268893-UUM-A-0407
6
Unitary Products Group
3.
Unplug the vent motor wires and ground wire. Remove the pres-
sure switch tubing at the top on the vent motor housing.
NOTE:
It is recommended that replacement gaskets be available
before removing vent motor.
4.
Remove six mounting screws that hold the vent motor to the
restrictor plate. The surface is gasketed and the gasket can be
reused if it is carefully removed. It is necessary to remove this
assembly to gain access to the restrictor plate mounting holes.
The assembly may be vacuumed if cleaning is necessary. If any
vent assembly parts are damaged, replace with an entire new
assembly (except for gaskets).
5.
Remove the perimeter screws attaching the restrictor plate assem-
bly to the vestibule panel. The surface is also gasketed. The
assembly, including the flue baffle plate (rear) may be vacuumed
or cleaned with hot water if necessary.
6.
The upper portion of the heat exchanger is now accessible. With a
long flexible wire brush, clean inside each tube at both the top and
bottom. The brush must pass around the rear heat exchanger
tubes. Vacuum loose scale and dirt from each tube.
7.
Clean - Replace all components in reverse order. Re-gasket all
surfaces which required a gasket. Reconnect all wiring. Reattach
vent pipe and gas supply lines before restoring service to furnace.
Restore electrical power, check gas supply piping for leaks, and
then verify furnace operation.
SEQUENCE OF OPERATION
The following describes the sequence of operation of the furnace. Refer
to Figure 1 for component location.
Continuous Blower
Cooling/heating thermostats have a fan switch that has an ON and
AUTO position. In the ON position the thermostat circuit is completed
between terminals R and G. The motor will operate on the speed tap
wire that is connected to the “HI COOL” cooling terminal on the control
board. To obtain a constant air circulation at lower flow rate, change the
high-speed wire to either the medium speed wire or the low speed wire.
Intermittent Blower - Cooling
Cooling/heating thermostats have a fan switch that has an ON and
AUTO position. In the AUTO position the thermostat circuit is completed
between terminals R and G when there is a call for cooling. The motor
will operate on the speed tap wire that is connected to the “HI COOL”
cooling terminal on the control board. The fan off setting is fixed at 60
seconds to improve cooling efficiency.
Heating Cycle
When the thermostat switch is set on HEAT and the fan is set on AUTO,
and there is a call for heat, a circuit is completed between terminals R
and W of the thermostat. When the proper amount of combustion air is
being provided, the pressure switch will close, the ignition control pro-
vides a 17-second warm-up period, the gas valve then opens, the gas
starts to flow, ignition occurs and the flame sensor begins its sensing
function. The blower motor will energize 30 seconds after the gas valve
opens, if a flame is detected. Normal furnace operation will continue
until the thermostat circuit between R and W is opened, which causes
the ignition system and gas valve to de-energize and the burner flames
to be extinguished. The vent motor will operate for 15 seconds and the
blower motor will operate for the amount of time set by the fan-off delay
jumper located on the control board. See Figure 8. The heating cycle is
complete, and ready for the start of the next heating cycle.
If the flame is not detected within 7 seconds of the gas valve opening,
the gas valve is shut off and a retry operation begins. If the flame is lost
for 2 seconds during the 10-second stabilization period, the gas valve is
shut off and a retry operation begins. During a retry operation, the vent
motor starts a 15 second inter-purge and the ignitor warm-up time is
extended to 27 seconds. If the flame is established for more than 10
seconds after ignition during a retry, the control will clear the ignition
attempt (retry) counter. If three retries occur during a call for heat, the
furnace will shut down for one hour. If at the end of the one hour shut
down there is a call for heat, the furnace will initiate a normal start cycle.
If the problem has not been corrected the furnace will again lockout
after three retries.
A momentary loss of gas supply, flame blowout, or a faulty flame probe
circuit will result in a disruption in the flame and be sensed within 1.0
seconds. The gas valve will de-energize and the control will begin a
recycle operation. A normal ignition sequence will begin after a 15 sec-
ond inter-purge. If during the five recycles the gas supply does not
return, or the fault condition is not corrected the ignition control will lock-
out for 60 minutes.
During burner operation, a momentary loss of power for 50 milliseconds
or longer will de-energize the gas valve. When the power is restored,
the gas valve will remain de-energized and the ignition sequence will
immediately restart.
Label all wires prior to disconnection when servicing
controls. Wiring errors can cause improper and danger-
ous operation. Verify proper operation after servicing.
FIGURE 9:
Furnace Control Board
PARK PARK
HI COOL
HEAT
EAC-H
L1
XFMR
NEUTRALS
HUM
TWIN
60
90
120
180
BLOWER
OFF
DELA
Y
Y/Y2
W
R
G
C
FAN OFF
ADJUSTMENT
JUMPER
RED-LOW
YELLOW-MED. LOW
BLACK-HI
BLUE-MED. HI
