5370596-YIM-C-0518
Johnson Controls Ducted Systems
47
Two-stage heating:
a. Upon a call for first stage heat by the thermostat, the
heater relay (RA) will be energized. After completing the
specified fan on delay for heating, the UCB will energize
the blower motor. If the second stage of heat is required,
heater relay (RB) will be energized. After completing the
specified fan on delay for heating, the UCB will energize
the blower motor.
b The thermostat will cycle the electric heat to satisfy the
heating requirements of the conditioned space.
Electric Heat Operation Errors
Temperature Limit
If the UCB senses zero volts from the high temperature limit,
the indoor blower motor is immediately energized.
This limit is monitored regardless of unit operation status, i.e.
the limit is monitored at all times.
If the temperature limit opens three times within one hour, it will
lock-on the indoor blower.
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Limit Switch (LS)
This control is located inside the heater compartment and is set
to open at the temperature indicated in the Electric Heat Limit
Setting Tables 24. It resets automatically. The limit switch
operates when a high temperature condition, caused by
inadequate supply air flow occurs, thus shutting down the
heater and energizing the blower.
Reset
Remove the call for heating by lowering the thermostat setting
lower than the conditioned space temperature.
Electric Heat Anticipator Setpoints
It is important that the anticipator setpoint be correct. Too high
of a setting will result in longer heat cycles and a greater
temperature swing in the conditioned space. Reducing the
value below the correct setpoint will give shorter “ON” cycles
and may result in the lowering of the temperature within the
conditioned space. Refer to Table 25 for the required electric
heat anticipator setting.
Gas Heating Sequence Of Operations
When the thermostat calls for the first stage of heating, the low-
voltage control circuit from “R” to “W1” is completed. A call for
heat passes through the UCB to the
Ignition Control Board
(ICB).
The UCB monitors the “W1” call and acts upon any call
for heat by monitoring the
Gas Valve (GV).
Once voltage has
been sensed at the GV, the UCB will initiate the fan on delay for
heating, energizing the indoor blower the specified delay has
elapsed.
When the thermostat has been satisfied, heating calls are
ceased. The GV is immediately closed. The blower is de-
Table 24: Electric Heat Limit Setting
UNIT (TONS)
VOLTAGE
HEATER
kW
LIMIT
SWITCH
OPENS °F
ZT037 (3)
208/230
3
155
6
155
9
170
15
170
ZT049 (4)
208/230
6
155
9
170
15
170
20
170
ZT061 (5)
208/230
6
155
9
170
15
170
20
170
24
170
ZT037 (3)
480
3
155
6
155
9
170
15
170
ZT049 (4)
480
6
155
9
170
15
170
20
170
ZT061 (5)
480
6
155
9
170
15
170
20
170
24
170
ZT037 (3)
600
9
170
15
170
ZT049 (4)
600
9
170
15
170
20
170
ZT061 (5)
600
9
170
15
170
20
170
24
170
Table 25: Electric Heat Anticipator Setpoints
SETTING, AMPS
W1
W2
0.13
0.1
Table 24: Electric Heat Limit Setting
UNIT (TONS)
VOLTAGE
HEATER
kW
LIMIT
SWITCH
OPENS °F
