54
SERVICING
e. Remove the 1/8" NPT hose barb fitting from the outlet
pressure tap. Replace the outlet pressure boss plug
and seal with a high quality thread sealer.
WARNING
HIGH
VOLTAGE
D
ISCONNECT
ALL
ELECTRICAL POWER AND SHUT OFF GAS SUPPLY
BEFORE SERVICING OR INSTALLING.
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 pres-
sure 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
HIGH
VOLTAGE
D
ISCONNECT
ALL
ELECTRICAL POWER AND SHUT OFF GAS
SUPPLY 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.
WARNING
HIGH
VOLTAGE
D
ISCONNECT
ALL
ELECTRICAL POWER AND SHUT OFF GAS SUPPLY
BEFORE SERVICING OR INSTALLING.
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 -
*MEC96 furnaces use a 120V
silicon nitride igniter for ignition. The normal op
erating tem-
perature is approximately 2156°F - 2678°F.
At room tem-
perature the igniter ohm reading should be from 37-68 ohms.
7. Place unit in heating cycle, measure current draw of ig-
nitor 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, re-
install 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 (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. In addition to the in-
ducer pressure switch, this furnace has a “front cover pres-
sure switch” wired in series with the gas valve. This pressure
switch keeps the gas valve from opening in the event of con-
densate backing up in the secondary heat exchanger. This
could occur from improperly connected drains or a plugged
drain tube.
1. Remove burner compartment door to gain access to pres-
sure switch(es).
2. Remove wires from the pressure switch(es) electrical ter-
minals.
3. Remove the pressure control hose from the control and
interconnect with an inclined 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.
