Johnson Controls
5586996-JSG-A-0120 | Start-Up & Operation Guide
56
Electric Heating
The unit can be ordered with optional factory installed
staged or modulating electric heat. There are several
KW sizes and steps available.
Sequences for staged electric and modulating electric
are different.
Table 25: Electric Heating Steps
Unit Size
kW
Low/High
Heating Steps
Low/High
25–30 Ton
50/120
3/6
40–50 Ton
60/160
4/8
Staged Electric Heat
When the unit has staged electric heat, the unit control-
ler sends 24 VAC from the main board to the electric
heat package installed.
Modulating Electric Heat
When the unit has modulating electric heat installed,
the first stage of electric heat is controlled by a silicon
controlled rectifier (SCR) controller.
The SCR controller provides full proportional control
over the entire heater range. The modulating electric
heat section is the first ON and last OFF.
SCR Operation
The first stage of electric heat is controlled by an
SCR. All other stages are staged ON/OFF.
The SCR receive a 0–10 VDC signal from the unit
controller to modulate the first stage of electric
heat. The unit controller varies this 0–10 VDC to
modulate the first stage of electric heat along with
other ON/OFF stages to maintain the active heat-
ing supply air temperature (SAT) setpoint.
The SCR has three LEDs for troubleshooting and
information on operation.
Green LED
• Steady ON when both supply voltage
and control voltage are present
• Flashes every half second when supply
voltage is present but control voltage is
not present
Yellow LED
• Steady ON whenever the SCR is ener-
gized and controlling the load
Red LED
• 2 flashes: Main voltage loss
• 3 flashes: Monitoring alarm (load loss,
SCR open circuit, SCR short circuit)
• 4 flashes: SCR internal fault
• ON steady: SCR over temperature
Hydronic Heating
The rooftop unit can be ordered with optional factory
installed hot water or steam coils. Both hot water and
steam coil options can be either low heat or high heat.
When hot water or steam coils are installed, there is a
factory installed freezestat. The freezestat is mounted
after the hot water/steam coil.
Factory installed valves can be ordered with the unit or
the end user can choose to use their own field installed
valves. That valves are connected to the building sys-
tem interface board at the heating valve output (HTGV-
O). The hot water/steam valve is controlled by a 0–10
VDC signal.
Freezestat Alarm
The freezestat is connected to the options control
board (LT-A/BI-4). It is a normally closed switch that
will open when the air temperature drops to 35.0°F.
When the freezestat opens, the following occurs:
• When the OAT is greater than 40.0°F (adjustable
between 20–40), no action is taken
• When the OAT is less than 40.0°F (adjustable be-
tween 20–40), the hot water/steam valve opens
100%, the supply fan is turned OFF, the outside
air dampers close to 0%, and the exhaust/return
fan is turned OFF if equipped
When the freezestat closes, the unit controller returns
to normal operation.
MZVAV Heating Operation
When the unit type is MZVAV, the unit controller con-
tinuously monitors the operational zone temperature.
The operational zone temperature is used to determine
whether the unit needs to provide cooling, heating, or
neither.
The operational zone temperature relies on which
of the following sensors are connected and reliable.
When more than one is connected and reliable, they
are used in order of priority.
