284806-YIM-B-0707
48
Unitary Products Group
When the thermostat calls for the first stage of heating,
the low-voltage control circuit from "R" to "W1" is com-
pleted. A call for heat passes through the UCB to the
ignition control board (ICB). The UCB monitors the
"W1" call and acts upon any call for heat. Once voltage
has been sensed at “W1”, the UCB will initiate the fan
on delay for heating, energizing the indoor blower after
the specified delay has elapsed.
When the thermostat has been satisfied, heating calls
are ceased. The GV is immediately de-energized. The
blower is de-energized after the fan off delay for heat-
ing has elapsed. The draft motor performs a 25-second
post purge.
GAS HEATING OPERATION ERRORS
TEMPERATURE LIMIT
If the UCB senses zero volts from the high temperature
limit, the indoor blower motor is immediately energized.
When the UCB again senses 24 volts from the temper-
ature limit, the draft motor will perform a 25-second
post-purge and the indoor blower will be de-energized
following the elapse of the fan off delay for heating.
This limit is monitored regardless of unit operation sta-
tus, i.e. the limit is monitored at all times.
If the temperature limit opens three times within one
hour, it will lock-on the indoor blower motor and a flash
code is initiated (See Table 29).
GAS VALVE
The UCB continuously monitors the GV. Any time the
UCB senses voltage at the GV without a call for heat
for a continuous five-minute period, the UCB will lock-
on the indoor blower and a flash code is initiated
(Table 29). When voltage is no longer sensed at the
GV, the UCB will de-energize the indoor blower follow-
ing the elapse of the fan off delay for heating.
If voltage has been sensed at the GV for at least 15
seconds during the fan on delay for heating and GV
voltage or "W1" is lost, the indoor blower is forced on
for the length of the fan off delay for heating.
SAFETY CONTROLS
The unit control board monitors the temperature limit
switch and the gas valve of gas furnace units.
The control circuit includes the following safety con-
trols:
1.
Limit Control (LS).
This control is located inside the heat exchanger
compartment and is set to open at the temperature
indicated in the Limit Control Setting Table 26. It
resets automatically. The limit switch operates
when a high temperature condition, caused by
inadequate supply air flow occurs, thus shutting
down the ignition control and closing the main gas
valves and energizing the blower.
2.
Centrifugal Switch (CS).
If the draft motor should fail, the centrifugal switch
attached to the shaft of the motor prevents the igni-
tion controls and gas valves from being energized.
3.
Redundant Gas Valve.
There are two separate gas valves in the furnace.
Each valve contains a main and a redundant valve.
The redundant valves are located upstream of the
main gas valves. Should either or both of the main
gas valves fail in the open position the redundant
valves serve as back-ups and shut off the flow of
gas.
4.
Flame Sensor Rod / 100% Ignition Control Lock-
Out.
The flame rods and controls are located per Proper
Flame Adjustment Figure 22. If an ignition control
fails to detect a signal from the flame sensor indi-
cating the pilot flame is properly ignited, then the
main gas valve will not open. It will continue to try
and ignite the pilot for a maximum of 85 seconds,
then if the pilot flame is not detected, the ignition
control will lock out furnace operation until 24V
power is removed from the module either at the
unit or by resetting the room thermostat.
FIGURE 19 - GAS VALVE PIPING
GAS
VALVE
GAS MAIN
MAIN VALVE
TO MAIN
BURNER
REDUNDANT
VALVE
TO PILOT BURNER
