TM 5-4520-253-13
1-12. ELECTRICAL SEQUENCE OF OPERATION
FUNCTIONS. The heater has five functions: (1) START, (2) RUN, (3) PURGE, (4) FAULT, and
(5) FAN.
START. With the heater connected to a 120 volt power source, potential is applied through circuit
breaker CB1, the line side of HEATER-OFF-FAN switch S1, and flame switch FS1. Circuit breaker CB1
protects the heater circuitry from direct shorts or overloads.
RUN. When HEATER-OFF-FAN switch is turned to the HEAT position, three parallel circuits are closed.
One circuit powers white HEAT light DS2. One circuit powers carburetor heater HR1 through carburetor
thermostat-S2. This circuit is completed when the temperature is below 400° F. The third circuit sends
power through connector J2 to the room thermostat. When the thermostat contacts are closed, calling
for heat, power passes back through connector J2 to circuit breaker CB2. When pressed, circuit breaker
CB2 allows current to flow to three circuits necessary for combustion: air, ignition, and fuel.
COMBUSTION. Combustion blower B2 supplies air needed for burning fuel. Combustion blower B2 is
electrically suppressed by two feed-through capacitors C1 and C2. Combustion blower B2 also in-
corporates normally closed thermal overload switch S5.
IGNITION. Ignition transformer T1 supplies the spark necessary to ignite the fuel for burning.
Transformer T1 increases the applied 120 volts to 6000 volts across igniter plug E1, which is much like
an automobile spark plug.
FUEL. The power from circuit breaker CB2 passes through two overheat switches S3 and S4 before
going to stepdown transformer T2 which decreases 120 volts to 24 volts. The 24 volts alternating cur-
rent is converted to 24 volts direct current by bridge rectifier CR1. The 24 volts direct current powers
fuel solenoid L1 and fuel pump L2, used to supply fuel for combustion.
HEAT. If all three items (spark, fuel, and combustion air) are present in their proper amounts, com-
bustion takes place in the heat exchanger, producing heat. If heat is sensed by flame switch FS1 within
a preset time after main HEATER-OF-FAN switch S1 is closed, the heater will continue to operate
and ventilating air motor B1 will operate.
FLAME SWITCH. Flame switch (FS1 ) is a normally open (NO), normally closed (NC) thermal switch. The NC
contacts complete the circuit for the time delay through the windings of the vent motor (B1). When the heat
exchanger reaches approximately 125°F the FS1 contacts reverse. The FS1 reversing action opens the circuit to
the time delay and activates the B1 motor to circulate warm air.
PURGE. When the contacts of room thermostat S7 are opened or HEATER-OFF-FAN switch S1 is turned to the
OFF position when the heater is hot, the unit will go into its purge cycle. Potential is applied through circuit
breaker CB1 to the normally open contacts of flame switch FS1, now closed because of the hot heat exchanger, to
ventilating air motor B1. Ventilating air motor B1 will stay in the circuit until the heat exchanger cools
enough to allow the contacts of flame switch FS1 to open.
FAN. When HEATER-OFF-FAN switch S1 is placed in the FAN position, ventilating air motor B1 will be started
with the aid of start capacitor C3. Capacitor C3 is disconnected by the RPM increase of ventilating air motor B1.
Ventilating air motor B1 is protected by overload switch S6, connected to the windings.
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