Specifications

57
CONTROLS (cont)
When the outdoor-air temperature is below the OAT setting and
the thermostat calls for Y1 and G (cooling), the economizer
damper moves to the minimum position when the evaporator
fan starts. The first stage of cooling is provided by the econo-
mizer. If the supply-air temperature is above 57 F, a switch on
the supply-air thermostat is closed between the T2 terminal and
the 24 vac terminal. This causes the damper to continue to
modulate open until the supply-air temperature falls below 55 F
or the damper reaches the fully open position.
When the supply-air temperature is between 55 F and 52 F, the
supply-air thermostat has open switches between the T2 and
24 vac terminals and between the T1 and 24 vac terminals. This
causes the economizer damper to remain in an intermediate
open position.
If the supply-air temperature falls below 52 F, a switch on the
supply-air thermostat is closed between the T1 terminal and the
24 vac terminal. This causes the damper to modulate closed
until the supply-air temperature rises above 55 F or the damper
reaches the minimum position.
When the supply-air temperature is between 55 F and 57 F, the
supply-air thermostat has open switches between the T2 and
24 vac terminals. This causes the economizer damper to
remain in an intermediate open position.
If the outdoor air alone cannot satisfy the cooling requirements
of the conditioned space, economizer cooling is integrated with
mechanical cooling, providing second stage cooling. Compres-
sor no. 1 and condenser fans will be energized, and the position
of the economizer damper will be determined by the supply-air
temperature. Compressor no. 2 is locked out.
When the second stage of cooling is satisfied, the compressor
and OFMs will be deenergized. The damper position will be
determined by the supply-air temperature.
When the first stage of cooling is satisfied, the IFM shuts off and
the damper will move to fully closed position.
Cooling, Units With EconoMi$er —
When the outdoor-air
temperature (OAT) is above the ECON SP set point and the
room thermostat calls for Stage 1 cooling (R to G + Y1), the
indoor (evaporator) fan motor (IFM) is energized and the
EconoMi$er damper modulates to minimum position. The com-
pressor contactor is energized to start the compressor and out-
door (condenser) fan motor (OFM). After the thermostat is
satisfied, the damper modulates to the fully closed position
when the IFM is deenergized.
When the OAT is below the ECON SP setting and the room
thermostat calls for Stage 1 cooling (R to G + Y1), the
EconoMi$er modulates to the minimum position when the IFM
is energized. The EconoMi$er provides Stage 1 of cooling by
modulating the return and outdoor air dampers to maintain a
55 F supply air set point. If the supply-air temperature (SAT) is
greater than 57 F, the EconoMi$er modulates open, allowing a
greater amount of outdoor air to enter the unit. If the SAT drops
below 53 F, the outdoor air damper modulates closed to reduce
the amount of outdoor air. When the SAT is between 53 and
57 F, the EconoMi$er maintains its position.
If outdoor air alone cannot satisfy the cooling requirements of
the conditioned space, and the OAT is above the MECH CLG
LOCKOUT set point, the EconoMi$er integrates free cooling
with mechanical cooling. This is accomplished by the strategies
below.
NOTE:
Compressor has a two-minute Minimum On, Minimum
Off, and Interstage delay timer.
1. If Y1 is energized, and the room thermostat calls for Y2
(2-stage thermostat), compressor no. 1 and OFM are ener-
gized. The position of the EconoMi$er damper is main-
tained at its current value.
2. If Y1 is energized for more than 20 minutes, and Y2 is not
energized (whether or not a 2-stage thermostat is used),
compressor no. 1 and OFM are energized. The position of
the EconoMi$er damper is maintained at its current value.
3. If Y1 is energized, and compressor no. 1 is already ener-
gized (see Step 2) and the room thermostat calls for Y2, the
compressor continues to operate. If Y2 remains energized
for more than 20 minutes, compressor no. 2 is energized.
NOTE:
Compressor no. 2 cannot be energized unless there is a
signal for Y2 from the space thermostat.
4. If compressor no. 2 is energized, and the Y2 signal from the
thermostat is satisfied, compressors 1 and 2 are deener-
gized. Reasserting Y2 will start compressor no. 1 and (after
a 20-minute interstage delay) compressor no. 2.
5. If compressor no. 1 is energized and the thermostat is satis-
fied, compressor no. 1, the OFM, and IFM are deenergized
and the EconoMi$er modulates closed.
When the OAT is below the MECH CLG LOCKOUT set point,
the compressors remain off.
Heating, Units Without Economizer —
Upon a call heating
through terminal W1, indoor (evaporator) fan contactor (IFC)
and heater contactor no. 1 (HC1) are energized. On units
equipped for 2 stages of heat, when additional heat is needed,
HC2 is energized through W2.
Heating, Units With Economizer —
When the room thermo-
stat calls for heat through terminal W1, the indoor (evaporator)
fan contactor (IFC) and heater contactor no. 1 (HC1) are ener-
gized. On units equipped for 2 stages of heat, when additional
heat is needed, HC2 is energized through W2. The economizer
damper moves to the minimum position. When the thermostat is
satisfied, the damper moves to the fully closed position.
OPERATING SEQUENCE — UNITS WITH PERFECT HUMID-
ITY™ DEHUMIDIFICATION PACKAGE —
When thermostat
calls for cooling, terminals G and Y1 and/or Y2 and the com-
pressor contactor(s) C1 (and C2) are energized. The indoor
(evaporator) fan motor (IFM), compressor(s), and outdoor (con-
denser) fan motor(s) (OFM) start. The OFM(s) runs continu-
ously while the unit is in cooling. As shipped from the factory,
dehumidification circuit is always energized. If Perfect Humidity
circuit modulation is desired, a field-installed, wall-mounted
humidistat or light commercial thermidistat is required.
If the Perfect Humidity humidistat is installed and calls for the Per-
fect Humidity subcooler coil to operate, the humidistat internal
switch closes. This energizes and closes the liquid line solenoid
valve coil(s) (LLSV) of the Perfect Humidity circuit, forcing the hot
liquid refrigerant of the liquid line to enter the subcooler coil (see
Dehumidification Option diagram on page 58). As the hot liquid
passes through the subcooler coil, it is exposed to the cold sup-
ply airflow coming off from the evaporator coil and the liquid is
further cooled to a temperature approaching the evaporator coil
leaving-air temperature. The state of the refrigerant leaving the
subcooler coil is a highly subcooled liquid refrigerant. The liquid
then enters a thermostatic expansion valve (TXV) where the liq-
uid is dropped to a lower pressure. The TXV does not have a
pressure drop great enough to change the liquid to a 2-phase
fluid. The TXV can throttle the pressure drop of the liquid refriger-
ant and maintain proper conditions at the compressor suction
valve over a wide range of operating conditions. The liquid then
enters a second fixed restrictor expansion device for a second
pressure drop to a 2-phase fluid. The liquid proceeds to the evap-
orator coil at a temperature lower than normal cooling operation.
This lower temperature is what increases the latent capacity of
the rooftop. The 2-phase refrigerant passes through the evapora-
tor and is changed into a vapor. The air passing over the evapo-
rator coil will become colder than during normal operation as a
result of the colder refrigerant temperatures. However, as it
passes over the subcooler coil, the air will be warmed slightly.