23
Verifying Operation of the Supply Air
Limit Switch
To verify operation of the supply air limit switch,
make sure that the blower door is in place and
that there is power to the furnace. Block the
return airflow to the furnace by installing a
close-off plate in place of or upstream of the
filter(s). Set the thermostat to a temperature
above room temperature and verify that the
Sequence of Operation is as described in
these instructions. The limit switch should
function to turn off the gas valve within
approximately four minutes (the exact time
depending on the efficiency of the close-off in
blocking the return air to the furnace). The
circulating air and combustion blowers should
continue to run when the limit switch opens.
Remove the close-off immediately after the
limit switch opens. If the furnace operates for
more than four minutes with no return air, set
the thermostat to a temperature below room
temperature, shut off the power to the furnace,
and replace the limit switch.
DESCRIPTION OF COMPONENTS
Figure 14 shows the location of each of the
functional components described below. Also,
refer to the Sequence of Operation Section of
this manual . If any component of the furnace
must be replaced, use only factory authorized
replacement parts. See the Replacement Parts
List for the factory authorized replacement for
each component.
Supply Air Limit Switch
The supply air limit switch prevents the air
temperature leaving the furnace from exceeding
the maximum allowable outlet air temperature.
Flame Sensor
The flame sensor acts to prove that flame has
carried over from the igniter to the opposite end
burner. If no flame is sensed, the furnace will
shut down within 7 seconds.
Flame Roll-Out Switch
The flame roll-out switch verifies that the burner
flames are drawn into the heat exchanger tubes.
If the burner flames are not properly drawn into
the heat exchanger, the flame roll-out switch will
open. The circulating air blower and combustion
blower will continue to operate if the flame roll-
out switch opens.
Gas Valve
The gas valve controls the flow of gas to the
burners. When the gas valve is energized it
automatically opens and regulates the gas
pressure in the manifold.
Pressure Switch
The pressure switch verifies that the inducer is
drawing the combustion gases through the heat
exchanger.
MAINTENANCE
It is recommended that the furnace be checked
yearly. At a minimum, this check should include
the following items.
!
WARNING:
To avoid electrical shock, personal injury,
or death, turn off the electric power at the
disconnect or the main service panel
before making any electrical
connections.
!
WARNING:
Products of combustion must not be
allowed to enter the living space. Failure
to prevent products of combustion from
being circulated into the living space
can create potentially hazardous
conditions including carbon monoxide
poisoning that could result in personal
injury or death.
Vent System
Check the vent pipe to ensure that it is not
corroded or blocked by debris. Any corroded
section of vent pipe must be replaced, and any
obstruction or blockage must be removed prior
to operating the furnace.
Air Filter(s)
!
WARNING:
Never operate the furnace without a filter
in place. Dust and lint in the return air can
build up on internal components,
resulting in loss of efficiency, equipment
damage, and possible fire.
