TM 5-4520-241-14
c. Running.
(1) When the flame switch is in the running position described above, the heater is in the normal running state.
Power to the fuel circuit is controlled by the room thermostat, and power to both motors and the ignition transformer is
obtained through the flame switch.
(2) In this condition, the room thermostat starts and stops fuel flow to the burner to control the temperature in the
enclosure. When the air temperature in the enclosure reaches the temperature set on the thermostat, the thermostat
contacts open, deenergizing the carburetor solenoid valve L1 and the external fuel pump. The combustion motor,
ventilating motor, and ignition transformer remain energized through the flame switch.
(3) When the air temperature in the enclosure drops below the thermostat setting, the thermostat contacts close,
energizing the fuel circuit, and combustion resumes. The heater will normally continue to cycle under control of the room
thermostat as long as fuel and power are available.
(4) During this cycling, if the fuel circuit remains deenergized long enough for the burner to cool
d. Shutdown
(1) When the HEATER-OFF-FAN switch S2 is moved to OFF, the fuel circuit is immediately deenergized,
stopping fuel flow to the burner head. As noted in paragraph b., the flame switch continues to supply power to both
blower motors and to the ignition transformer.
(2) The continuing spark and combustion air flow ensure the combustion of any residual fuel, and ventilating air
flow scavenges heat from the heat exchanger. When the temperature of the heat exchanger falls below the flame switch
setting, the flame switch trips, deenergizing both motors and the ignition transformer.
e. Overheat .
(1) If the heater overheats because of a reduced supply of ventilating air, excessive fuel flow or other cause, one
or both overheat thermostats (S5 and S6) will function.
(2) Both thermostats are designed to open at 270F (132C). When the thermostats open, the solenoid valve
and fuel pump are deenergized and fuel flow stops.
(3) The ventilating motor and combustion motor will continue to run to cool the heater. When the overheat
thermostat which opened senses a temperature of 245F (118 C), it will close, and fuel will again be available to continue
normal running.
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