TM 5-4520-256-14
Section III. TECHNICAL PRINCIPLES OF OPERATION
Para.
Para.
General .............................................................. 1-1
Fuel System ....................................................... 1-13
Electrical System ............................................... 1-12
Airflow System ................................................... 1-14
1-11. GENERAL. This section provides the theory of operation and a functional description of components comprising
the heater. Three systems generally 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. (Refer to Figure 1-6).
The electrical system provides for two modes of operation: the heating mode and the ventilating mode. A functional
description of these modes is described in the following paragraphs.
a.
Ventilation Mode. The ventilation mode is initiated when mode selector switch S 1 is set to vent. Initially, 120-
vac, 50/60 Hertz, single phase power is applied through main power plug P 1 and circuit breaker CB 1 to open contacts
of mode selector switch S1, which power the motor.
b.
Heating Mode. The heating mode is initiated when mode selector switch S 1 is set to heat and the K8 reset
button is set. Initially, 120-vac power is applied through main power plug P and circuit breaker CB 1 to mode selector
switch S 1, which powers the motor B 1. Continued operation of the heating mode is dependent upon combustion control
relay K8 and its associated controls. In addition to the 120-vac power input (black lead) and the neutral (white lead), two
input circuits and two output circuits are provided by relay combustion control K8. One input, namely the flame detector
circuit, includes the safety devices. The other input, namely the thermostat circuit, includes the temperature control
devices and the control relay contacts. Both the flame detector and thermostat circuits must be completed to provide an
output for the fuel solenoid. Functional descriptions of these circuits are contained in the paragraphs that follow.
(1) Flame Detector. Components comprising the circuit are temperature limit switch S2 and flame detector D 1.
After depressing K8 reset button the flame detector 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 K8. When ignition occurs, flame detector D1 will
complete the flame detector circuit. Should the flame detector circuit open due to an overheat condition in
the heat exchanger, or a flameout, power will be removed from solenoid valve L1, safety release light DS 1
will illuminate, and audible alarm EMI will sound.
1-10 Change 3
