Carrier SINGLE PACKAGE ROOFTOP UNITS 48PG20-28 Installation And Service Instructions Manual page 45

When ignition occurs the IGC board will continue to moni-
tor the condition of the rollout and limit switches, the hall effect
sensor, as well as the flame sensor. If the unit is controlled
through a room thermostat set for fan auto., 45 seconds after
ignition occurs, the indoor-fan motor will be energized (and the
outdoor-air dampers will open to their minimum position). If
for some reason the overtemperature limit opens prior to the
start of the indoor fan blower, on the next attempt, the
45-second delay will be shortened to 5 seconds less than the
time from initiation of heat to when the limit tripped. Gas will
not be interrupted to the burners and heating will continue.
Once modified, the fan on delay will not change back to
45 seconds unless power is reset to the control.
When additional heat is required, W2 closes and initiates
power to the second stage of the main gas valve. When the
thermostat is satisfied, W1 and W2 open and the gas valve
closes, interrupting the flow of gas to the main burners. 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 (and the outdoor-air dampers will close).
If the overtemperature limit opens after the indoor motor is
stopped 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 opera-
tion. The IGC is located by removing the side panel and view-
ing the IGC through the view port located in the control box
access panel. During normal operation, the LED is continuous-
ly on. For information on troubleshooting refer to page 57.
COOLING, UNITS WITH ECONOMI$ER IV — When free
cooling is not available, the compressors will be controlled by
the zone thermostat. When free cooling is available, the
outdoor-air damper is modulated by the EconoMi$er IV
control to provide a 50 to 55 F mixed-air temperature into the
zone. As the mixed-air temperature fluctuates above 55 or
below 50 F, the dampers will be modulated (open or close) to
bring the mixed air temperature back within control.
If mechanical cooling is utilized with free cooling, the
outdoor-air damper will maintain its current position at the time
the compressor is started. If the increase in cooling capacity
causes the mixed-air temperature to drop below 45 F, then the
outdoor-air damper position will be decreased to the minimum
position. If the mixed-air temperature continues to fall, the
outdoor-air damper will close. Control returns to normal once
the mixed-air temperature rises above 48 F.
If optional power exhaust is installed, as the outdoor-air
damper opens and closes, the power exhaust fans will be ener-
gized and deenergized.
If field-installed accessory CO
EconoMi$er IV control, a demand controlled ventilation strate-
gy will begin to operate. As the CO
above the CO set point, the minimum position of the damper
2
will be increased proportionally. As the CO
because of the increase in fresh air, the outdoor-air damper will
be proportionally closed.
For EconoMi$er IV operation, there must be a thermostat
call for the fan (G). This will move the damper to its minimum
position.
When the EconoMi$er IV control is in the occupied mode
and a call for cooling exists (Y1 on the thermostat), the control
will first check for indoor fan operation. If the fan is not on,
then cooling will not be activated. If the fan is on, then the
control will open the EconoMi$er IV damper to the minimum
position.
sensors are connected to the
2
level in the zone increases
2
level decreases
2
On the initial power to the EconoMi$er IV control, it will
take the damper up to 2
itself. Any change in damper position will take up to 30 sec-
onds to initiate. Damper movement from full closed to full
open (or vice versa) will take between 1
If free cooling can be used as determined from the appropri-
ate changeover command (switch, dry bulb, enthalpy curve,
differential dry bulb, or differential enthalpy), then the control
will modulate the dampers open to maintain the mixed-air tem-
perature set point at 50 to 55 F.
If there is a further demand for cooling (cooling second
stage — Y2 is energized), then the control will bring on
compressor stage 1 to maintain the mixed-air temperature set
point. The EconoMi$er IV damper will be open at maximum
position. EconoMi$er IV operation is limited to a single
compressor.
HEATING, UNITS WITH ECONOMI$ER IV —
the room temperature calls for heat, the heating controls are
energized as described in the Heating, Units Without Econo-
mizer section. The IFM is energized and the EconoMi$er IV
damper modulates to the minimum position. When the thermo-
stat is satisfied and W1 and W2 are deenergized, the IFM
continues to run, and the economizer damper modulates to the
minimum position.
SUBCOOLER HEAT EXCHANGER (SHX) — The purpose
of the subcooler heat exchanger (SHX) and the subcooler TXV
is to increase the capacity of the evaporator by sending
subcooled liquid refrigerant into the evaporator. Normally, the
condenser subcools the liquid refrigerant by approximately
10 F. The SHX adds another 10 F to 15 F degrees of subcooling
before the refrigerant reaches the evaporator. This allows the
refrigerant to absorb more heat from the mixed air, thereby
providing more cooling capacity.
The subcooler functions by taking a small percentage of the
high pressure, low temperature liquid refrigerant which exits
the condenser and converting it into a low pressure, low tem-
perature gas. This is accomplished by using the subcooler
TXV. The gas is then routed through a reverse-flow heat
exchanger (SHX), which transfers heat from the remaining
liquid refrigerant in the liquid line to the reverse flow gas
refrigerant. The moderate temperature gas is then sent to the
compressor to complete the loop and the cooler liquid refriger-
ant continues its normal path to the main TXV and the
evaporator. See Fig. 30.
SUBCOOLER
HX
SUBCOOLER TXV
Fig. 30 — Subcooler Operation
45
1 / minutes before it begins to position
2
1 1
/ and 2 / minutes.
2 2
COMPRESSOR
COND
EVAP
MAIN TXV
When