TM 5-4520-241-14
e. Fuel Pump Power Supply. The fuel pump power supply rectifies the 120 VAC lines current to 24 VDC to operate
both the solenoid and the external fuel pump.
4-35.
DESCRIPTION OF ELECTRICAL SYSTEM OPERATION.
a. Startup.
(1) One hundred twenty VAC power is applied to the heater through the POWER RECEPTACLE, through circuit
breaker CB1, and through normally closed reset switch S1. See figure 4-25.
(2) As soon as HEATER-OFF-FAN switch S2 is moved to HEATER, white indicator DS2 is illuminated and power
is applied to terminal A of ROOM THERMO receptacle J2, and through fuel thermostat S7 to the cartridge heater HR1 in
the carburetor body. If the ambient temperature is less than approximately 40F (4C), the fuel heater will be energized to
preheat the carburetor body and hence the fuel flowing through it.
(3) As long as the temperature of the enclosure is less than the temperature set on the room thermostat, the
thermostat contacts will be closed, applying 120 volts to the fuel circuit and to the flame switch. Overheat thermostats S5
and S6 are normally closed as long as the temperature of the heat exchanger remains within safe limits, and 24 VDC
power from the fuel pump power supply (transformer T2 and rectifier CR1) is applied to carburetor solenoid valve L1 and
through EXTERNAL FUEL PUMP RECEPTACLE J4 to the external fuel pump (not supplied with the heater).
(4) Before the burner ignites, the flame switch S3 is cool, and is in its normally closed position. The normally
closed contacts apply 120 volts power from the room thermostat to combustion motor B2 (through suppression capacitors
C1 and C2 and thermal cutout S4) and to ignition transformer T1, which supplies high voltage for spark at igniter E1 in the
burner. Normally closed flame switch contacts also apply 120 volts to time delay transformer T3. The output of this
transformer is rectified by CR2, and the resulting DC voltage is applied to the bimetal heater/sensor HR in the time delay
assembly to time the ignition sequence.
b. Transition.
(1) The electrical operation described above ensures that fuel, combustion air, and spark are available in the
burner. Under most conditions, the heater will ignite normally within 60 to 90 seconds, and the flame switch S3 will
transfer from its normally closed to its normally open contacts.
(2) The time delay assembly times this ignition sequence, and will shut down the heater if the burner fails to ignite
within the allotted time. Bimetal heater/sensor HR2 begins to heat as soon as S2 is moved to HEATER (as described
above). Reset switch S1 is thermally operated by the heat produced by HR2, so that if HR2 continues to heat, S1 will
open to disconnect power from the operating circuits, and to cause red indicator DS1 to light.
4-66
