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Johnson Controls Unitary Products
SAT CONTROL FOR COOLING
This control has priority over any other zone temperature or SAT
control and is used at all times, during CV as well as VAV
operation. The Excessive SAT Control state is entered any time
the SAT drops below the trip point of a compressor (as the trip
points are programmed in an ascending sequence, the
compressor that trips will always be the highest- numbered one).
While in this state, the control will continue monitoring the SAT
and turning off compressors any time the SAT drops below the
respective trip point. There is a 2 minute time delay between
compressor trips in cases when the SAT drops below trip points
of multiple compressors. This assures that multiple
compressors will not be turned off simultaneously.
ECONOMIZER LOADING OPERATION DURING AN EXCES-
SIVE SAT FOR COOLING:
CV ECONOMIZER OPERATION
Economizer dampers allow mixing of outdoor and return air.
The dampers are coupled and controlled with a single actuator
such that when the Outdoor Air damper is fully closed, the
Return Air damper is fully open (and vice versa). The position of
the Economizer dampers is controlled based on:
• Energy considerations (“free cooling”)
• Ventilation considerations (minimum Outdoor Air damper
position and Demand Ventilation)
• Space static pressure considerations (minimum Outdoor
Air damper position).
Economizer dampers are also controlled in certain situations to
perform “economizer loading” - which minimizes SAT temperature
swings resulting from turning cooling, or heating stages on / off.
This function is separate from normal economizer operation, and is
separately described at the end of this section.
WHEN IS THE ECONOMIZER OPERATION USED?
If the rooftop unit is equipped with an economizer (and that
option is selected), and free cooling is available (“economizer
suitable”), then the Economizer Operation as specified in this
section will be used in the following operation modes:
CV cooling with thermostat, or zone temperature sensor control
(note that in the zone sensor mode, the compressor staging
algorithm defined in this Economizer section will replace the
zone sensor based control described in Sensor Operation in CV
Operation section, earlier in this document).
MINIMUM VENTILATION POSITION SETTING
The minimum position setting represents the minimum opening
of the outdoor air damper (% open). This setting will be
maintained any time the unit is in Occupied mode. The
minimum position setting will be determined by an “Economizer
Min Position” programmable parameter set by a Palm Pilot. The
minimum position setting will be ignored during the Unoccupied
mode. During the Unoccupied mode, the minimum position is
0% (the Economizer may not remain closed during the
Unoccupied mode, in case the temperature control to an
unoccupied setpoint can use Outside Air for free cooling).
MINIMUM POSITION DURING HEATING AND OCCUPIED
MODE
During heating while in Occupied mode, the economizer will be
at its programmed minimum position.
MINIMUM POSITION DURING COOLING AND OCCUPIED
MODE
During cooling while in Occupied Mode, the economizer may be
at its programmed minimum or may be modulated between its
minimum position and 100% open position by the economizer
control.
SAT SETPOINTS USED DURING COOLING WITH
ECONOMIZER OPERATION
As long as the Economizer Operation is enabled and “free
cooling” is available, the economizer will be controlled (with, or
without any compressors running) to maintain the following SAT
setpoints: In CV cooling mode:
• With a call for first stage cooling, a programmed Econo-
mizer First Stage Setpoint. This setpoint is programmable
in the range of 40 °F to 65 °F, default setting is 55 °F.
• With a call for second stage cooling, a programmed Econo-
mizer Second Stage Setpoint. This setpoint is programma-
ble in the range of 40 °F to 65 °F, default setting is 50 °F.
Criteria for Economizer Suitable decision Sensor
availability:
There are three different methods of deciding whether the
economizer is suitable:
•
Differential enthalpy
(highest preference from energy
viewpoint)
•
Outside enthalpy
(middle preference) and
•
Outside temperature method
(lowest preference)
The choice of a method with highest preference is automatic
(“self-configuration”) based on availability of appropriate
sensors. If a sensor fails/ goes unreliable, a fault is indicated
and, again, the next highest preference method will be
automatically selected (“fault tolerance”).
There are two ON/OFF programmable parameters related to
the choice of an economizer method:
• “OAH Sensor Enable”
• “RAH Sensor Enable”
These parameters are set to reflect the installed sensors that
can be used by the “self-configuration” feature and control
sensor failure alarms.
The OAH sensor, if available, allows use of Outside Enthalpy
method for deciding on free cooling availability.
