349218-JIM-F-0512
Johnson Controls Unitary Products
51
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 22. 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.
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 24. If an ignition control fails to detect a
signal from the flame sensor indicating the pilot flame is
properly ignited, then the main gas valve will not open. It will
continue to try and ignite the pilot for a maximum of 85 seconds,
then if the pilot flame is not detected, the ignition control will
lock out furnace operation until 24V power is removed from the
module either at the unit or by resetting the room thermostat.
Rollout Switch
This switch is located above the main burners in the control
compartment, which in the event of a sustained main burner
rollout shuts off and locks out both ignition controls closing both
gas valves. The ignition controls lock out furnace operation until
24V power is removed from the controls either at the unit or by
resetting the room thermostat.
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.
The ICB monitors the Pressure and Rollout switches of gas
heat units.
The control circuit includes the following safety controls:
Figure 23: Gas Valve and Controls
Flash Codes
The UCB will initiate a flash code associated with errors within
the system. Refer to UNIT CONTROL BOARD FLASH CODES
Table 25.
Resets
Remove the call for heating by lowering the thermostat setting
lower than the conditioned space temperature. This resets any
flash codes.
Gas Heat Anticipator Setpoints
It is important that the anticipator setpoint be correct. Too high
of a setting will result in longer heat cycles and a greater
temperature swing in the conditioned space. Reducing the
value below the correct setpoint will give shorter “ON cycles
and may result in the lowering of the temperature within the
conditioned space. Refer to Table 23 for the required gas heat
anticipator setting.
Table 22: Gas Heat Limit Control Setting
Units
(Tons)
Capacity, MBH
Limit Control
Opens, ºF
Input
Output
15, 17.5, 20 & 25
300
240
195
15, 17.5, 20 & 25
400
320
195
Burner Compartment
Sensor #2
Sensor #1
Rollout
Switch
Ignitor #2
Ignitor Control #1
Ignitor Control #2
GV2
Gas
Valve
GV1
Gas
Valve
Ignitor #1
