5168276-XIM-B-0216
Johnson Controls Unitary Products
41
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 31).
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 31). 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 27. 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 (AUX)
This control is located inside the heat exchanger compartment
and is set to open at 190°F. It is a manual reset switch. If AUX
trips, then the primary limit has not functioned correctly.
Replace the primary limit.
NOTE:
Access to (LS) and (AUX) switches is through the
compressor access panel on side discharge
applications and through the discharge air duct cover
on down discharge applications.
Figure 25: Gas Heat Limit And Auxiliary Location Down
Discharge Application
Figure 26: Gas Heat Limit and Auxiliary Location Side
Discharge Application
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 28. If an ignition control fails to detect a
Auxilary
Limit
Limit
Switch
Limit
Access
