66389-BIM-C-1208
Johnson Controls Unitary Products
51
On calls for heating, the supply air blower operates but the
draft motor does not (the room thermostat fan switch is in the
“AUTO” position).
1.
The draft motor has inherent protection. If the motor
shell is hot to the touch, wait for the internal overload to
reset.
2.
If the motor shell is cold with the room thermostat calling
for heat, check for line voltage at the motor leads. If line
voltage is present, replace the draft motor.
3.
If line voltage is not present, check for line voltage on the
ignition control at the “inducer” terminal draft motor relay
(DMR or DMC) contacts in the main control box and
check to see if the (DMR or DMC) is pulled in.
The draft motor runs but the furnace does not light and the
spark ignitor does not spark.
1.
Check for 24 volts at the spark ignitor from the ignition
control board (ICB). Check the 24-volt wiring from the
ICB to the spark ignitor. Check for 24 volts at the ICB
spark ignitor terminal.
2.
Check the ground wiring for the ICB and the gas valve is
intact and making good electrical connection. Check the
ceramic insulator on the spark ignitor for breaks or
cracks. Replace the spark ignitor if damaged.
3.
With the draft motor running, check for 24 volts at the
pressure switch terminal on the ICB. If not present,
check for 24 volts on the terminal from the pressure
switch. If present, go to step 4. If 24 volts is not present,
the either pressure or rollout switch is not closed. Or the
draft motor is not sufficiently evacuating the heat
exchanger tubes or the pressure switch has failed.
Check the operation of the pressure switch. Check the
line voltage to the unit; if line voltage is low, call the local
power company. If the problem persists, the draft motor
may need replacement.
4.
If the furnace is hot, it may be out on a high temperature
limit open; wait for limit reset.
5.
If all are intact replace the ICB.
The draft motor runs and the spark ignitor sparks at the
burner, but the burner does not ignite and a gas odor is not
detected at the draft motor outlet.
1.
Check to ensure gas is being supplied to the unit. Con-
firm that the gas pressure to the unit is within the proper
limits as described in the “POST START CHECKLIST”.
2.
Check the voltage at the gas valve and at the gas valve
terminals on the ICB. Check all wiring between the ICB
and the gas valve. Check to make sure the ground con-
nections are intact.
3.
If 24 volts is present, remove the pilot burner and the ori-
fice. The removal procedure is described in
“BURNER/ORIFICE INSTRUCTIONS.” Inspect the ori-
fice for obstruction. If it is clear, replace the gas valve.
Main burners light but exhibit erratic flame characteristics.
4.
Check the main burner orifices for obstruction and align-
ment. The removal procedure is described in
“BURNER/ORIFICE INSTRUCTIONS.” Clean or replace
burner orifices and burners as needed.
