66
SERVICING
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.
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/GMVM97 and A/GCVM97
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 backing
up in the secondary heat exchanger. This could occur from
improperly connected drains or a plugged drain tube.
WARNING
HIGH
VOLTAGE
D
ISCONNECT
ALL
POWER BEFORE SERVICING OR
INSTALLING THIS UNIT.
M
ULTIPLE POWER SOURCES MAY
BE PRESENT.
F
AILURE TO DO SO MAY CAUSE PROPERTY
DAMAGE, PERSONAL INJURY OR DEATH.
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 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 complete circuit
across the pressure switch. If the switch is not closed,
compare the negative pressure to the closing point speci
-
fied for the particular switch. Either the switch is defective
or the inducer/venting system is inadequate.
OPEN TO
ATMOSPHERE
INCLINED MANOMETER
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
(90% Upflow Furnace Shown, Counterflow Similar)
HIGH ALTITUDE APPLICATION (USA)
When furnaces are installed at high altitude, the appropriate
High Altitude Orifice or Pressure Switch Kit must be applied.
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 altitude
ranges and corresponding manufacturer's high altitude (Nat
-
ural or Propane Gas) orifice or pressure switch kits.
Do
not
derate the furnace by adjusting the manifold pressure
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 drawing the prop
-
er amount of air into the burner. This may cause incomplete
combustion, flashback, and possible yellow tipping.
In some areas the gas supplier may artificially derate the gas
in an effort to compensate for the effects of altitude. If the
gas is artificially derated the appropriate orifice size must be
determined based on the BTU/ft
3
content of the derated gas
and the altitude. Refer to the National Fuel Gas Code, NFPA
54/ANSI Z223.1, and information provided by the gas supplier
to determine the proper orifice size.
