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5121835-TIM-J-1118
Two-stage heating:
a. When there is a call for first stage heat by the thermostat,
the heater relay (RA) is energized. After completing the
specified fan on delay for heating, the UCB energizes the
blower motor. If the second stage of heat is required,
heater relay (RB) is energized. After completing the
specified fan on delay for heating, the UCB energizes the
blower motor.
b The thermostat cycles 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, that
is, the limit is monitored at all times.
If the temperature limit opens three times within one hour, it
locks on the indoor blower motor.
Safety controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Limit switch
The limit switch (LS) is located inside the heater compartment
and is set to open at the temperature indicated in Table 26,
Electric heat limit setting 50 in. cabinet, on page 54 and
Table 27, Electric heat limit setting 42 in. cabinet, on page 54. It
resets automatically. The limit switch operates when a high
temperature condition caused by inadequate supply air flow
occurs. This shuts down the heater and energizes the blower.
Table 26: Electric heat limit setting 50 in. cabinet
Unit (tons)
ZKT08, T10, T12
(8.5, 10, 12.5)
ZKT08, T10, T12
(8.5, 10, 12.5)
ZKT08, T10, T12
(8.5, 10, 12.5)
54
Heater
Limit switch
Voltage
kW
opens °F
18
150
24
150
208/230
34
150
54
130
18
150
24
150
480
34
150
54
130
18
150
24
150
600
34
150
54
130
Table 27: Electric heat limit setting 42 in. cabinet
Unit (tons)
ZKT06, T07 (6.5, 7.5)
ZKT06, T07 (6.5, 7.5)
ZKT06, T07 (6.5, 7.5)
Reset
Remove the call for heating by lowering the thermostat setting
lower than the conditioned space temperature.
Electric heat anticipator setpoints
The anticipator setpoint must be correct. Too high of a setting
results in longer heat cycles and a greater temperature swing in
the conditioned space. Reducing the value below the correct
setpoint gives shorter ON cycles and may result in the lowering
of the temperature within the conditioned space. See Table 28
for the required electric heat anticipator setting.
Table 28: Electric heat anticipator setpoints
Setting, amps
W1
0.13
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 on any call for heat by
monitoring the gas valve (GV). When voltage is sensed at the
GV, the UCB initiates the fan on delay for heating, energizing
the indoor blower the specified delay has elapsed.
When the thermostat is satisfied, heating calls are ceased. The
GV is immediately closed. The blower is de-energized after the
fan off delay for heating has elapsed. The draft motor performs
a 30-second post purge.
Ignition control board
First stage of heating
When the ICB receives a call for first stage of heating, W1, the
draft motor is energized. Once the draft motor has been proven,
a 30-second purge is initiated. At the end of the purge, the GV
is opened and the spark igniter is energized for 10 seconds.
The ICB then checks for the presence of flame. If flame is
detected, the ICB enters a flame stabilization period. If flame is
not detected, the GV closes and a retry operation begins.
Heater
Limit switch
Voltage
kW
opens °F
9
18
208/230
24
34
9
18
480
24
34
9
18
600
24
34
W2
0.1
Johnson Controls Ducted Systems
135
150
165
190
135
150
165
185
135
150
150
185
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