NORMAL
OPERATING
SEQUENCE
201842
1. When the room temperature drops below the thermostat setting, the thermostat
calls for heat. A 24-volt circuit is completed through the time delay relay coil.
After three to five seconds, the contacts in the relay close. The blowers come on.
2. One motor drives both the combustion air blower and the circulating room air
blower. The combustion air blower draws air from outside through the intake
air vent. This air discharges into the combustion chamber. The combustion air
blower then forces the combustion products out of the exhaust tube(s). The
Combustion air blower is sealed so that air does not pass between it and the
circulating room air blower.
3. When the blower motor reaches 75% of normal r.p.m. (three to five seconds),
air from the blower turns on the microswitch. When the microswiteh activates,
the electrical circuit is completed through the limit switch to the control
module. The glo bar begins to glow. In about 15 seconds, the glo bar becomes
hot enough to ignite the burner.
4. Another 24-volt circuit is completed through the module to the main gas valve.
The valve opens. Gas passes through the valve to the main burner. The gas is
ignited by the glo bar.
5. After the main burner ignites, the flame detector senses the presence of flame.
This opens the circuit to the glo bar. The glo bar shuts down.
6. The circulating room air blower pulls return air in from the living area. This air
is heated as it flows past the combustion chamber. The heated air is then blown
out into the room.
7. When the room temperature rises above the thermostat setting, the circuit to
the valve opens and the burner shuts off.
8. The blower continues to run about two to three minutes until the time delay
relay coil cools. When the coil cools, the circuit to the motor opens. The
blower shuts off.
Limit Switch
I "'°° I
Oo not operate furnace without a properly-working limit switch.
The limit switch will turn off the gas to the main burner if the furnace becomes too
hot. when this happens, the main burner will go out and the blower will continue to
run for two to three minutes. This usually happens when circulating air is partially
or completely blocked.
Occasional cycling of the limit switch is not bad. This indicates safe operation.
Limit switch cycling will most likely occur on warm days. If this happens too often
or for a long period of time, the circulating air system should be thoroughly cleaned
by a qualified service person.
If the circulating air system is cleaned and the limit switch continues to shut down
the furnace, the limit switch is defective. A defective limit switch cannot be re-
paired. It must be replaced. For safety reasons, never operate the furnace without a
limit switch.
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