66389-BIM-C-1208
Johnson Controls Unitary Products
37
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 25. 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 Gas Heat
Limit Control Settings Table 25. 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-ener-
gized, the ignition cycle is aborted, and the ICB flashes the
appropriate code. See Table 31 Ignition Control Flash Codes.
The draft motor is energized until the pressure switch closes
or “W1” is lost.
ROLLOUT SWITCH (ROS)
The rollout switch is wired in series with the pressure switch.
As such, the ICB cannot distinguish the rollout switch opera-
tion from that of the pressure switch.
Consequently, the control will only respond in the same man-
ner as outlined above under “Pressure Switch”. An open roll-
out will inhibit the gas valve from actuating.
INTERNAL MICROPROCESSOR FAILURE
If the ICB detects an internal failure, it will turn cease all out-
puts, ignore inputs, and display the proper flash code for con-
trol replacement. The ICB remains in this condition until
replaced.
FLASH CODES
The UCB will initiate a flash code associated with errors
within the system. Refer to UNIT CONTROL BOARD FLASH
CODES Table 29.
RESETS
Remove the call for heating by lowering the thermostat set-
ting 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 26 for the required gas
heat anticipator setting.
TABLE 25: GAS HEAT LIMIT CONTROL SETTINGS
1
1.
Rollout = 300°F, Auxiliary Limit = 200°F
# of HX Tubes
Main Limit Setting
6
195°F
8
160°F
TABLE 26: GAS HEAT ANTICIPATOR SETPOINTS
SETTING (AMPS)
W1
W2
0.65
0.1
FIGURE 26 - TYPICAL FLAME
H E A T E X C H A N G E R T U B E
G A S
S U P P L Y
P I P E
B U R N E R
B U R N E R B R A C K E T
I G N I T O R
B U R N E R F L A M E
( B L U E O N L Y )
