5567611-BIM-B-1218
64
Johnson Controls Ducted Systems
If there is a call for dehumidification and no call for cooling, the
unit automatically reverts back to the normal hot gas reheat
mode only allowing refrigerant stage one to run in reheat mode
and refrigerant stage two remains off.
If there is a call for dehumidification and a call for cooling, the
unit remains in the alternate hot gas reheat mode allowing
refrigerant stage one to run in hot gas reheat or
dehumidification mode and refrigerant stage two runs in
cooling.
Electric heating sequence of operations
The following sequence describes the operation of the electric
heat section.
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.
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,
on page 64. 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.
Reset
Reset removes 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 27
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 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 when 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,
Table 26: Electric heat limit setting
Unit (tons)
Voltage
Heater
kW
Limit switch
opens °F
ZV-06 (6.5)
ZV-07 (7.5)
ZV-08 (8.5)
208/230
9
150
18
150
24
150
36
150
ZV-10 (10)
ZV-12 (12)
18
150
24
150
36
150
54
130
ZV-06 (6.5)
ZV-07 (7.5)
ZV-08 (8.5)
480
9
150
18
150
24
150
36
150
ZV-10 (10)
ZV-12 (12)
18
150
24
150
36
150
54
130
Table 27: Electric heat anticipator setpoints
Setting, Amps
W1
W2
0.13
0.1
