Carrier WeatherExpert 48LC Series Installation Instructions Manual page 42

Economizer (Optional)
When the economizer is in Free Cooling Mode and a demand for
cooling exists (Y1 on the thermostat), the economizer will modu-
late the outdoor-air damper to provide a 50°F (10°C) to 55°F
(13°C) mixed-air temperature into the zone and run the indoor-fan
at high speed. As mixed-air temperature fluctuates above 55°F
(13°C) or below 50°F (10°C) dampers will be modulated (open or
close) to bring the mixed-air temperature back within control.
Upon more call for cooling (Y2 on the thermostat), the outdoor-air
damper will maintain its current position, compressor C1 will run
and the outdoor-fan will run at low speed. If there is further de-
mand for cooling, the outdoor-air damper will maintain its current
position, compressor C2 will run and the outdoor-fan will run at
medium speed. The VFD controlled indoor-fan will operate at
high speed regardless of the cooling demand.
If the increase in cooling capacity causes the mixed-air tempera-
ture to drop below 45°F (7°C), the outdoor-air damper will return
to the minimum position. If the mixed-air temperature continues
to fall, the outdoor-air damper will close. Control returns to nor-
mal once the mixed-air temperature rises above 48°F (9°C). The
power exhaust fans will be energized and de-energized, if in-
stalled, as the outdoor-air damper opens and closes.
If field-installed accessory CO
omizer, a demand controlled ventilation strategy will begin to op-
erate. As the CO level in the zone increases above the CO
2
point, the minimum position of the damper will be increased pro-
portionally. As the CO level decreases because of the increase of
2
fresh air, 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 oper-
ate 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 be-
tween 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 out-
door fans will run in the standard cooling mode. If the Outdoor
Fan Select Switch (see Fig. 56) 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 out-
door fans will run in the Minimum Fan Speed Mode (MIN) set to
160 rpm regardless of the cooling demand.
48LC**14-26 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. 57). 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 fac-
tory Low Ambient Speed.
Table 11 shows the operation of the outdoor fan for each unit.
OF SPEED
Fig. 56 — Outdoor Fan Speed Select Switch
sensors are connected to the econ-
2
SELECT
ON
FCS
MIN
OFF
Fig. 57 — Schematic of SPST Low Ambient Switch
Table 11 — Low Ambient Temperature Outdoor
48LC NO. OF
UNIT FANS ON
3
14
4
17
4
20
6
24
6
set- 26
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 con-
trolled 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 ig-
nite 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 mo-
tor is stopped, but within 10 minutes of W1 becoming inactive, on
the next cycle the time will be extended by 15 seconds. The maxi-
mum 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 sat-
isfied, the gas valve closes, interrupting the flow of gas to the main
burners.
42
JUMPER ON
PS PS
TS OF
LAS
SPST
Fan Control
OUTDOOR FAN
NO. OF
SWITCH SELECT
FANS OFF
SWITCH
0 SPST
0 SPST
0 SPST
0 SPST
0 SPST
RPM
Up 250
Up 250
Up 250
Up 250
Up 250