Carrier AC350 - 2.3 Modes of Operation

2--2
T--304 04/08
2.3 MODES OF OPERATION
2.3.1 Temperature Control
Temperature is controlled by maintaining the return air
temperature measured at the return air grille. To
maintain cooling, turn the temperature control knob
towards the minus (--) symbol. To start heating cycle,
turn the temperature control knob towards the plus (+)
symbol (See Figure 2.1).
2.3.2 Cooling Mode
Cooling is accomplished by energizing the compressor
and condenser fans, opening the liquid line solenoid
valve and closing the heating valve. Once interior
temperature reaches t he desired set point, the system
will de--energize the compressor clutch and allow the
system to operate in the vent mode until further cooling
isrequired.Thetemperaturewillbemaintained within2°
C. or 3.6° F.
A controller programed for reheat will mantain
compressor operation and cycle the heat valve to allow
reheating of the return air . In the reheat mode interior
temperature is maintained at the desired set point while
additional dehumidification takes place.
2.3.3 Heating Mode
In the heat mode the liquid line solenoid is closed and
thecompressorandcondenserfans areshutdown.The
heat valve is opened to allow a flow of engine coolant
through the heat section of the evaporator coil. The
evaporator fans speed is varied as required to circulate
air over the evaporator coil based on the temperature
difference from setpoint.
Operatingin theheatingmodeiscontrolledby thewater
temperature switch (WTS). The WTS is located on the
engine block of the vehicle and is provided by theOEM.
It senses the engine collant temperature and reverses
its contacts on temperature rise at 105° F. The switch
prevents the circulation of cooler air throughout the
vehicle as the engine comes up to temperature.
2.3.4 Boost Pump (Optional)
Whentheunit isintheheatmode,andif aboost pumpis
supplied by the coach manufacturer, the boost pump
relay is energized, providing 24 VDC to activate the
boost pump.
2.3.5 Vent Mode
Once the temperature is satisfied, there is a window
whentheunitwillgointoaventmode.Thisis whenthere
is neither heating or cooling. Only the evaporator fans
are operating. The range of the V ent mode is 2°C. or
3.6°F. from the set point. The compressor clutch is
disengaged at this time.
2.3.6 Compressor Unloader Control (Only with
05G or 05K Compressors)
When operating in cooling, the unloaders are used to
reduce system capacity as return air temperature
approaches set point. Operation of the unloaders
balances system capacity with the load and thereby
prevents overshoot from set point.
Relay Board mounted unloader outputs control the
capacity of the compressor by energizing or
de--energizing unloader solenoid valves. The model
05Gcompressor hasthreebanksof two cylinders each.
Enercizingavalvede--activatesabankofcylinders.The
outboard cylinder banks of the 05G are equipped with
unloader valves (UV1 and UV2), each controlling two
cylinders; this allows the 05G to be operated with two,
four or six cylinders.
The unloaders are used to control system capacity by
controlling compressor capacity.
Control of the unloaders is with the pressure switches.
a. Suction Pressure
The unloaders are used to control suction pressure and
thereby prevent coil frosting:
1. Compressor Unloader UV1 Relay
-- When the suc-
tion pressure decreases below 26 psig (R--134a),
unloader UV1 is energized, unloading a cylinder
bank (2 cylinders); this output will remain energized
until the pressure increases to above 34 psig
(R--134a).
2. Compressor Unloader UV2 Relay
-- When suction
pressure decreases below 23 psig (R--134a), un-
loaderUV2 isenergized,unloadingthesecondcom-
pressor cylinder bank; this output will remain ener-
gized until the pressure increases to above 31 psig
(R--134a).
b. Discharge Pressure
Discharge pressure is also controlled by the unloaders:
1. Compressor Unloader UV1 Relay
-- When the dis-
charge pressure increases above setpoint A (see
Table 2.1), unloader UV1 is energized; this unloader
will remain energized until the pressure decreases
below set point B (see Table 2.1).
Table 2.1 Unloader UV1 Relay
HP Switch
(PSIG)
Set Point A
(PSIG)
Set Point B
(PSIG)
300 (R--134a) 275 220
350 (R--134a)
(High Ambient)
325 270
2. CompressorUnloaderUV2Relay -- On R--134asys-
tems when the discharge pressure increases above
setpoint A (see Table 2.2), unloader UV2 is ener-
gized; this unloader will remain energized until the
pressure decreases below set point B (see
Table 2.2).
Table 2.2 Unloader UV2 Relay
HP Switch
(PSIG)
Set Point A
(PSIG)
Set Point B
(PSIG)
300 (R--134a) 285 225
350 (R--134a)
(High Ambient)
330 275
2.3.7 Override Mode -- AC310 (Dehumidification)
Whenin theheat modethecompressorwill notoperate.
The thermostat will allow only COOL, VENT or HEAT
modes independently. An override switch has been
installed in the return air area to allow the compressors
torunwhenintheHEAT mode. Moving theswitch tothe
ON position will energize both clutch relays energizing
the clutches. There are two temperature sensors that
are in series with the clutch relay circuit, Ambient
Sensor (mounted in the condenser) and Humidity
Control Sensor (mountedin thereturn air area). Aslong
as the return air temperature is above 60° F and the
ambient is above 25° F, the override circuit will function
when energized, providing dehumidification. (See
section 1.4 for sensor specs)