Carrier 48NHT024 - Page 12

Table 9 shows the approved temperature rise range for each
unit, and the air delivery Cfm at various temperature rises.
The heating operation airflow must produce a temperature
rise that falls within the approved range.
Refer to Step 9, part C, of these instructions to adjust heat¬
ing airflow, when required.
G. Safety Check of Limit Control
The control shuts off the combustion gas supply and ener¬
gizes 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.
H. Heating Sequence of Operation See Figs. 12,13 or 14
Room thermostat calls for heat closing circuit between “ R”
and “W” 24 volt control circuit terminals. (Power to the
“R” terminal is supplied through “CB” Circuit Breaker and
“LS, ALS and FL” safety switches) “PC2” inducer control
board is energized which starts the inducer motor “IM”.
The inducer motor comes up to speed, the vacuum in the col¬
lector box increases, opening the normally closed and clos¬
ing the normally open contacts of the contacts of the pres¬
sure switch “PS” energizing the circuit to the “IGN” igni¬
tion control and the pilot valve “PV”. If the flame sensor
proves the presence of the pilot flame the internal switching
of the ingition control de-energizes the spark generator and
energizes the main gas valve, “MV” and the “IFR2” elec¬
tronic timer. Gas flows to the main burners and is ignited
by the pilot flame. The “PCI” electronic timer will close the
“IFR2” relay between 60 to 90 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 con¬
trol and the “IGN” ignition module which causes the main
gas valve to close instantly and the inducer motor is de¬
energized. The electronic timer “PCI” will keep the “IFM”
blower motor running an additional 1 to 3 minutes. Then
the blower stops and the unit is on standby until another
call for heat.
NOTE: If the main limit switch opens due to the unit over
heating, the blower motor is turned on thru the electronic
board.
If the pilot fails to fight within a 50 second trail for ignition
period from the initial call for heat the ignition control
“IGN” will lockout the system and prevent further fighting
attempts. To reset, open the “R” - “W” thermostat circuit
for 30 seconds and re-close.
I. Limit Switches
Furnace limit switch “LS” (See Figs. 12, 13 or 14.) closes
the gas valve if the leaving-air temperature exceeds the
maximum allowable temperature.
Normally closed limit switch “LS” completes the control
circuit through the thermostat “R” circuit. See Figs. 12, 13
or 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 blower motor contin¬
ues to run until the time-delay sequence of blower relay
“PCI” is completed.
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 ignition system cycles and the unit returns to normal
heating operation.
J. Auxiliary Limit Switch
Auxiliary limit switch “ALS” is a temperature-actuated
manual reset switch and is connected in series with the limit
switch “LS.” The function of the switch is to prevent abnor¬
mal blower compartment temperatures. The switch is
mounted on the blower housing. When the temperature at
the auxiliary switch reaches the maximum allowable tem¬
perature the “R” control circuit “breaks”, closing the gas
valve and stopping gas flow to the burners and pilot. To
reset switch push in on red push button. If it cycles again,
shut down the unit and call for service.
K. Fusible Link
Fusible Link “FL” is a temperature-actuated device con¬
nected in series with the limit switch “LS.” It is located in
the wire bundle adjacent to the burner manifold.
The function of the device is to prevent abnormally high
burner compartment temperatures. The fink will melt if an
overheating condition caused by inadequate combustion air
supply or improper venting occurs. Do not jumper this fuse.
Correct the condition and replace the fuse with an identical
part.
Step 9—Cooling Section Startup and Adjustments
A CAUTION
Complete the required procedures given in Step 7, “Pre¬
paring Unit for Startup,” before starting the unit.
Do not jumper any safety devices when operating the
unit.
Do not operate the compressor when the outdoor tem¬
perature is below 55 F (unless accessory low-temp kit is
installed).
Do not rapid-cycle the compressor. Allow 5 minutes
between “on” cycles to prevent compressor damage.
A. Checking Cooling Control Operation
Start and check the unit for proper cooling control operation
as follows:
1. Place room thermostat SYSTEM switch in OFF posi¬
tion. Observe that blower motor starts when FAN
switch is placed in ON position and shuts down when
FAN switch is placed in AUTO position.
2. Place SYSTEM switch in COOL position and FAN
switch in AUTO position. Set cooling control below
room temperature. Observe that compressor, con¬
denser fan, and evaporator blower motors start.
Observe that cooling cycle shuts down when control
setting is satisfied. Blower motor has off delay of
approximately one minute on shut down.
3. When using an autochangeover room thermostat, place
both SYSTEM and FAN switches in AUTO positions.
Observe that unit operates in heating mode when tem¬
perature control is set to “call for heating” (above
room temperature) and operates in cooling mode when
temperature control is set to “call for cooling” (below
room temperature).
NOTE: 060 size 460V models are equipped with a 3-phase
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