5835254-YIM-A-1119
Johnson Controls Ducted Systems
45
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
UCB de-energizes the indoor blower following the elapse of the
fan off delay for heating.
If voltage is 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.
During a call for heat, if the UCB does not sense voltage at the
GV for a continuous five-minute period, the UCB initiates a error
message. The indoor blower motor is not locked on while there
is no GV voltage.
Safety controls
The UCB monitors the temperature limit switch of gas heat
units.
The control circuit includes the following safety controls:
Limit switch
The limit switch (LS) is located inside the gas heat compartment
and is set to open at the temperature indicated in Table 11. It
resets automatically. The limit switch operates when a high
temperature condition, caused by inadequate supply air flow
occurs. This shuts down the heater and energizes the blower.
Auxiliary limit switch
The auxiliary limit switch (ALS) is wired in series with the limit
switch. As such, the UCB cannot distinguish the auxiliary limit
and the gas heat limit switch operation except the auxiliary is
manual reset. Consequently, the control responds in the same
manner as outlined in
The ICB monitors the pressure and roll out switches of gas heat
units.
The control circuit includes the following safety controls:
Pressure switch
When the draft motor has reached full speed and closes the
pressure switch (PS) during a normal ignition sequence, if the
pressure switch opens for 2 seconds, the GV is de-energized,
the ignition cycle is aborted, and the ICB flashes the
appropriate code. For information on the ignition control flash
codes, see Table 31 on page 56. The draft motor is energized
until the pressure switch closes or W1 is lost.
Roll-out switch
The roll-out switch (ROS) is wired in series with the pressure
switch. As such, the ICB cannot distinguish the roll-out switch
operation from that of the pressure switch.
Consequently, the control only responds in the same manner
as outlined in
on page 45. An open roll-out
inhibits the gas valve from actuating.
Internal microprocessor failure
If the ICB detects an internal failure, it ceases all outputs,
ignores inputs, and displays the proper flash code for control
replacement. The ICB remains in this condition until it is
replaced.
Table 26: Gas heat limit control settings
1
1.Roll-out = 300°F, Auxiliary Limit = 200°F.
Unit
Main limit setting
°F
Size
Opt.
ZZ15
N18
195
N24
160
