TM 9-4520-258-14
Section III. PRINCIPLES OF OPERATION
1-11. GENERAL.
This section provides the principles of operation and functional description of the components comprising the ASH Unit.
Three systems comprise the heater: the electrical system, the fuel system, and the airflow system. These systems are
described in the paragraphs that follow.
1-12. ELECTRICAL SYSTEM.
The electrical system (refer to FO-1, ASH Electrical Schematic) provides for three modes of operation: the ventilation
mode, the auto heating mode, and the manual heating mode. A functional description of these modes are described in
the following paragraphs.
a.
Ventilation Mode. Initially, 120 vac, 50/60 Hz, single-phase power is applied through the main power
plug (P1) and the POWER circuit breaker (CB1) to the contacts of the mode selector switch (S1).
(1)
When the MODE SWITCH (S1) is placed in the VENT position, the POWER light (DS1) will illuminate.
The ventilation fan motor (B1) and the ignition transformer (TR1) also begin operating. Once the
ventilation fan motor (B1) is operating, a position pressure is built up inside the enclosure which closes
the air pressure switch (S5). The air pressure switch (S5) must close to allow the combustor air fan
motor (B2) to operate in the HEAT modes (B2 does not operate in the VENT mode).
(2)
Power is also available at the PURGE SWITCH (S4). The air purge switch (S4) is used to purge air
from the fuel system. The (S4) is spring loaded to the OFF position, holding it to the ON position
operates the 3-way purge valve solenoid (L2).
b.
Heating Mode. The heating mode is initiated when MODE SWITCH (S1) is set to AUTO HEAT or MANUAL
HEAT. Continued operation of the heating mode is dependent upon combustion control relay (K1), its
associated controls, and the setting of the remote thermostat. In addition to the 120-vac 50/60 Hz power
input (black lead) and the neutral (white lead), two input circuits and two output circuits are provided by relay
combustion control (K1). One input, the flame sensor circuit, includes the safety devices. The other input,
the thermostat circuit, includes the temperature control devices and the control relay contacts. Both the
flame sensor and thermostat circuits must be completed to provide an output for the two-way fuel solenoid
valve (L1). Functional descriptions of these circuits are contained in the paragraphs that follow:
(1)
Flame Sensor. Components comprising the circuit are temperature limit switch (S2) and flame sensor
(DL).
(a)
After depressing FLAME RESET button (F7), the flame sensor circuit will be overridden for
approximately 10 to 13 seconds. During this time, if the temperature controller thermostat
circuits are closed, power will be applied to fuel solenoid valve (L1) through combustion control
relay (K1). When ignition occurs, flame sensor (D1) will complete the flame detector circuit.
(b)
Should the flame sensor circuit open due to an overheat condition in the heat exchanger, a
flameout, or lack of fuel, power will be removed from solenoid valve (L1), and safety flameout
(DS3) will illuminate.
1-13
