59
The position of the EconoMi$er damper is main-
tained at its current value.
3. If Y1 is energized, and compressor no. 1 is already
energized (see Step 2) and the room thermostat calls
for Y2, compressor no. 1 continues to operate. If Y2
remains energized for more than 20 minutes, com-
pressor no. 2 is energized.
NOTE: Compressor no. 2 cannot be energized unless
there is a signal for Y2 from the space thermostat.
4. If compressor no. 2 is energized, and the Y2 signal
from the thermostat is satisfied, compressor no. 1
and 2 are deenergized. Reasserting Y2 will start com-
pressor no. 1 and (after a 20-minute interstage delay)
compressor 2.
5. If compressor no. 1 is energized and the thermostat is
satisfied, compressor no. 1, the OFM, and IFM are
deenergized and the EconoMi$er modulates closed.
When the OAT is below the MECH CLG LOCKOUT set
point, the compressors remain off.
Heating, units without economizer —
When the ther-
mostat calls for heating, terminal W1 is energized. In order
to prevent thermostat short-cycling, the unit is locked into
the Heating mode for at least 1 minute when W1 is ener-
gized. The induced-draft motor (IDM) is then energized and
the burner ignition sequence begins. The indoor (evapora-
tor) fan motor (IFM) is energized 45 seconds after a flame
is ignited. When additional heat is needed, W2 is energized
and the high-fire solenoid on the main gas valve (MGV) is
energized. When the thermostat is satisfied and W1 and
W2 are deenergized, the IFM stops after a 45-second time-
off delay.
Heating, units with economizer/two-position
damper —
When the thermostat calls for heating, termi-
nal W1 is energized. In order to prevent thermostat short-
cycling, the unit is locked into the Heating mode for at
least 1 minute when W1 is energized. The induced-draft
motor is then energized and the burner ignition sequence
begins. The indoor (evaporator) fan motor (IFM) is ener-
gized 45 seconds after a flame is ignited and the damper
moves to the minimum position. If the accessory two-
position damper is used, the outdoor-air damper opens to
the minimum position whenever the evaporator fan runs.
When additional heat is needed, W2 is energized and the
high-fire solenoid on the main gas valve (MGV) is ener-
gized. When the thermostat is satisfied and W1 and W2
are deenergized, the IFM stops after a 45-second time-off
delay. The economizer damper then moves to the fully
closed position. When using continuous fan, the damper
will remain in the minimum position.
Operating sequence — Units with
MoistureMiser dehumidification package
When thermostat calls for cooling, terminals G and Y1
and/or Y2 and the compressor contactor(s) C1 (and C2)
are energized. The indoor (evaporator) fan motor (IFM),
compressor(s), and outdoor (condenser) fan motor(s) (OFM)
start. The OFM(s) runs continuously while the unit is in
cooling. As shipped from the factory, MoistureMiser dehu-
midification circuit is always energized. If MoistureMiser
circuit modulation is desired, a field-installed, wall-mounted
humidistat is required.
If the MoistureMiser humidistat is installed and calls for
the MoistureMiser subcooler coil to operate, the humidistat
internal switch closes. This energizes and closes the liquid
line solenoid valve coil(s) (LLSV) of the MoistureMiser cir-
cuit, forcing the hot liquid refrigerant of the liquid line to
enter the subcooler coil (see Typical Piping and Wiring sec-
tion on page 55). As the hot liquid passes through the sub-
cooler coil, it is exposed to the cold supply 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 liquid 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 refrigerant and maintain proper condi-
tions 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 evaporator 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
evaporator and is changed into a vapor. The air passing
over the evaporator coil will become colder than during
normal operation as a result of the colder refrigerant tem-
peratures. However, as it passes over the subcooler coil,
the air will be warmed slightly.
As the refrigerant leaves the evaporator, the refrigerant
passes a low-pressure switch in the suction line. This low-
pressure switch will de-activate the MoistureMiser package
when the suction pressure reaches 60 psig. The low-pres-
sure switch is an added safety device to protect against
evaporator coil freeze-up. The low-pressure switch will
only de-activate and open the liquid line solenoid valve in
the MoistureMiser circuit. The compressors will continue to
run as long as there is a call for cooling, regardless of the
position of the low-pressure switch. The solenoid valve and
the MoistureMiser package will be re-activated only when
the call for cooling has been satisfied, the low-pressure
switch has closed, and a new call for cooling exists. The
crankcase heaters on the scroll compressors provide addi-
tional protection for the compressors due to the additional
refrigerant charge in the subcooler.
When the humidistat is satisfied, the humidistat internal
switch opens, cutting power to and opening the LLSV.
The refrigerant is routed back through the evaporator and
the subcooler coil is removed from the refrigerant loop.
When the thermostat is satisfied, C1 (and C2) is deener-
gized and the compressor(s) and OFM(s) shut off. After a
30-second delay, the IFM shuts off. If the thermostat
fan selector switch is in the ON position, the IFM will run
continuously.
