SERVICING
61
Gas Line
Gas Shutoff Valve
Gas Line
To Furnace
Drip Leg Cap
With Fitting
Manometer Hose
Manometer
Open To
Atmospere
MEASURING INLET GAS PRESSURE
2. Remove the pressure tap fitting at the manifold if provided
or from the gas valve and install fitting to connect another
manometer or gauge.
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
tmosphere
MEASURING MANIFOLD GAS PRESSURE
With Power ON:
WARNING
L
INE 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 pres-
sure 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.
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 partial
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 pressure 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 control sees
that the pressure switch is closed before the induced draft
blower is energized, the LED will flash a code of "3" (to indi-
cate the pressure switch is stuck closed) and the inducer
will remain off until the pressure switch opens.
Continued on page 66
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