TM 55-1520-240-23-10
13-1
ENVIRONMENTAL SYSTEM
(Continued)
13-1
The switch is on the cockpit overhead panel.
The controller is in the heater compartment
on the side of the HEATER CONTROL
RELAY box at sta. 95. The thermostat is on
the left side of the cabin at sta. 335.
f.
Power is provided through the switch to the
controller and thermostat when relay 082K5
is energized (See step b). At the same
time, power passes through the controller
and thermal cycling switch 082A7 to open
the solenoid valve in fuel control 082A2,
permitting fuel to be delivered to the heater
as described in step c.
g.
The cabin thermostat consists of a column
of mercury surrounded by a heater winding.
Current passes through the variable resistor
within the temperature controller to heat
the winding. When the winding raises the
temperature of the mercury to 93ºF (34ºC),
it opens contacts within the temperature
controller relay. This removes power from
the solenoid valve within the fuel control,
shutting off fuel to the heater. It also removes
power from the heater winding, allowing the
mercury column to cool and contract. When
this happens, the relay contacts open, power
is restored to the fuel solenoid valve, and the
heater starts.
h.
Protection for the heater is provided by
three thermal switches at the base of the
heater. Cycling switch 082A7 interrupts the
circuit to the fuel control solenoid valve when
temperature of the air leaving the heater
goes over 250ºF (121ºC). Overheat switch
08285 is a backup switch that functions if the
cycling switch fails. It opens to deenergize
relay 082K3 whenever air in the transition
assembly below the heater exceeds 350ºF
(177ºC). This shuts off the heater, but lets the
fan continue to operate. At the same time,
the HEATER HOT caution light on the master
caution panel comes on. When this happens,
the heater cannot be operated again until
the transition assembly has cooled below
250ºF (121ºC). At that time, the heater can
be started again by pressing the HEATER
START button. A fan overrun switch 08286
keeps the fan operating after the heater has
been shut down until the temperature within
the heater drops below 120ºF (49ºC). If the
temperature rises to 120ºF (49ºC) again, the
overrun switch will close and the fan will come
on again. This cycling will continue until the
temperature stays below 120ºF (49ºC).
i.
Additional heater protection is provided by
differential air pressure switch 082A4, on the
sta. 95 bulkhead. This switch shuts down
the fuel and ignition systems by deenergizing
relay 082K5 if air pressure through the fan
drops below 1.25 to 1.75 inches of water.
j.
When the switch on the cockpit overhead
panel is set to VENT FAN ONLY, only those
contacts within the switch that energize the
heater fan are closed. The blower then forces
unheated ventilating air through the ducting.
13-6
