88
Rev. 2
SERVICING
WARNING
Disconnect Electrical Power and Shut Off Gas Supply:
8. Disconnect manometer after turning off gas at manual
shutoff valve. Reinstall gas valve outlet pressure tap plug
before turning on gas to furnace.
9. Turn on gas to furnace and check for leaks. If leaks are
found, repair and then reinstall burner compartment door.
10. 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.
For natural gas, the manifold pressure must be between 3.0
and 3.6 inches water column (3.5 nominal). The low stage
manifold pressure must be between 1.6 and 2.2 inches wa-
ter column (1.9 nominal).
Propane Gas Adjustments
WARNING
Disconnect Electrical Power and Shut Off Gas Supply:
1. After turning off gas to furnace at the manual gas shutoff
valve external to the furnace, remove burner compart-
ment door to gain access to the gas valve.
2. Connect a calibrated water manometer (or appropriate
gas pressure gauge) at the gas valve outlet pressure
tap. Refer to previous
Measuring Manifold Gas Pressure
figure for proper connection.
WARNING
LINE VOLTAGE NOW PRESENT.
3. Turn ON the gas and electrical power supply and oper-
ate the furnace.
4. Adjust the high stage regulator to required manifold pres-
sure setting.
5. Reinstall high stage cap screw and recheck manifold
pressure setting with cap on.
6. Adjust low stage regulator to required manifold pressure
setting.
7. Reinstall low stage cap screw and recheck manifold pres-
sure setting with cap on.
WARNING
Disconnect Electrical Power and Shut Off Gas Supply:
8. Disconnect manometer after turning off gas at manual
shutoff valve. Reinstall gas valve outlet pressure tap plug
before turning on gas to furnace.
9. Turn on gas to furnace and check for leaks. If leaks are
found, repair and then reinstall burner compartment door.
10. 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.
For propane gas, the manifold pressure must be between
9.7 and 10.3 inches water column (10.0 nominal).The low
stage manifold pressure must be between 5.7 and 6.3 inches
water column (6.0 nominal).
S-19
CHECKING FOR DELAYED IGNITION
Delayed ignition is a delay in lighting a combustible mixture
of gas and air which has accumulated in the combustion
chamber.
When the mixture does ignite, it may explode and/or rollout
causing burning in the burner venturi.
If delayed ignition should occur, the following should be
checked:
1. Improper gas pressure - adjust to proper pressure (See
S-18 CHECKING GAS PRESSURE).
2. Improper burner positioning - burners should be in locat-
ing slots, level front to rear and left to right.
3. Carry over (lighter tube or cross lighter) obstructed - clean.
4. Main burner orifice(s) deformed, or out of alignment to
burner - replace.
S-20
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-18 CHECKING GAS PRESSURE)..
2. Check burner for proper alignment and/or replace burner.
3. Improper orifice size - check orifice for obstruction.
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