Piper APACHE PA-23-235 Service Manual page 1063

PIPER AZTEC SERVICE MANUAL
shut off and re-cycle if an overheat condition, due to restriction of ventilating
air, should occur since this condition will not normally affect the thermal fuse.
If an unsafe operating condition should occur, due to a lack of combustion
air through the heater while it is burning, the hot gases at the combustion air in-
let will quickly melt the fusible link of the thermal fuse assembly. This breaks
the circuit from the heater switch through terminal No. 4 of the terminal strip,
and is equivalent to turning off the heater switch. The heater will then shut down
in the normal manner described in the preceding paragraphs, and cannot be re-
started until the thermal fuse has been replaced, since the starting circuit will
remain open regardless of heater switch position. In the event of such a shut
down, the relay will be de-energized at the moment the overheat condition oc-
curs, and the safety valve and fuel solenoid shutoff valve will both close at the
same time without regard to flame detector switch position. The combustion air
blower and ventilating air blower will continue to run until the flame detector
switch cools and transfers.
13-27. ELECTRICAL SYSTEM (Model 940-DB and 940-K Series). The Models
listed above have electrical systems similar to the system of the Model 940-D,
except that an additional safety device, the lockout overheat switch, is connected
in series with the thermal fuse in the starting circuit, and a cycling temperature
limit switch is connected into the restriction solenoid circuit.
(Refer to Figure
13-11 .) The purpose of the lockout overheat switch is to prevent re-cycling the
heater after an overheat condition, and the purpose of the cycling limit switch is
to prevent needless operation of the lockout overheat switch.
These heaters start in the same manner described for the 940-D Series
heaters. Heater outlet temperature
will then be controlled by action of the ther-
mostat which opens and closes the restriction solenoid in the normal manner. If
the thermostat should fail in the closed position, or should fail to sense duct tem-
perature for any reason, the cycling switch will automatically limit heat output,
since it is connected into the thermostat circuit, and serves the same purpose.
This feature prevents heater shut down by the lockout overheat switch and con-
sequent loss of heat, due to a defective thermostat or duct system.
The lockout overheat switch protects the heater and the airplane from an
overheat condition caused by excessive fuel flow, or by a combinedfailure of ven-
tilating air and the cycling limit switch. The lockout overheat switch is set at a
higher temperature than the cycling limit switch, and like the thermal fuse, will
operate only under extreme conditions.
In the event of such an overheat condi-
tion, the heater will shut down the same as if the heater switch were turned off
and cannot be restarted until the overheat switch plunger has been reset.
HEATING AND VENTILATING SYSTEM
5C5
Reissued: 2/18/81
5C5