TM 5-4520-240-14
tion and fuel pump power circuits are broken, stopping
(4) The glow plug G1 and the timer heater S7 re-
ignition and fuel flow.
main energized.
(2) Power is still available to thermocouple relay
(5) The combination of heat from the glow plug
(flame switch S3) to keep control relay K2 energized.
and spark from the igniter make possible fuel ignition.
Power is thus available to the relay to run blower B1
c. Run (Burner Lighted).
until all fuel in the burner is consumed. When the
(1) If ignition proceeds normally in the transition
flame extinguishes, the blower circulates air to cool the
stage, the heater lights, and heat from the burner is
detected by the thermocouple which actuates the ther-
heater.
mocouple relay (flame switch S3), closing the relay con-
tacts.
pollers cools the heat exchanger until the thermocouple
(2) When flame switch S3 closes, it energizes con-
reaches its cutout setting. The thermocouple relay con-
trol relay K2. When control relay K2 is energized, it dis-
tacts open, deenergizing control relay K2. This shuts
connects time delay heater S7 and bypasses the time
off power to the blower and prepares the heater for the
delay switch S8.
next startup.
(3) When energized, the control relay also sends
g. Overheat.
power to the ignition power supply, fuel pump, and
(1) If the heater overheats because of reduced ven-
blower motor. With the time delay circuit disconnected,
tilation air flow, excessive fuel flow, or other causes,
the glow plug and timer heater shut off and cool, but
overheat thermostat S5 opens. This breaks the flow of
the flame in the burner is sustained as long as fuel is
current to the fuel pump and carburetor solenoid valve,
supplied.
shutting off the flow of fuel.
d. Ignition Failure.
(2) Current still flows through control relay K2 to
(1) If the burner fails to light for any reason, the
operate the blower, cooling down the heat exchanger.
thermocouple will not actuate the thermocouple relay,
When the heat exchanger temperature reaches a safe
and the contacts will not close. Control relay K2 will
level, the overheat switch closes to restart fuel flow and
not energize to operate it in the run stage described in
ignition. Because the heat exchanger and burner head
subparagraph c, above.
are still quite warm, ignition is rapid and the glow plug
(2) In this situation, the glow plug and timer
and time delay assembly are not utilized.
heater will temporarily remain energized. After a total
4-26. Thermocouple.
of 60 to 70 seconds, heat from the timer heater will trip
RESET switch S2 to shut off all power to the heater to
a. Removal.
prevent damage to the glow plug or to the time delay
WARNING
mechanism.
The ignition system of this space
(3) If the heater runs out of fuel, or if the fuel flow
heater contains dangerous voltages
stops for any reason, the thermocouple and relay will
which can cause severe electrical
also act to shut off the heater when the heat exchanger
shock. Be sure to disconnect the power
cools.
plug before repairing the heater.
e. Thermostatic Control.
(1) Disconnect the power plug and remove the top
(1) When ON-OFF switch S1 is turned on, the ther-
and side cover panels and warm air louver to provide
mostat will operate the heater automatically to control
access to the thermocouple and its connection at the
room temperature. When the room temperature
thermocouple relay.
reaches the thermostat setting, the thermostat con-
(2) Disengage the grommet (10, fig. 4-13) from the
tacts open, starting the shutdown stage of the heater
hole in the heat exchanger housing. Unscrew the com-
cycle.
pression nut (12) on the thermocouple (13) and remove
(2) When the room temperature falls below the
the assembled compression sleeve (11), compression
thermostat setting, the thermostat contacts close, caus-
nut, and thermocouple (13) from the heat exchanger.
ing the heater to enter the startup stage of its cycle.
Slide the grommet (10) from the thermocouple. Do not
(3) The thermostat will continue to turn the heater
attempt to disassemble the compression sleeve, nut,
on and off to control room temperature as long as fuel
and thermocouple.
and power are available to the heater.
(3) Remove the connector bolt at the top of the
f. Shutdown.
thermocouple relay. Disconnect and remove the ther-
(1) When the operator turns ON-OFF switch S1 to
mocouple lead from the relay.
OFF, or when the thermostat contacts open, the igni-
4-20
