DESCRIPTION AND THEORY OF OPERATION
ARMY SPACE HEATER H-140
THEORY OF OPERATION
The electrical system provides for three modes of operation: ventilation mode, automatic heating mode, and manual
heating mode. Refer to Figure FO-1, ASH Electrical System Schematic Diagram.
Initially, 120 Vac, 50/60 Hz, single-phase power is applied through main power plug P1 and main POWER CIRCUIT
BREAKER CB1 to the contacts of MODE SWITCH S1.
When MODE SWITCH S1 is placed in the VENT position, POWER indicator DS1 will light. Ventilation air fan motor
B1 will also begin operating. When ventilation air fan motor B1 is operating, a positive pressure is built up inside the
ASH enclosure which closes air pressure switch S5. Air pressure switch S5 must close to allow the combustion air fan
motor B2 to operate in the heating modes. Combustion air fan motor B2 does not operate in the ventilation mode.
Power is also available at fuel PURGE SWITCH S4 and is used to purge air from the fuel system. Fuel PURGE
SWITCH S4 is spring-loaded to the off position; holding it to the on position operates three-way purge solenoid valve
L2. Whenever fuel PURGE SWITCH S4 is held in the up position, fuel is drawn from the fuel source and filtered and
pumped back to the fuel tank through fuel transfer valve V1 and purge solenoid valve L2.
The heating modes are initiated when MODE SWITCH S1 is set to HEAT MANUAL MODE or HEAT AUTO MODE.
Continued operation in the heating mode is dependent upon the combustion control relay K1, its associated controls, and
the setting of the remote control box. In addition to the 120 Vac, 50/60 Hz power and neutral inputs, the combustion
control relay K1 provides two input circuits and two output circuits. One of these inputs, the flame detector circuit,
includes the safety devices. The other input, the control box control circuit, includes the temperature control devices and
the control relay contacts. Both the flame detector and thermostat circuits must be completed to activate two-way fuel
solenoid valve L1.
Flame Detector Circuit
Components comprising the flame detector circuit are overheat thermostat S2 and flame detector Dl.
After positioning FLAME RESET switch S7 to the up position (spring return down), the flame detector circuit will be
overridden for approximately 10 to 13 seconds. During this time if the temperature control circuits are closed, power will
be applied to fuel solenoid valve L1 through the combustion control relay K1. When ignition occurs, flame detector D1
will complete the flame detector circuit. If the flame detector circuit opens due to an overheat condition in the heat
exchanger or a flameout, power will be removed from fuel solenoid valve L1 and FLAME OUT indicator DS3 will light.
Temperature Control Circuits
Components comprising the temperature control circuits are discharge air thermostat S3, overheat thermostat S2, and
remote thermostat S6.