290699-YIM-B-0607
44
Unitary Products Group
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 32).
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 32). 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 heat-
ing off delay.
The gas valve is of the redundant type. If for any rea-
son 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 cen-
trifugal 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 33). 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 centrifu-
gal 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 33). 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 opera-
tion for 5 minutes.
TABLE 29: SINGLE STAGE GAS HEAT LIMIT
CONTROL SETTING
Units
(Tons)
Capacity, MBH
Limit Control
Opens, ºF
Input
Output
3
50
40
165
4
75
60
165
3
100
80
165
4
125
100
165
5
100
80
165
5
125
100
165
6
100
80
165
6
125
100
165
