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178113-BIM-D-0906
When the thermostat calls for the second stage of heating, the
low-voltage control circuit from "R" to "W2" is completed, thru
the UCB. Heat relay "RW2" is energized. The "RW2-1" contact
is closed energizing the second stage ignition module "IC2".
"IC2" will immediately start the second stage igniter sparking
and will open the redundant valve located inside the second
stage main gas valve "GV2" to allow a flow of gas to the second
stage carryover tube. Only after the pilot flame has been ignited
and the presence of pilot flame detected at "IC2" by a signal
sent back through the flame sensor is sparking terminated and
the main gas valve opened.
Gas flows into each of the second stage main burners and is
ignited from the carryover tube flame.
If "IC2" fails to detect a pilot flame, it will continue to try for a
maximum of 85 seconds to ignite the pilot tube. If the pilot flame
is not detected, then "IC2" will lock out first stage furnace
operation for five minutes or until 24V power is removed from
the module either at the unit or by resetting the room
thermostat.
NOTE: That the second stage furnace can operate even if first
stage has locked out.
When the thermostat satisfies de-energizing the "RW2"and
"RW1", thus opening all gas valves. The blower motor will
continue to run after the furnace is shut down until the specified
fan off delay for heating has been satisfied. The UCB will de-
energize the blower motor.
Redundant valve
Gas main
Gas Valve
Figure 21: Gas Valve Piping
When the thermostat calls for the first stage of heating, the low-
voltage control circuit from "R" to "W1" is completed. 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 heating 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 temperature limit, the draft
motor will perform a 25-second post-purge and the indoor
Unitary Products Group
Main valve
To main burner
To pilot burner
blower will be de-energized following the elapse of the fan off
delay for heating.
This limit is monitored regardless of unit operation status, i.e.
this 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 flash code is initiated (See
Table 24).
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 24). When voltage is no longer
sensed at the GV, the UCB will de-energize the indoor blower
following 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 UCB monitors the temperature limit switch of gas heat units.
The control circuit includes the following safety controls:
Limit Switch (LS)
This control is located inside the gas heat compartment and is
set to open at the temperature indicated in the Gas Heat Limit
Control Settings Table 21. It resets automatically. The limit
switch operates when a high temperature condition, caused by
inadequate supply air flow occurs, thus shutting down the
heater and energizing the blower.
Centrifugal Switch (CS)
If the draft motor should fail, the centrifugal switch attached to
the shaft of the motor prevents the ignition controls and gas
valves from being energized.
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.
Flame Sensor Rod / 100% Ignition Control Lock-Out.
The flame rods and controls are located per Proper Flame
Adjustment Figure 23. If an ignition control fails to detect a
signal from the flame sensor indicating 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.
45
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