Johnson Controls
5586996-JSG-A-0120 | Start-Up & Operation Guide
52
Dry Bulb
The unit controller continuously monitors the outside
air dry bulb temperature
•
Economizer Active:
When the OAT is less than
the economizer OAT enable setpoint minus 1.0°F
•
Economizer Inactive:
When the OAT is greater
than the economizer OAT enable setpoint
Single Enthalpy
The unit controller continuously monitors the outside
air dry bulb temperature and the outside air relative hu-
midity.
•
Economizer Active:
When the outside air en-
thalpy is less than the OA enthalpy setpoint minus
1 BTU/lb and the OAT is less than the high limit
shutoff minus 1.0°F
•
Economizer Inactive:
When the outside air en-
thalpy is greater than the OA enthalpy setpoint or
the OAT is greater than the high limit shutoff
Dual Enthalpy
The unit controller continuously monitors the outside
air enthalpy and compares it to the return air enthalpy.
•
Economizer Active:
When the outside air en-
thalpy is less than the return air enthalpy minus
1 BTU/lb and the OAT is less than the high limit
shutoff minus 1.0°F
•
Economizer Inactive:
When the outside air en-
thalpy is greater than the return air enthalpy or the
OAT is greater than the high limit shutoff
Differential Dry Bulb
The unit controller continuously monitors the outside
air dry bulb temperature and space dry bulb tempera-
ture.
•
Economizer Active:
When the OAT is less than
the operational zone temperature minus 1.0°F
•
Economizer Inactive:
When the OAT is greater
than the operational zone temperature
Auto
The unit controller continuously monitors all the tem-
perature and humidity sensors installed and determine
which economizer method to use depending on the re-
liability of the sensors.
For example, if the unit was ordered as a dual enthal-
py economizer and the return air humidity sensor be-
comes unreliable, the unit controller starts to control
the economizer in single enthalpy.
Heating
The rooftop unit offers five factory installed heating
package options.
• Staged gas heat
• Modulating gas heat
• Staged electric heat
• Modulating electric heat
• Hydronic heat (hot water or steam)
The heating package must be ordered with the unit and
is factory installed. There are no field installation kits
available for heating.
Gas Heating
The unit can be ordered with optional factory installed
staged gas heat or modulating gas heat. There are
several MBH inputs that can be selected.
Sequences for staged gas and modulating gas are dif-
ferent.
Table 22: Gas Heating Stages and Turndown Ratios
Heat Type
MBH Input
Stages
25–30 T
on
Staged
250
2
500
2
750
4
Modulating
250
10:1
500
20:1
750
30:1
40–50 T
on
Staged
500
2
750
4
1250
6
Modulating
500
20:1
750
30:1
1250
50:1
Staged Gas Heat
The following sequence refers to any burner that is
non-modulating. This depends on the heating options
ordered for the unit.
This sequence could refer to all burners installed in the
unit or either one, two, or three burners. It could also
refer to the staged burners of a modulating gas unit
(burner 2 and/or burner 3).
The staged gas controller receives a demand for heat
from the main unit controller through a Modbus signal.
The controller performs a system check to ensure
that all safeties are closed, the APS is open, and the
2-stage gas valve is closed.