5188235-YIM-B-1016
Johnson Controls Unitary Products
55
state, then the ICB immediately closes the pilot and main gas
valves. The ICB will determine the device that is signaling a
fault and flash a code for that device. A primary limit trip,
centrifugal switch trip or flame sense fault triggers a temporary
lock out. An auxiliary limit or a roll out switch trip requires
intervention to reset the ICB. The UCB also monitors the
primary limit and gas valve.
Temperature Limits
The primary limit is located such that the temperature
sensitive switch can sense the temperature of the heat
exchanger tubes. On single or two stage models the limit is
mounted just above the inlet of the heat exchanger tubes on
the right side. If a primary limit (LS) fault occurs (the primary
limit opens due to excessive heat exchanger temperature),
then the ICB will flash the appropriate code (Table 31) and
monitor the primary limit. The UCB will energize the indoor
blower and the ICB will energize the draft motor while the
primary limit is open. When the primary limit closes and the
call for heat still exists, the ICB will start the ignition sequence
over and the UCB will de-energize the blower for 45 seconds.
However, the auxiliary limit is in series with the primary limit
and it takes first control. If the excessive heat has been high
enough to cause the auxiliary limit (AUX) to open, then the
ICB will flash the primary limit code but the furnace will not
retry ignition during the same call for heat. The auxiliary limit is
of the manual reset type and is mounted in the upper right
hand corner of the panel between the burner manifold and the
flue gas collector box just behind the draft motor. If the
auxiliary switch has opened, then special attention should be
paid to the primary limit as it may be faulty as well. However,
the auxiliary switch is sized such that multiple trips of the
primary limit due to complete blower failure will cause enough
heat to build up and trip the auxiliary.
f the primary limit opens three times within one hour, then the
UCB will lock on the indoor blower and flash a flash a code on
the Simplicity Lite Control (Table 30) or sends error message
to the LCD Smart Equipment™ Control.
Gas Valve
The UCB monitors the gas valve (GV). Any time the UCB senses
voltage at the GV without a call for heat for a continuous five-
minute period, the UCB will lock on the indoor blower and flash a
code on the Simplicity Lite Control (Table 30) or sends error
message to the LCD Smart Equipment™ Control. When the
UCB no longer senses voltage at the GV the UCB will de-
energize the indoor blower after the expiration of the indoor
blower heating off delay.
If the voltage has been sensed at the GV for at least 15
seconds during the fan on delay for heating and the UCB no
longer senses voltage at the GV (W1 call for heat removed or
an ICB fault exists), then the UCB forces the indoor blower on
for the indoor blower heating off delay.
The gas valve is of the redundant type. If for any reason the
main gas valve fails in the open position, then the redundant
valve ahead of the main gas valve will shut off the flow of gas to
both the pilot and main gas valves.
Centrifugal Switch
The centrifugal switch is mounted on the end of the draft motor
and it is an integral part of the motor assembly. On a call for
heat, the ICB checks the centrifugal switch (CS) for open state
before it energizes the draft motor. If it is closed, then the ICB
will lock out the furnace and flash a code (Table 31). If open,
then the ICB will energize the draft motor and verify that the
switch closes before initiating the purging and ignition
sequence. If at any time during furnace operation the
centrifugal switch opens, then the ICB will de-energize the pilot
and main gas valves and monitor the centrifugal switch. If the
centrifugal switch closes and the call for heat still exists, then
the ICB will retry the purging and ignition sequence.
Roll-out Switch
This temperature sensitive switch is located in the burner
vestibule just above the right hand side of the burner assembly.
In the event of the flame spilling out into the burner manifold
area the roll-out switch will open, the ICB will close both the
main and pilot gas valves and flash a code (Table 31). The ICB
will not retry the ignition sequence during the same call for heat.
Flame Sense Circuit
The flame sensor is mounted on the left hand side of the burner
assembly and is positioned such that the pilot flame surrounds
the tip of the sensor. On a call for heat the ICB checks for the
flame sense circuit to be open. If open, then the ICB initiates the
purging and ignition sequence. Once the pilot flame is present,
the ICB monitors the flame sense circuit for pilot flame stability.
If the pilot flame is unstable or lost completely, then the ICB will
immediately close both the pilot and main gas valves. The ICB
will retry the purging and ignition sequence. If the flame is
unstable or lost more than 16 times during the same call for
heat, then the ICB will lock out furnace operation for 5 minutes.
Table 23: Single Stage Gas Heat Limit Control Setting
Unit
(Tons)
Capacity, MBH
Limit Control Opens, °F
Input
Output
Direct Drive
Belt Drive
3
50
40 240
240
3
100
80
170
210
4
75
60 210
240
4
125
100
165
210
5
100
80 170
210
5
125
100
165
210
Table 24: 2 Stage Gas Heat Limit Control Setting
Unit
(Tons)
1st Stage
Capacity
2nd Stage
Capacity
Limit Control Opens,
°F
Input
(MBH)
Output
(MBH)
Input
(MBH)
Output
(MBH)
Direct
Drive
Belt
Drive
3
45
36 75
61
210
240
3
69
55
115
92
200
200
4
45
36 75
61
210
240
4
75
60
125
101
165
210
5
45
36 75
61
210
210
5
75
60
125
101
165
210
