689670-BIM-A-0311
46
Johnson Controls Unitary Products
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 (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
energized, 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.
Gas Valve
The UCB and ICB continuously monitor the GV.
If the ICB senses voltage at the GV when not requested, the
ICB will energize the draft motor. The ICB will 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 will lock-on
the indoor blower and a flash code is initiated (Table 27). 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.
During a call for heat, if the UCB does not sense voltage at the
GV for a continuous five-minute period the UCB will initiate a
flash code (Table 27). The indoor blower motor will not be
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 (LS)
This control is located inside the gas heat compartment and is
set to open at the temperature indicated in the Unit Physical
Data Table 9. 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.
Auxiliary Limit Switch (ALS)
This control is located inside the supply air compartment and is
set to open at the temperature indicated in the Unit Physical
Data Table 9. It resets manually. 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.
The auxiliary limit switch 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 will respond in the same
manner as outlined above under “Limit Switch”.
The ICB monitors the Pressure and Rollout switches of gas
heat units.
The control circuit includes the following safety controls:
Pressure Switch (PS)
Once the draft motor has reached full speed and closes the
pressure switch during a normal ignition sequence, if the
pressure sw opens for 2 seconds, the GV will be de-energized,
the ignition cycle is aborted, and the ICB flashes the
appropriate code. See Table 28 Ignition Control Flash Codes.
The draft motor is energized until the pressure switch closes or
“W1” is lost.
