56
SERVICING
1. Remove burner compartment door to gain access to the
ignitor.
2. Ignitor cool - approximately 70 - 77°F.
3. Disconnect the ignitor from the Ignition Control.
4. Using an ohmmeter measure the resistance of the igni-
tor:
Mini Igniter : Carbide Mini Igniter should read between
30 to 300 ohms.
5. Reconnect ignitor.
L
INE VOLTAGE NOW PRESENT
WARNING
6. Place unit in heating cycle, measure current draw of ig-
nitor during preheat cycle.
Models using 120V Mini Igniter :
Should read approximately 1 amp maximum. The steady
state current at 120V is a nominal of .7 plus or minus .3
amps.
7. After checking and/or replacing of hot surface ignitor,
reinstall burner compartment door and verify proper unit
operation.
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 CHECKING GAS PRESSURE
)..
2. Check burner for proper alignment and/or replace burner.
3. Improper orifice size - check orifice for obstruction.
S-310 CHECKING PRESSURE SWITCH
The pressure switch a safety device to prevent the combus-
tion cycle from occurring with inadequate venting caused by
a restricted or blocked vent pipe on the 80% and 90% fur-
naces.
1. Remove burner compartment door to gain access to
pressure switch(es).
2. Remove wires from the pressure switch(es) electrical
terminals.
3. Using a VOM check from common terminal to NC (Nor-
mally Closed) - should read open.
If switch reads as above proceed to Step 4, otherwise re-
place control.
4. Remove the pressure control hose from the control and
interconnect with an inclined manometer as shown in
the following figures.
Induced Draft Blower
Pressure Switch
Pressure Switch
Hose
1/4" Tee
Hose to Induced
Draft Blower Tap
Inclined Manometer
ID Blower Pressure Switch
Negative Pressure Measurement
(80% Upflow Single-Stage Furnace Shown,
Counterflow Similar)
S-311 HIGH ALTITUDE APPLICATION (USA)
Check your furnace rating plate to determine its rated alti-
tude (either 4500 or 5500 ft). Do not attempt to increase
the firing rate by changing orifices or increasing the mani-
fold pressure below 4500/5500 feet. This can causepoor
combustion and equipment failure. High altitude installa-
tions above 4500/5500 feet may require both a pressure
switch and an orifice change. These changes are neces-
sary to compensate for the natural reduction in the density
of both the gasfuel and the combustion air at higher alti-
tude.
