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SERVICING
5. Reconnect ignitor.
WARNING
L
INE VOLTAGE NOW PRESENT
6. 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.
7. 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 at-
tempt should be made to determine which has occurred.
If flashback should occur, check for the following:
1. Improper gas pressure - adjust to proper pressure (See
S-307 CHECKING GAS PRESSURE)..
2. Check burner for proper alignment and/or replace burner.
3. Improper orifice size - check orifice for obstruction.
S-310 CHECKING PRESSURE CONTROL
The pressure control is a safety device to prevent the com-
bustion cycle from occurring with inadequate venting caused
by a restricted or blocked vent pipe. In addition to the high
fire and low fire pressure switches, A/GMVC96 and A/GCVC
model furnaces have a "front cover pressure switch" wired
in series with the gas valve. This pressure switch keeps
the gas valve from opening in the event of condensate back-
ing up in the secondary heat exchanger. This could occur
from improperly connected drains or a plugged drain tube.
HIGH VOLTAGE
D
ALL
ISCONNECT
POWER BEFORE SERVICING OR
M
INSTALLING THIS UNIT.
ULTIPLE POWER SOURCES MAY
F
BE PRESENT.
AILURE TO DO SO MAY CAUSE PROPERTY
DAMAGE, PERSONAL INJURY OR DEATH.
1. Remove burner compartment door to gain access to pres-
sure switch(es).
2. Remove wires from the pressure switch(es) electrical
terminals.
72
WARNING
3. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown in
the following figure.
4. With an ohm meter connected across the pressure
switch terminals and with the inducer running, the switch
should close and the ohm meter should show a com-
plete circuit across the pressure switch. If the switch is
not closed, compare the negative pressure to the clos-
ing point specified for the particular switch. Either the
switch is defective or the inducer/venting system is in-
adequate.
*
*
*
M
1
P
3
O
F
*
F
C
2
O N
*
*
*
COIL COVER
PRESSURE SWITCH
INDUCED
DRAFT
BLOWER
Blower Pressure Switch
Negative Pressure Measurement
(90% Upflow Furnace Shown, Counterflow Similar)
S-311 HIGH ALTITUDE APPLICATION (USA)
When furnaces are installed at high altitude, the appropriate
High Altitude Orifice or Pressure Switch Kit must be ap-
plied. This is required due to the natural reduction in the
density of both the gas fuel and combustion air as altitude
increases. The High Altitude Orifice Kit will provide the proper
design certified input rate within the specified altitude range.
High Altitude Orifice or Pressure Switch Kits are purchased
according to the installation altitude and usage of either natu-
ral or propane gas. Refer to the Technical Manual or product
Specification Sheet for a tabular listing of appropriate alti-
tude ranges and corresponding manufacturer's high altitude
(Natural or Propane Gas) orifice or pressure switch kits.
Do not derate the furnace by adjusting the manifold pres-
sure to a lower pressure than specified on the furnace rating
plate. The combination of the lower air density and a lower
manifold pressure will prohibit the burner orifice from draw-
ing the proper amount of air into the burner. This may cause
incomplete combustion, flashback, and possible yellow tip-
ping.
INDUCED DRAFT BLOWER
PRESSURE SWITCH
PRESSURE SWITCH
HOSE
OPEN TO
ATMOSPHERE
1/4" TEE
INCLINED MANOMETER
HOSE TO INDUCED
DRAFT BLOWER TAP
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