335224-BIM-B-1208
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Johnson Controls Unitary Products
GAS HEATING SEQUENCE OF OPERATIONS
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 by monitoring the
Gas Valve (GV)
. Once
voltage has been sensed at the GV, the UCB will initiate the
fan on delay for heating, energizing the indoor blower the
specified delay has elapsed.
When the thermostat has been satisfied, heating calls are
ceased. The GV is immediately closed. The blower is de-
energized after the fan off delay for heating has elapsed. The
draft motor performs a 30-second post purge.
IGNITION CONTROL BOARD
FIRST STAGE OF HEATING
When the ICB receives a call for first stage of heating, “W1,”
the draft motor is energized. Once the draft motor has been
proven, a 30-second purge is initiated. At the end of the
purge, the GV is opened, and the spark ignitor is energized
for 10 seconds. The ICB then checks for the presence of
flame. If flame is detected, the ICB enters a flame stabiliza-
tion period. If flame was not detected, the GV closes, and a
retry operation begins.
During the flame stabilization period, a loss of the flame for 2
seconds will cause the GV to close and the retry operation to
begin. After the flame stabilization period, a loss of flame for
3/4 second will cause the GV to close and the retry operation
to begin.
At the conclusion of the flame stabilization period, the ICB will
operate the gas heat in high fire for an additional 60 seconds
(for a total for 120 seconds of high fire operation). After this
60 seconds, the ICB will then use the call for the second
stage of heat to control second stage operation of the GV.
When “W1” is satisfied, both valves are closed.
SECOND STAGE OF HEATING
When the ICB receives a call for the second stage of heating,
“W2,” the ICB conducts a complete first stage ignition
sequence. If this sequence is satisfied, the second main
valve of the GV is opened.
When “W2” is satisfied, the second main valve is closed.
RETRY OPERATION
When a flame is lost or is not detected during an attempt to
achieve ignition, a retry operation occurs. A 30-second purge
is performed between ignition attempts.
If the unit fails after three ignition attempts, the furnace is
locked-out for one hour. The furnace is monitored during this
one-hour period for unsafe conditions.
RECYCLE OPERATION
When a flame is lost after the flame stabilization period, a
recycle operation occurs. If the unit fails after five recycle
attempts, the furnace is locked-out for one hour.
GAS HEATING OPERATION ERRORS
LOCK-OUT
A one-hour lockout occurs following three retries or five recy-
cles. During the one-hour lockout, flame detection, limit con-
ditions, and main valves are tested. Any improper results will
cause the appropriate action to occur. Recycling the low volt-
age power cancels the lock-out.
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 15-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 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 41).
FLAME SENSE
Flame sensing occurs at all times. If “W1” is not present and
a flame is sensed for 2 seconds, the draft motor is energized
and the GV is kept off. The ICB halts any operation until a
flame is not detected. Once the flame detection is lost, the
ICB performs a post-purge. Normal operation is allowed con-
currently with the purge (i.e. this purge can be considered the
purge associated with a call for “W1”).
If “W1” is present, a flame is sensed, but the GV is not ener-
gized, the draft motor is energized until the flame detection is
lost. Normal operation is now allowed.
The flame detection circuitry continually tests itself. If the ICB
finds the flame detection circuitry to be faulty, the ICB will not
permit an ignition sequence and the draft motor is energized.
If this failure should occur during an ignition cycle the failure
is counted as a recycle.
TABLE 36: ELECTRIC HEAT ANTICIPATOR
SETPOINTS
SETTING, AMPS
W1
W2
0.13
0.1
