Honeywell AUTOMATIC CONTROL Engineering Manual page 261

FEATURES
1. This application uses zone control of heating and cooling.
2. Deck temperatures dictated by zones with greatest heating
and cooling demand and with a deadband.
CONDITIONS FOR SUCCESSFUL OPERATION
All zones should be connected to load analyzer program to
satisfy total load requirements. However, in larger systems it
may be good practice to connect only selected diverse zone
loads. Zones that may be allowed to go out of control (storage
rooms, etc.) should not be connected to the load analyzer
program.
SPECIFICATIONS
See FAN SYSTEM START-STOP CONTROL.
Anytime the supply fan runs, control system shall be enabled,
and mixing dampers shall position to minimum ventilation
position during scheduled occupancy periods.
Each zone space temperature PI loop shall modulate its zone
mixing dampers to maintain its space temperature setpoint.
The zone with the greatest temperature deviation below
setpoint shall reset the hot deck temperature setpoint from 55°F
to 97°F as required to maintain the zone space temperature 2
degrees below setpoint.
The zone with the greatest temperature deviation above
setpoint shall reset the cold deck temperature setpoint from
78°F to 55°F as required to maintain the zone space temperature
2 degrees above setpoint.
The hot deck PID loop shall modulate the hot deck hot water
valve to maintain the hot deck temperature setpoint.
The cold deck PID loop shall modulate the OA/RA mixing
dampers in sequence with the cold deck chilled water valve to
maintain the cold deck temperature setpoint.
Anytime the economizer decision program determines that
OA is unsuitable to assist in cooling, the OA/RA mixing
dampers shall be returned to their minimum ventilation position.
PSYCHROMETRIC ASPECTS
In the summer, zone space temperature is maintained by
mixing air from the hot deck with cold deck air (the temperature
of which is dictated by the zone with the greatest demand for
cooling). The zone with the greatest demand for cooling gets
100% cold deck air.
AIR HANDLING SYSTEM CONTROL APPLICATIONS
In the winter, zone space temperature is maintained by mixing
air from the cold deck with hot deck air (the temperature of
which is dictated by the zone with the greatest demand for
heating). The zone with the greatest demand for heating gets
100% hot deck air. The zone with the greatest demand for
cooling (assuming a zone space temperature rises two degrees
above setpoint and demands cooling) gets 100% cold deck air
and dictates the cold deck temperature maintained via the
economizer cycle.
In the following chart it is assumed that:
1. Desired space condition is 75°F DB.
2. Outdoor air condition is 75°F DB and 80% RH.
3. The mixed air is 25 percent outdoor air during the
cooling cycle.
4. Coil leaving air temperature is 55°F (at least one zone
demands full cooling).
3
4
5
COLD
DECK
55°F DB
The following results are obtained:
Item
No. Explanation
1 OA temperature at example time.
2 Mixed air is 25% OA and 75% RA. This is
also the hot deck air, assuming no zone
temperature has dropped two degrees and
demanded heating.
3 This line represents the cooling process of the
cold deck air. The zone demanding the most
cooling dictates how far the process goes from
Point 2 to Point 4.
4 55°F DB is the minimum cold deck setpoint
set up in the program, set by a zone requiring
full cooling.
5 Discharge air to a zone requiring half cold
deck air and half mixed air.
6 The space cooling process line.
7 Return air is 75°F DB and 60% RH (humidity
rises because humidity is high outdoors and
only partial supply airflow is dehumidified).
251 ENGINEERING CMANUAL OF AUTOMATION CONTROL
1
OA 75°F DB, 80% RH
2
MA AND HOT DECK
RA 75°F DB, 60% RH
7
6
M10466