5835254-YIM-A-1119
44
Johnson Controls Ducted Systems
Limit switch
The limit switch (LS) is located inside the heater compartment
and is set to open at the temperature indicated in Table 24,
Electric heat limit setting 50 in. cabinet,
Table 25,
Electric heat limit setting 42 in. cabinet,
on page 57. 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
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 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 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.
During the flame stabilization period, a loss of the flame for 2
seconds causes the GV to close and the retry operation to
begin. After the flame stabilization period, a loss of flame for 3/4
second causes the GV to close and the retry operation to begin.
At the conclusion of the flame stabilization period, the ICB
operates the gas heat in high fire for an additional 60 seconds,
for a total for 120 seconds of high fire operation. After this 60
seconds, the ICB then uses the call for the second stage of heat
to control second stage operation of the GV.
When W1 is satisfied, both valves are closed.
Second stage of heating
When the ICB receives a call for the second stage of heating,
W2, the ICB conducts a complete first stage ignition sequence.
If this sequence is satisfied, the second main valve of the GV is
opened.
When W2 is satisfied, the second main valve is closed.
Retry operation
When a flame is lost or is not detected during an attempt to
achieve ignition, a retry operation occurs. A 30-second purge is
performed between ignition attempts.
If the unit fails after three ignition attempts, the furnace is locked
out for one hour. The furnace is monitored during this one-hour
period for unsafe conditions.
Recycle operation
When a flame is lost after the flame stabilization period, a recy-
cle operation occurs. If the unit fails after five recycle attempts,
the furnace is locked out for one hour.
Gas heating operation errors
Lockout
A one-hour lockout occurs following three retries or five
recycles. During the one-hour lockout, flame detection, limit
conditions, and main valves are tested. Any improper results
Table 24: Electric heat limit setting 50 in. cabinet
Unit (tons)
Voltage
Heater
kW
Limit switch
opens °F
ZZ15
(15)
208/230
18
150
24
150
34
150
54
130
ZZ15
(15)
480
18
150
24
150
34
150
54
130
Table 25: Electric heat anticipator setpoints
Setting, amps
W1
W2
0.13
0.1
For units with VFD and gas heat, the speed of the indoor
blower motor continues to be controlled by duct static
pressure via the Smart Equipment™ control board.
If there are VAV boxes present in the duct system, the
boxes must be driven to the full-open position using a
customer-supplied power source to assure adequate
airflow across the heat exchanger tubes.
