74
SERVICING
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 Igniter -
furnaces use a 120V silicon
nitride igniter for ignition. The normal operating temperature
is approximately 2156°F - 2678°F.
D
ISCONNECT
ALL
POWER BEFORE SERVICING.
WARNING
1. Remove burner compartment door to gain access to the
ignitor.
2. Ignitor cool - approximately 70 - 77°F.
3. Disconnect the ignitor from the Ignition Control.
4. Using an ohmmeter measure the resistance of the igni-
tor:
120 Volt Silicon Nitride : 120V Nitride Igniter should read
between 37 to 68 ohms.
5. Reconnect ignitor.
L
INE VOLTAGE NOW PRESENT
WARNING
6. Place unit in heating cycle, measure current draw of
ignitor during preheat cycle.
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 SWITCH
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 on the 80% and 90%
furnaces. Also on the 90% furnaces there is a pressure
control that will prevent the combustion cycle from occur-
ring with inadequate condensate drainage due to a partial
or blocked recuperator coil or drain.
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. Using a VOM check from common terminal to NC (Nor-
mally Closed) - should read open.
If switch reads as above proceed to Step 4, otherwise re-
place control.
4. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown in
the following figures.
