Carrier 48HDT - Manual (Page 17)

any safety trip, and indicator light on thermostat
comes on. Check reason for safety trip.
Compressor restart is accomplished by manual
reset at the thermostat by turning the selector switch
to OFF and then to ON.
Heating
1. Purge gas supply line of air by opening union
ahead of gas valve. When gas odor is detected,
tighten union and wait 5 minutes before
proceeding.
2. Turn on manual gas valve and unit electrical
supply.
3. Set system switeh selector at FI EAT and fan
switch at AUTO, or ON. Set heating temperature
lever at above room temperature.
4. The induced draft combustion air fan will start.
Pilot lights within 30 seconds. Refer to Service,
Pilot Light if pilots do not light.
5. Main gas valve will open and main burners ignite
within 2 minutes from the pilot light ignition.
Natural gas burners are factory adjusted and
should burn with a soft blue flame. Flame should
not lift off burner face. Refer to Service, Main
Burners, if required.
6. Adjust airflow to obtain a temperature rise with­
in the range specified on the unit nameplate.
TO SHUT OFF UNIT — Set system selector switch
at OFF or set heating selector lever below room
temperature.
Safety Relief — A soft solder joint at the service
Schrader port provides pressure relief under
abnormal temperature and pressure conditions.
Ventilation (Continuous Fan) — Set fan and sys­
tem selector switches at ON and OFF, respectively.
Indoor air fan operates continuously to provide
constant air circulation.
Economizer Operating Sequence
COOLING MODE — When there is no power
applied to the indoor fan motor, the outdoor
damper is closed.
If outdoor ambient temperature is above the
outdoor air thermostat (OAT.) setting, the OAT.
contact is open. No power is supplied to DR2. Upon
a first-stage call for cooling, the indoor and outdoor
fans and compressor energize. The economizer
goes to vent position.
If outdoor ambient temperature is below the
outdoor air thermostat (OAT.) setting, the OAT.
contact is closed and power is supplied to DR2
(normally open contacts close and normally closed
contacts open). Upon a first-stage call for cooling
through Y1, power goes through contacts 4 and 6 of
DR2 causing DRl to energize (normally open con­
tacts close and normally closed contacts open). The
economizer switches from vent or closed position
to modulation mode.
Modulation is accomplished by using the mixed
air thermostat (MAT.) in conjunction with the stall
circuitry inside the actuator. If the mixed air
temperature is above the MAT. setting, C-Tl and
C-T2 are made causing the damper motor to open
(which causes the air temperature to fall). As the
mixed air temperature falls to the MAT. setting
(54 L), C-T2 opens causing the motor to stall. When
mixed air temperature goes 4L below the MAT.
setting, C-Tl opens causing the spring to close the
damper. This in turn raises the mixed air tempera­
ture and causes C-Tl to again stall the motor into
a less than full open position. If over-shoot should
occur, this process would oscillate a few times until
the mixed air temperature stabilizes.
Upon a second-stage call for cooling through Y2,
power goes through contacts 1,3 and 5 of DR2 and
CLO and the compressor is energized. Mechanical
cooling is integrated with economizer cooling.
Economizer continues in modulation mode.
When outdoor ambient temperature goes below
50 E, the cooling lockout switch (CLS) opens and
mechanical cooling is locked out.
An economizer control thermostat (ECT) on the
evaporator coil guards against abnormally low
suction temperatures while operating mechanical
cooling in conjunction with economizer. Low
suction temperatures can lead to frost on the evap­
orator coil. If frost buildup is detected, thermostat
de-energizes economizer, thus closing the outdoor
air damper. This raises air temperature entering the
evaporator coil and melts frost. Once frost is melted,
the economizer is re-energized.
When cooling load is satisfied, the damper motor
is de-energized causing the spring to close the
damper.
During unoccupied periods, if a field-supplied
night switch is used and is opened, the outdoor
air damper closes.
HEATING MODE — With no power applied to the
indoor fan motor, the outdoor air damper is closed.
Upon a call for heat through Wl, the indoor fan
contactor is energized and provides power to the
indoor fan motor and economizer.
If the night set-back switch is closed, the outdoor
dampers will open to the manually preset vent
position by power passing through contacts 1 and 2
of DR 1 and goingto switches SW1 and SW2through
terminal XV of actuator. The switches are set with
3 degree angular difference between each other,
acting as a dead spot or stall region. Thus, if the
damper is below vent position, SW2 is closed power­
ing the damper open until SWl closes and SW2
opens which stalls the motor. If the damper is too far
open, both SWl and SW2 are open causing the
spring to close the damper until SWl closes, again
stalling the motor.
When heating load is satisfied, the damper motor
is de-energized causing the spring to close the
damper.
During unoccupied periods, if a night set-back
switch is used and opened, the damper closes.
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