66399-YIM-C-0405
66
Unitary Products Group
blower is de-energized after the fan off delay for heat-
ing has elapsed. The draft motor performs a 25-second
post purge.
GAS HEATING OPERATION ERRORS
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 temper-
ature limit, the draft motor will perform a 25-second
post-purge and the indoor blower will be de-energized
following the elapse of the fan off delay for heating.
The temperature 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 a flash
code is initiated (See Table 55).
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 55). When voltage is no longer sensed at the
GV, the UCB will de-energize the indoor blower follow-
ing 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 unit control board monitors the temperature limit
switch and the gas valve of gas furnace units.
The control circuit includes the following safety con-
trols:
1.
Limit Control (LS) - This control is located inside
the heat exchanger compartment and is set to
open at the temperature indicated in the Gas Heat
Limit Control Setting Table 52. It resets automati-
cally. The limit switch operates when a high tem-
perature condition, caused by inadequate supply
air flow occurs, thus shutting down the ignition con-
trol and closing the main gas valve and energizing
the blower.
2.
Centrifugal Switch (CS) - If the draft motor should
fail, the centrifugal switch attached to the shaft of
the motor prevents the ignition control and gas
valve from being energized.
3.
Redundant Gas Valve - This valve is an integral
part of the main gas valve and is located up stream
of the main gas valve. Should the main gas valve
fail in the open position, the redundant valve
serves as a back-up and shuts off the flow of gas.
4.
Flame Sensor Rod / 100% Ignition Control Lock-
Out - The flame rods and controls are located per
Proper Flame Adjustment Figure 20. If an ignition
control fails to detect a signal from the flame sen-
sor indicating the pilot flame is properly ignited,
then the main gas valve will not open. It will con-
tinue 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.
5.
Rollout Switch (RS) - This switch is located in the
burner vestibule. In the event of a sustained main
burner flame rollout, it shuts off the ignition control
and closes the main gas valve.
NOTE:
The auto reset rollout switch must reset before
allowing furnace operation.
6.
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.
TABLE 52: GAS HEAT LIMIT CONTROL SETTING
Units
(Tons)
Capacity, MBH
Limit Control
Opens, ºF
Input
Output
3
50
40
165
4
75
60
165
3
100
79
165
4
125
99
165
5
100
79
165
5
125
99
165
6
100
79
165
6
125
99
165
