SERVICING
39
2. Turn on gas to furnace and check for leaks. If leaks are
found, repair and then reinstall burner compartment door.
3. Turn on electrical power and verify proper unit operation.
Make sure furnace operates at the proper manifold
pressure at both high and low stage outputs.
Gas
Rate
Range
Nominal
High Stage 3.2 to 3.8" w.c. 3.5" w.c.
Low Stage 1.6 to 2.2" w.c. 1.9" w.c.
Manifold Gas Pressure
Natural Gas
WARNING
H
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GH
VOLTAGE
D
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SCONNECT
ALL
ELECT
RI
CAL
P
O
W
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R
AN
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SHUT OFF GAS SU
PP
LY
BEFO
R
E SE
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V
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C
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NG O
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NG.
Gas
Rate
Range
Nominal
High Stage 9.7 to 10.3" w.c. 10.0" w.c.
Low Stage 5.7 to 6.3" w.c. 6.0" w.c.
Manifold Gas Pressure
Propane Gas
S-308 Checking Hot Surface Ignitor
120V Silicon Nitride Ignitor -
*M/CEC80
use a 120V silicon
nitride ignitor for ignition. The normal op
erating temperature
is approximately 2156°F - 2678°F.
At room temperature the
igniter ohm reading should be from 37-68 ohms.
7. Place unit in heating cycle, measure current draw of ignitor
during preheat cycle.
The steady state current at 120V is 0.37 to 0.68 amps.
8. After checking and/or replacing of hot surface ignitor,
reinstall burner compartment door and verify proper unit
operation.
S-309 Checking For Flashback
Flashback will also cause burning in the burner venturi, but is
caused by the burning speed being greater than the gas-air
flow velocity coming from a burner port.
Flashback may occur at the moment of ignition, after a burner
heats up or when the burner turns off. The latter is known as
extinction pop.
Since the end results of flashback and delayed ignition can
be the same (burning in the burner venturi) a definite attempt
should be made to determine which has occurred.
If flashback should occur, check for the following:
1. Improper gas pressure - adjust to proper pressure.
2. Check burner for proper alignment and/or replace burner.
3. Improper orifice size - check orifice for obstruction.
S-310 Checking Pressure Switch
The pressure control is a safety device to prevent the
combustion cycle from occurring with inadequate venting
caused by a restricted or blocked vent pipe.
1. Remove burner compartment door to gain access to
pressure switch(es).
2. Remove wires from the pressure switch(es) electrical
terminals.
3. Remove the pressure control hose from the control and
interconnect with an manometer as shown in the following
figures.
4. With an ohm meter connected across the pressure switch
terminals; with the inducer running the switch should close
and the ohm meter should show a complete circuit across
the pressure switch. If the switch is not closed, compare
the negative pressure to the closing point specified for
the particular switch. Either the switch is defective or the
inducer / venting system is inadequate.
-.
OPEN TO
ATMOSPHERE
HOSE TO INDUCED
DRAFT BLOWER TAP
1/4" TEE
INDUCED DRAFT BLOWER
PRESSURE SWITCH
PRESSURE SWITCH
HOSE
INDUCED
DRAFT
BLOWER
*
*
*
*
*
*
*
M
1
2
3
P
C
ON
O
F
F
COIL COVER
PRESSURE SWITCH
Blower Pressure Switch
Negative Pressure Measurement
