5808011-JIM-A-0320
Johnson Controls Ducted Systems
63
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 on any call for heat by
monitoring the gas valve (GV). When voltage is sensed at the
GV, the UCB initiates the fan on delay for heating, energizing
the indoor blower the specified delay has elapsed.
When the thermostat is 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 igniter is energized for 10 seconds.
The ICB then checks for the presence of flame. If flame is
detected, the ICB enters a flame stabilization period. If flame is
not detected, the GV closes and a retry operation begins.
During the flame stabilization period, a loss of the flame for 2
seconds causes the GV to close and the retry operation to
begin. After the flame stabilization period, a loss of flame for 3/4
second causes the GV to close and the retry operation to begin.
At the conclusion of the flame stabilization period, the ICB
operates 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 then uses 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 recy-
cle operation occurs. If the unit fails after five recycle attempts,
the furnace is locked out for one hour.
Gas heating operation errors
Lockout
A one-hour lockout occurs following three retries or five
recycles. During the one-hour lockout, flame detection, limit
conditions, and main valves are tested. Any improper results
will cause the appropriate action to occur. Recycling the low
voltage power cancels the lockout.
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 performs a 15-second post-purge and the indoor blower
is de-energized following the elapse of the fan off delay for
heating.
This limit is monitored regardless of unit operation status, that
is, this limit is monitored at all times.
If the temperature limit opens three times within one hour, it
locks on the indoor blower motor.
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 concurrently with the
purge. That is, 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 energized
and 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 does not
permit an ignition sequence and the draft motor is energized. If
this failure occurs during an ignition cycle, the failure is counted
as a recycle.
Gas valve
The UCB and ICB continuously monitor the GV.
If the ICB senses voltage at the GV when not requested, the
ICB energizes the draft motor. The ICB does not operate the
furnace until voltage is no longer sensed at the GV. The draft
motor is stopped when voltage is not sensed at the GV.
Any time the UCB senses voltage at the GV without a call for
heat for a continuous five-minute period, the UCB locks on the
indoor blower. When voltage is no longer sensed at the GV, the
Table 25: Electric heat anticipator setpoints
Setting, Amps
W1
W2
0.13
0.1
