Carrier 48NLT - Manual (Page 13)

PILOT FLAME BURNER FLAME
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SAFETY CHECK OF LIMIT CONTROL - The control
shuts off the gas supply and energizes the circulating-air
blower motor if the furnace overheats.
The recommended method of checking this limit control
is to gradually block off the return air after the furnace has
been operating for a period of at least 5 minutes. As soon
as the limit control functions, the return-air opening should
be unblocked to permit normal air circulation. By using this
method to check the limit control, it can be established that
the limit is functioning properly and the furnace will “fail­
safe” if there is a restricted circulating air supply or motor
failure. If the limit control does not function during this
test, the cause must be determined and corrected.
HEATING SEQUENCE OF OPERATION - See Fig. 13
for single-phase operation and Fig. 14 for 3-phase opera­
tion. Room thermostat calls for heat, closing circuit be­
tween R and W 24-V control circuit terminals. (Power to
the R terminal is supplied through LS and ALS safety
switches.) The PC2 inducer control board is energized through
the normally closed set of contacts of pressure switches (CPS),
which starts the inducer motor (IM). The IM comes up to
speed, and the vacuum in the collector box increases, open­
ing the normally closed and closing the normally open
contacts of the pressure switch (PS), energizing the circuit
to the ignition control (IGN) and the pilot valve (PV). If
theflame sensor proves the presence of the pilot flame, the
internal switching of the ignition control deenergizes the
spark generator and energizes the main gas valve (MV) and
the IFR2 electronic timer. Gas flows to the main burners
and is ignited by the pilot flame. The PCI electronic timer
will close the IFR2 relay 60 seconds after the burners are
ignited and the blower motor (IFM) will start. When the
thermostat is satisfied, the R and W circuit is opened and
power is removed from the PC2 inducer control and the ig­
nition module (IGN), which causes the MV to close in­
stantly and the IM is deenergized. The electronic timer PCI
will keep the IFM running an additional 60 to 90 seconds.
Then the blower stops and the unit is on standby until an­
other call for heat.
NOTE; If the main limit switch opens due to the unit over­
heating, the IFM is turned on through the electronic board.
NOTE: When the unit is initially powered, IER2 will close
and run the IFM for the duration of the off-delay cycle (60
to 90 seconds).
If the pilot fails to light within a 120-second trial for ig­
nition period from the initial call for heat, the IGN will go
into a Retry mode after a period of approximately 5 min­
utes (following the 2-minute trial for ignition period). If the
pilot again fails to light, IGN will go into Retry mode; this
cycle will be repeated until the pilot light ignites. If the pi­
lot flame has been established but then extinguishes, the
IGN will immediately reset as if it were the initial call for
heat. If this occurs more than 3 times, the IGN will lock
out the system, and the diagnostic LED (located on the IGN)
will flash. To reset, open the R-W thermostat circuit for 30
seconds and reclose. If the diagnostic LED glows con­
stantly, replace control.
LIMIT SWITCHES — Normally closed limit switch LS com­
pletes the control circuit through the thermostat R circuit.
See Fig. 13 and 14. Should the leaving-air temperature rise
above the maximum allowable temperature, the limit switch
opens and the R control circuit “breaks.” Any interruption
in the R control circuit instantly closes the gas valve and
stops gas flow to the burners and pilot. The IFM continues
to run until the LS resets.
When the air temperature at the limit switch drops to the
low-temperature setting of the limit switch, the switch closes
and completes the R control circuit. The electric-spark ig­
nition system cycles and the unit returns to normal heating
operation.
BLOWER AUXILIARY LIMIT SWITCH - Blower aux­
iliary limit switch ALSl is a temperature-actuated auto, re­
set switch and is connected in series with the limit switch
LS. The function of the switch is to prevent abnormal blower
compartment temperatures. The switch is mounted on the
blower housing. When the temperature at the auxiliary switch
reaches the maximum allowable temperature, the R control
circuit “breaks”, closing the gas valve and stopping gas
flow to the burners and pilot. The switch will automatically
reset when the blower compartment temperature returns to
normal. The IFM continues to run until ALSl resets.
ROLLOUT AUXILIARY LIMIT SWITCH - Rollout aux­
iliary limit switch ALS2 is a temperature-actuated manual
reset switch connected in series with limit switch LS and
blower auxiliary limit switch ALSl. The function of the
switch is to close the main gas valve in the event of flame
rollout. The switch is located above the main burners. When
the temperature at the auxiliary switch reaches the maxi­
mum allowable temperature, the R control circuit trips, clos­
ing the gas valve and stopping gas flow to the burners and
pilot. To reset the switch, push in the red button. If the
switch cycles again, shut down the unit and call for service.
The IFM continues to run until ALS2 is reset.
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