SERVICING
57
If operating pressures differ from above, make necessary
pressure regulator adjustments, check piping size, etc., and/
or consult with local utility.
S-308 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-307)
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-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
.
2. Check burner for proper alignment and/or replace burner.
33. Improper orifice size - check orifice for obstruction.
S-310 CHECKING PRESSURE CONTROL
A pressure control device is used to measure negative
pressure at the induced draft blower motor inlet to detect a
partial or blocked flue.
Pressure Switch Operation (DSI Direct Spark System)
The pressure switch is ignored unless there is a call for heat.
When the control receives a call for heat, the control checks
to see that the pressure switch is open. If the control sees
that the pressure switch is closed before the induced draft
blower is energized, the LED will flash a code of "2" (to
indicate the pressure switch is stuck closed) and the inducer
will remain off until the pressure switch opens.
If the pressure switch opens before the ignition period, the
induced draft blower will remain on and the control will stay
in pre-purge until the pressure switch is closed for an entire
15 second pre-purge period. The LED will flash a code of "3"
to indicate open pressure switch.
If the pressure switch opens after the gas valve has been
energized, the control will de-energize the gas valve and run
the indoor blower through the heat off delay. The inducer
stays on until the pressure switch re-closes. Then the
control makes another ignition attempt.
HIGH VOLTAGE!
Disconnect ALL power before servicing
or installing this unit. Multiple power
sources may be present. Failure to do so
may cause property damage, personal injury
or death.
1. Remove wires from the electrical terminals.
2. Using a VOM check from Common to NO (Normally
Open) - should read open.
If switch reads as noted proceed to Step 3, otherwise
replace control.
3. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown:
PRESSURE SWITCH
INCLINED
MANOMETER
1/4" COPPER TEE
HOSE
TO J-TUBE
Reconnect wires to the Common and NO terminals.
With Power ON:
WARNING
Line Voltage now present.
4. Energize furnace for heating cycle. The induced draft
blower motor will begin to run. The inclined manometer
should read approximately -1.2" ± 0.3" W.C with no
combustion.
5. Remove and check the two electrical wires and using the
VOM check from Common to NO (Normally Open), it
should read closed (with I.D. motor running). If not as
above, replace pressure control.
6. Reconnect all wires to the control and place in heating
cycle.
7. As the unit fires on high stage, the inclined manometer
negative pressure will drop to -1.0" ± 0.3" W.C.
