Johnson Controls
Start-Up & Operation Guide | 5586996-JSG-A-0120
51
Operation
Modulating HGRH
3-Way Stepper
Valve
LD20880
Figure 22: HGRH Schematic
Economizer
The rooftop unit can have a factory installed econo-
mizer as an option.
Economizer operation is often referred to as free cool-
ing because it uses outside air instead of mechanical
cooling to satisfy the cooling demands when the out-
side air conditions are suitable.
NOTE: There is a difference between economizer op-
eration and ventilation operation.
• Ventilation operation is active when the unit
mode is OCC and the supply fan is operating
when a ventilation option is installed
• Economizer operation is only active when
there is a demand for cooling and the out-
side air conditions are suitable for meeting
the demands utilizing outdoor air
• Ventilation and economizer options both use
the outside air damper in their respective se-
quences
There are four economizer options that can be in-
stalled: dry bulb, singly enthalpy, dual enthalpy, and
differential dry bulb.
Changeover Method
The unit can be ordered with one of the three econo-
mizer options:
• Dry Bulb: Measures the dry bulb temperature of
the outside air to determine if conditions are suit-
able for economizer operation
• Single Enthalpy: Measures the outside air en-
thalpy (temperature and humidity) to determine if
conditions are suitable for economizer operation
• Dual Enthalpy: Measures the outside air enthal-
py against the return air enthalpy to determine if
conditions are suitable for economizer operation
There are also two options that can be selected in the
unit setup.
• Differential Dry Bulb: Measures the dry bulb
temperature of the outside air against the dry bulb
temperature of the space
• Auto: The unit controller checks to see what
sensors are installed and reliable and determine
which of the four methods are available to use for
economizer operation
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