5725848-XIM-A-0319
Johnson Controls Ducted Systems
41
Temperature limits
The primary limit is located such that the temperature sensitive
switch can sense the temperature of the heat exchanger tubes.
On single stage models the limit is mounted to the condenser
partition panel. On 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 27) 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.
If the primary limit opens three times within one hour, then the
UCB will lock on the indoor blower and flash a code on the
Simplicity Lite Control (Table 26) 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 a
flash code on the UCB is initiated (Table 26). 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 27). 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.
Rollout 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 rollout switch will open, the ICB will close both the
main and pilot gas valves and flash a code (Table 27). 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.
Flash codes or error messages
The UCB will initiate a flash code associated with errors within
the system. Refer to flash a code on the Simplicity Lite Control
(See Table 26) or sent error message to the LCD Smart
Equipment™ Control.
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.
Table 23: Limit control setting
Unit
(Tons)
Capacity, MBH
Limit Control Opens, °F
Input
Output
Direct Drive
Belt Drive
3
15 (50)
12 (40)
115 (240)
-
4
22 (75)
18 (60)
99 (210)
-
5, 6.3
29 (100)
29 (80)
-
99 (210)
Table 24: Gas heat anticipator setpoints
Gas Valve
Anticipator Setpoint
Honeywell VR8204M
0.60 amp
White-Rodgers 36E36
0.54 amp
