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Table 13. Gas Flow Rate
control board. Reconnect the desired blower tap
to the piggyback quick connect.
The blower control is designed to start the
circulating air blower 30 seconds after the gas
valve is opened. The blower control is factory
wired to turn the blower motor off 120 seconds
after the gas valve is closed. This timing can be
changed using the BLOWER OFF timing switch
opposite the terminal block on the control board.
(See Figure 28.)
Verifying Burner Operation
To verify operation of the burners, make sure
that the blower compartment door is in place
and that there is power to the furnace. Set the
thermostat above room temperature and ob-
serve the ignition sequence. The fl ame can be
observed through the small clear window on the
burner box. The burner fl ame should carry over
between all burners. The fl ames should be blue,
without yellow tips. Flames should extend from
each burner without lifting, curling, or fl oating.
After verifying ignition, set the thermostat below
room temperature and verify that the burner fl ame
extinguishes completely.
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. Completely
block the return airfl ow to the furnace by installing
a close-off plate in place of or upstream of the
fi lter(s). Set the thermostat above room tempera-
ture and verify that the Sequence of Operation
is as described in these instructions. The supply
air limit switch should function to turn off the
gas valve within approximately fi ve minutes. The
circulating air and combustion blowers should
continue to run when the supply air limit switch
opens. Remove the close-off plate immediately
after the supply air limit switch opens. If the
furnace operates for more than fi ve minutes
with no return air, set the thermostat below room
temperature, shut off the power to the furnace,
and replace the supply air limit switch.
DESCRIPTION OF COMPONENTS
Figure 28 shows the location of each of the
functional components described below. If any
component of the furnace must be replaced,
use only factory authorized replacement parts.
See the Replacement Parts List for each com-
ponent.
Flame Sensor – The fl ame sensor acts to prove
that fl ame has carried over from the igniter to the
opposite end burner. If no fl ame is sensed, the
furnace will be shut down automatically.
Gas Valve – The gas valve controls the fl ow
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 verifi es
that the inducer is drawing the combustion gases
GAS FLOW RATE (CUBIC FEET PER HOUR)
TIME FOR
ONE REVOLUTION
(SECONDS)
CUBIC FEET PER REVOLUTION OF
METER
TIME FOR
ONE REVOLUTION
(SECONDS)
CUBIC FEET PER REVOLUTION OF
METER
1 5 10 1 5 10
24 150 750 1500 80 45 225 450
26 138 692 1385 82 44 220 439
28 129 643 1286 84 43 214 429
30 120 600 1200 86 42 209 419
32 113 563 1125 88 41 205 409
34 106 529 1059 90 40 200 400
36 100 500 1000 92 39 196 391
38 95 474 947 94 38 191 383
40 90 450 900 96 38 188 375
42 86 429 857 98 37 184 367
44 82 409 818 100 36 180 360
46 78 391 783 102 35 176 353
48 75 375 750 104 35 173 346
50 72 360 720 106 34 170 340
52 69 346 692 108 33 167 333
54 67 333 667 110 33 164 327
56 64 321 643 112 32 161 321
58 62 310 621 114 32 158 316
60 60 300 600 116 31 155 310
62 58 290 581 118 31 153 305
64 56 281 563 120 30 150 300