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will begin to operate. As the CO
increases above the CO
set- - point, the minimum position
2
of the damper will be increased proportionally. As the
CO
level decreases because of the increase of fresh air,
2
the outdoor- - air damper will be proportionally closed. For
economizer operation, there must be a thermostat call for
the fan (G). If the unit is occupied and the fan is on, the
damper will operate at minimum position. Otherwise, the
damper will be closed.
Low Ambient Cooling Operation down to 40_F (4_C) —
In Low Ambient RTU conditions when the temperature is
between 55_F (13_C) and 40_F (4_C), the Low Ambient
Switch (LAS) will be active and the outdoor fans will run
to the pre- - set factory outdoor- - fan speed. When the
temperature is greater than 65_F (18_C), the Low
Ambient Switch will deactivate and the outdoor fans will
run in the standard cooling mode. If the Outdoor Fan
Select Switch (see Fig. 46) is in the ON position, the
outdoor fans will run in the Fan Cycle Speed Mode (FCS)
set to 250 rpm. If the Outdoor Fan Select Switch is in the
OFF position, the outdoor fans will run in the Minimum
Fan Speed Mode (MIN) set to 160 rpm regardless of the
cooling demand.
LC Size 07 units have a SPST normally open Low
Ambient Switch wired across the TS and OF terminal and
a jumper placed across the PS terminal (see Fig. 47).
When the LAS is active, the switch will close making
contact to the OF terminal. This is done for units that
require all outdoor fans to run at the same pre- - set factory
Low Ambient Speed.
OF SPEED
SELECT
Fig. 46 - - Outdoor Fan Speed Select Switch
JUMPER ON
PS
TS
Fig. 47 - - Schematic of SPST Low Ambient Switch
The Low Ambient Temperature Outdoor Fan Control
Table (below) shows the operation of the outdoor fan for
size 07 units.
level in the zone
2
ON
FCS
MIN
OFF
a48--- 9341
PS
OF
LAS
SPST
Table 7 – Low Ambient Temperature Outdoor Fan Control
No. of
LC Size
Fans On
07
2
Heating —
In the Heating Mode (W1 and G on the thermostat), the
ISC board sends power to W on the IGC board. Assuming
the unit is controlled through a room thermostat set for fan
auto, the indoor- - fan motor will energize and the
outdoor- - air dampers will open to their minimum position.
The ISC board upon seeing W1 and G ON will turn the
indoor fan to high speed.
The IGC board starts its gas ignition process. A check is
made to ensure that the rollout switch and limit switch are
closed. If the check was successful, the induced draft motor is
energized, and when its speed is satisfactory, as proven by the
flue gas pressure switch, the ignition activation period begins.
The burners will ignite within 5 seconds. If the burners do not
light, there is a 22- - second delay before another 5- - second
attempt. This sequence is repeated for 15 minutes or until the
burners light. If, after the 15 minutes, the burners still have
not lit, heating is locked out. To reset the control, break
24VAC power to the thermostat.
When gas ignition occurs, the IGC board will continue to
monitor the condition of the rollout switch, the limit
switches, the flue gas pressure switch, as well as the flame
sensor.
When W1 is turned OFF, the IGC board turns off the gas
valve. The IGC board has a delay time before it turns
IFO=OFF. At this time, the ISC board sees W1=OFF and
IFO=ON. The ISC will keep the indoor fan ON high
speed. Once the IGC board delay times out, the ISC board
will see W1=OFF and IFO=OFF, which then turns the
indoor fan OFF.
If the call for W1 lasted less than 1 minute, the heating
cycle will not terminate until 1 minute after W1 became
active. If the unit is controlled through a room thermostat
set for fan auto, the indoor fan motor will continue to
operate for an additional 45 seconds then stop. If the over
temperature limit opens after the indoor motor is stopped,
but within 10 minutes of W1 becoming inactive, on the
next cycle the time will be extended by 15 seconds. The
maximum delay is 3 minutes. Once modified, the fan OFF
delay will not change back to 45 seconds unless power is
reset to the control. A LED indicator is provided on the
IGC to monitor operation.
When additional heat is required, W2 closes and initiates
power to the second stage of the main gas valve. When
the thermostat is satisfied, the gas valve closes,
interrupting the flow of gas to the main burners.
C13328
32
No. of
Outdoor Fan
Switch
Fans Off
Select Switch
0
SPST
Up
RPM
250
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