROL
3
ENTERING
AIR
2 3
4
C2550-2