Service Manual

SERVICING
55
1. Connect a water manometer or adequate gauge to the
inlet pressure fitting of the gas valve.
2. Remove the pressure tap fitting at the manifold if provided
or check at the gas valve outlet fitting and connect another
manometer or gauge.
Outlet
Inlet
Outlet Pressure
Tap
Inlet Pressure
Tap
Gas Valv
e
On/Off
Switch
Manometer
Open to
Atmosphere
Manometer
Hose
Open to
Atmosphere
Manometer
Hose
High Pressure
Adjust
Low Pressure
Adjust
WHITE ROGERS MODEL 36G 2-STAGE GAS VALVE
MEASURING INLET AND 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. (2 stage heat models only) Manifold pressure on
low stage should be 2.0” w.c. ± .3"w.c.
c. Manifold pressure for 1 stage heat models and
high stage for 2 stage heat models should be 3.5”
± .3"w.c.
For PROPANE GAS:
a. Inlet pressure should be a nominal 11" w.c.
b. (2 stage heat models only) Manifold pressure on
low stage should be 6" w.c.
c. Manifold pressure for 1 stage heat models and
high stage for 2 stage heat models should be 10”
w.c.
Manifold Gas Pressure
Natural Gas 3.5" w.c.
Propane Gas 10.0" w.c.
Single Stage
Range Nom ina l
Low Stage
1.7
- 2.3" w.c. 2.0" w.c.
High Stage 3.2 - 3.8" w.c. 3.5" w.c.
Low Stage 5.7 - 6.3" w.c. 6.0" w.c.
High Stage 9.7 - 10.3" w.c. 10.0" w.c.
Manifold Gas Pressure
Ga s
Propane
Natural
Two Stage
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.