437642-UIM-C-0210
24
Johnson Controls Unitary Products
SECTION VIII: SAFETY CONTROLS
CONTROL CIRCUIT FUSE
A 3-amp fuse is provided on the control circuit board to protect the 24-
volt transformer from overload caused by control circuit wiring errors.
This is an ATO 3, automotive type fuse and is located on the control
board.
BLOWER DOOR SAFETY SWITCH
This unit is equipped with an electrical interlock switch mounted in the
burner compartment. This switch interrupts all power at the unit when
the panel covering the blower compartment is removed.
Electrical supply to this unit is dependent upon the panel that covers the
blower compartment being in place and properly positioned.
ROLLOUT SWITCH CONTROLS
These controls are mounted on the burner assembly. If the temperature
in the area surrounding the burner exceeds its set point, the gas valve
is de-energized. The operation of this control indicates a malfunction in
the combustion air blower, heat exchanger or a blocked vent pipe con-
nection. Corrective action is required. These are manual reset controls
that must be reset before operation can continue.
PRESSURE SWITCHES
This furnace is supplied with a pressure switch, which monitors the flow
through the combustion air/vent piping system. This switch de-ener-
gizes the gas valve if any of the following conditions are present.
1.
Blockage of vent piping or terminal.
2.
Failure of combustion air blower motor.
LIMIT CONTROLS
There is a high temperature limit control located on the furnace vesti-
bule panel near the gas valve. This is an automatic reset control that
provides over temperature protection due to reduced airflow. This may
be caused by:
1.
A dirty filter.
2.
If the indoor fan motor should fail.
3.
Too many supply or return registers closed or blocked off.
The control module will lockout if the limit trips 5 consecutive times. If
this occurs, control will reset & try ignition again after 1 hour.
SEQUENCE OF OPERATION
The following describes the sequence of operation of the furnace. Refer
to Owners Manual 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 continuously on the
speed selected on the “FAN SPEED” jumper on the main control board.
Intermittent Blower - Cooling
On cooling/ heating thermostats with a fan switch, when the fan switch
is set in the auto position and the thermostat calls for cooling, a circuit is
completed between the R, Y and G terminals. The motor is energized
through the Y1 cool terminal and runs on the speed selected on the
COOL tap of the CFM Timer board. The fan off setting is fixed at 60
seconds for SEER enhancement. The control board can accommodate
two-stage cooling. When a two-stage cool thermostat is connected to
the Y1 and Y2 terminals on the board, the blower will operate on LOW
COOL speed when Y1 is energized and on HI COOL speed when Y1
and Y2 are energized.
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 ignitor 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 24. The heat-
ing cycle is now 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. Also, 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.
Hot Surface Ignition System
Main power to the unit must still be interrupted at the main power
disconnect switch before any service or repair work is to be done to
the unit. Do not rely upon the interlock switch as a main power dis-
connect. Blower and burner must never be operated without the
blower panel in place.
HOT SURFACE IGNITION SYSTEM
Do not attempt to light this furnace by hand (with a match or any
other means). There may be a potential shock hazard from the
components of the hot surface ignition system. The furnace can
only be lit automatically by its hot surface ignition system.
