826081-XIM-C-0214
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Johnson Controls Unitary Products
satisfied, the ICB closes the pilot and main gas valves and
performs a 30 second purging of the heat exchanger by
continuing the operation of the draft motor. The UCB continues
the operation of the indoor blower for a configurable amount of
time after the call for heat is satisfied.
Two Stage Furnace Only
If a W1 only call for heat from a two stage thermostat is present,
then a two stage furnace will start on high fire for 1 minute and
then reduce to low fire until the call for heat is satisfied or a W2
call for heat is received. If a W2 call for heat is received while in
low fire operation, then the ICB will immediately move to high
fire operation. If a W1 and W2 call for heat is present, then the
furnace will remain on high fire operation until the W2 call for
heat is satisfied.
Automatic staging of a two stage furnace using a single stage
thermostat is possible. To achieve automatic staging of a two
stage furnace using a single stage thermostat, a jumper is
installed between R and W2 on the UCB with the single stage
thermostat heat control connected to W1. Wired in this manner,
the ICB will interpret the continuous power on W2 as automatic
two stage operation of the two stage furnace. The unit will
operate the same as a W1 only call for heat for 10 minutes. If
the call for heat is not satisfied in the 10 minutes, then the ICB
will move to high fire operation until the W1 call for heat has
been satisfied.
Gas Heat Operation Errors
During furnace operation, the ICB monitors the flame sense
circuit, the centrifugal switch, the primary limit switch and the
roll out switch. If a signal from any of the inputs moves to a fault
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 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 25) 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 fault code (Table
24).
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 24). 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 25). 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
Figure 19: Gas Valve Piping
GAS
VALVE
GAS MAIN
MAIN VALVE
TO MAIN
BURNER
REDUNDANT
VALVE
TO PILOT BURNER
