Service manual

SERVICING
48
Manometer Hose
O
F
F
ON
M
P
C
1
3
2
WR
INLET
OUTLET
Pressure Regulator Adjustment
(Under Cap Screw)
Inlet Pressure Tap
(Side of Valve)
Outlet (Manifold)Pressure Tap
(Side of Valve)
Gas Valve Control
ON/Off Switch
Manometer
Open to
Atmosphere
MEASURING MANIFOLD GAS PRESSURE
With Power ON:
WARNING
LINE VOLTAGE NOW PRESENT.
3. Put furnace into heating cycle and turn on all other gas
consuming appliances.
For NATURAL GAS:
a. Inlet pressure should be a nominal 7" w.c.
b. Manifold pressure should be 3.5 ± .3"w.c.
(Canadian - Sea Level 4.2" ± .3" w.c.)
For PROPANE GAS:
a. Inlet pressure should be a nominal 11" w.c.
b. Manifold pressure should be a nominal 10" w.c.
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 lo-
cating 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 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.
2. Check burner for proper alignment and/or replace
burner.
3. Improper orifice size - check orifice for obstruction.
S-310 CHECKING PRESSURE CONTROL
A pressure control device is used to measure negative pres-
sure at the induced draft blower motor inlet to detect a par-
tial or blocked flue.
Pressure Switch Operation (Honeywell Smart Valve
System)
The pressure switch is ignored unless there is a call for
heat. When the control receives a call for heat, power for
the smart valve is routed through the pressure switch (wires
VT-18 - YL-11). The valve is not energized until the pres-
sure switch is closed. If at any time the pressures opens
power to the gas valve is removed. The inducer motor will
continue to run as long as there is a call for heat at the
thermostat.
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 con-
trol sees that the pressure switch is closed before the in-
duced draft blower is energized, the LED will flash a code
of "3" (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
"2" 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 in-
ducer stays on until the pressure switch re-closes. Then
the control makes another ignition attempt.
WARNING
Disconnect Electrical Power Supply:
1. Remove wires from the electrical terminals.
2. Using a VOM check from Common to NO (Normally
Open) - should read open.