SERVICING
53
V O L T / O H M
M E T E R
R E D
R E S E T
B U T T O N
C O L O R
ID E N T IF Y IN G
T A B
TESTING AUXILIARY LIMIT CONTROL
S-302 CHECKING FLAME ROLLOUT SWITCH
APG/GPG units are equipped with a temperature-activated
manual reset control. This control is mounted to the manifold
assembly and is wired in series with the auxiliary limit and
gas valve. The control is designed to open should a flame roll
out occur. An over firing condition or flame impingement on
the heat shield can also cause the control to open.
If the rollout control has opened, the circuit between the
ignition control and gas valve will be interrupted and the
ignition control module will go into lockout. The servicer
should reset the ignition control by opening and closing the
thermostat circuit. The servicer should look for the ignitor
glowing which indicates there is power to the ignition control.
The servicer should measure the voltage between each side
of the rollout control and ground while the ignition control is
try to power the gas valve.
VO LT / O H M
M ETER
R E D
R E S E T
B U T TO N
C O L O R
ID EN TIFYIN G
TA B
CHECKING FLAME ROLLOUT SWITCH
Limit Switch Operation (Applies to Primary, Auxiliary,
and Roll Out Limits) DSI systems.
If a limit switch opens, the indoor blower is energized on heat
speed and the induced draft blower is energized. The LED
on the control flashes "4" to indicate an open limit switch.
The blower and inducer remain on while the limit switch is
open. The gas valve is de-energized. Power to the thermo-
stat "R" is removed while the limit switch is open.
When the limit switch re-closes, the induced draft motor runs
through its post purge and the indoor blower goes through the
heat off delay.
If a call for heat exists when the limit switch re-closes, the
control goes through a pre-purge period and then makes an
ignition attempt. The indoor blower remains on (for the delay
off time) during the re-ignition attempt.
1. If no voltage is measured on either side of control it
indicates ignition control or wiring to control problem.
2. If voltage is measured on one side of the control and
not the other, it indicates the control is open.
3. If voltage is measured on both sides of the control the
wiring to gas valve or valve is at fault.
Servicing procedure with furnace not firing.
1. Confirm that the outer door was in place and all screws
tightened. (No leaks under the door.)
2. Check to see if any damage was done to the furnace
especially the wiring.
3. Confirm that heat exchanger is not obstructed by feeling
for discharge air from the flue hood when the combustion
blower is running but the unit is not firing.
If the above steps do not suggest the reason the control has
tripped the furnace should be fired.
1. Remove the heating compartment door.
2. Turn of the power or open the thermostat circuit.
3. Reset the rollout control.
4. Turn power on and put the unit into a call for heating.
CAUTION
Flame rollout could occur. Keep face and hands a
safe distance from burner area.
5.
Look under the heat shield as the unit is running.
Flames should be drawn into firing tubes.
a.
If only one burners flame is not drawn into the tube,
that tube is restricted.
b.
If, without the air circulation blower running, all
flames are not drawn into the tubes either the
collector box, combustion blower, or flue outlet is
obstructed. If the combustion blower or flue outlet
is obstructed, the pressure switch should have
opened preventing the unit from firing, also inspect
the unit pressure switch and wiring.
c.
If the burner flame is not drawn into the tube only
when the air circulation blower is running, then a
cracked heat exchanger tube is present.
