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5121817-TIM-A-0515
Redundant valve
Gas main
Gas Valve
Figure 21: Gas Valve Piping
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, cut the 440/GY
wire that runs from pin 2 of the S3 connector on the (UCB) unit
control board to P2-4 on the ignition control board. Cut the wire
1/2" from the S3 mate-n-lock connector. Wire nut or tape off the
1/2" wire. Attach the other end of the 440/GY wire to the R
terminal on the UCB, or to the TB1 power strip.
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 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 30) 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
46
Main valve
To main burner
To pilot burner
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 send an error
message.
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 28). 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 30). 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 30). The ICB
will not retry the ignition sequence during the same call for heat.
Johnson Controls Unitary Products
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