64
Ventilation
In the Ventilation/Fan Mode (G on the thermostat), the indoor
fan will run at low speed and the damper will operate at mini
-
mum position.
Cooling
In the Cooling Mode, the small and large compressors will be se
-
quenced to maintain the thermostat/DDC temperature setpoint.
Table 39 shows the cooling operation based on the following
conditions.
The outdoor fan and VFD controlled indoor-fan will operate at
low, medium and high speed. The indoor-fan speed (rpm) is fac
-
tory set by the CFM and static pressure requirements for the unit
installed.
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, only 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. Once the mixed air tem
-
perature rises above 48°F (9°C), the control returns to normal. 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
2
sensors are connected to the econ
-
omizer, a demand controlled ventilation strategy will begin to op
-
erate. As the CO
2
level in the zone increases above the CO
2
set
-
point, the minimum position of the damper will be increased pro
-
portionally. As the CO
2
level decreases because of the increase of
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. 62) is in the up 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 down position, the out
-
door fans will run in the Minimum Fan Speed Mode (MIN) set to
160 rpm regardless of the cooling demand.
The 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. 63). 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.
Fig. 62 — Outdoor Fan Speed Select Switch
Fig. 63 — Schematic of SPST Low Ambient Switch
Table 40 shows the operation of the outdoor fans for each unit.
Table 39 — Cooling Operation
INPUT
OUTPUT
Thermostat
Compressor
C1
Compressor
C2
Indoor
Fan
Speed
Outdoor
Fan
Speed
First Stage
Cooling (Y1)
On
Off
Low
Low
(700 rpm)
Second Stage
Cooling (Y2)
Off
On
Medium
Medium
(800 rpm)
Third Stage
Cooling (Y3)
On
On
High
High
(1000 rpm)
OF SPEED
SELECT
ON
FCS
MIN
OFF
JUMPER ON
LAS
SPST
PS
PS
TS
OF
Table 40 — Low Ambient Temperature Outdoor Fan Control
48LC**
NO. OF FANS ON NO. OF FANS OFF
SWITCH
LAS Fig. No.
OF SELECT
SWITCH
RPM
14
3
0
(1) SPST
Up
250
17
4
0
(1) SPST
Up
250
20
4
0
(1) SPST
Up
250
24
6
0
(1) SPST
Up
250
26
6
0
(1) SPST
Up
250
