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46
SEQUENCE OF OPERATION
General
The sequence below describes the sequence of operation
for an electro--mechanical unit with and without a factory
installed EconoMi$er IV (called “economizer” in this
sequence ). For information regarding a direct digital
controller, see the start--up, operations, and
troubleshooting manual for the applicable controller.
Electro--mechanical units with no economizer
Cooling —
When the thermostat calls for cooling, terminals G and Y1
are energized. As a result, the indoor--fan contactor (IFC)
and the compressor contactor (C1) are energized, causing
the indoor--an motor (IFM), compre ssor #1, and outdoor
fan t o start. If the unit has 2 stages of cooling, the
thermostat will additionally energize Y2. The Y2 signal
will energize compressor contactor #2 (C2), causing
compressor #2 to start. Regardless of the number of
stages, the outdoor--an motor runs conti nuously while unit
is cooling.
Heating
NOTE: The 558J is sold as cooling only. If electric
heaters are required, use only factory--approved electric
heaters. They will operate as described below.
Units have either 1 or 2 stages of electric heat. When the
thermostat calls for heating, power is applied to the W1
terminal at the unit. The unit control will energize the
indoor fan contactor and the first stage of elect ric heat. On
units with two--stage heating, when additional heating is
required, the second stage of electric heat (if equipped)
will be energized when power is applied at the W2
terminal on the unit.
Electro--mechanical units with an economizer
Cooling —
When free cooling is not availa ble, the compressors will
be controlled by the zone thermostat. When free cooling is
avai lable, the outdoor--air damper is modulated by the
EconoMi$er IV cont rol to provide a 50_F(10_C) to 55_F
(13_C) mixed-- air temperature into the zone. As the mixed
air temperature fluctuates above 55_F(13_C) or below
50_F(10_C) 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(7_C), then the outdoor--air damper position
will be decreased to the minimum position. If the
mixed--air temperature continues to fall, the outdoor--air
dampe r will close. Control returns to normal once the
mixed--air temperature rises above 48_F(9_C). The
power exhaust fans will be energized and de--energized, if
installed, as the outdoor--air damper opens and closes.
If field--installed accessory CO2 sensors are connected to
the EconoMi$er IV control, a demand c ontrolled
ventilation strategy will begin to operate. As the CO2
leve l in the zone increases above the CO2 set point, the
minimum position of the damper will be increased
proportionally. As the CO2 l evel decreases because of the
increase in fresh air, the outdoor--air damper will be
proportionally closed. For EconoMi$er IV operation, there
must be a thermostat call for the fa n (G). If the unit is
occupied and the fan is on, the damper will operate at
minimum position. Otherwise, the damper will be closed.
When the EconoMi$er IV control is i n the occupied mode
and a call for cooling exists (Y1 on the thermostat), the
control will first check for indoor fan operation. If the fan
is not on, then cooling will not be activat ed. If the fan is
on, then the c ontrol will open the EconoMi$er IV damper
to the minimum position.
On the initial power to the E conoMi$er IV control, it will
take the damper up to 2 1/2 minutes before it begins to
position itself. After the initial power-- up, further changes
in damper position can take up to 30 seconds to initiate.
Damper movement from full closed to full open (or vice
versa) will take between 1 1/2 and 2 1/2 minutes. If free
cooling can be used as determined from t he appropriate
change over command (switch, dry bulb, enthalpy curve,
differential dry bulb, or differential enthalpy), then the
control will modulate the dampers open to maintain the
mixed--air temperature set point at 50_F(10_C) to 55_F
(13_C). If there is a further demand for cooling (cooling
second stage -- Y2 is energized), then the control will
bring on compressor stage 1 to maintain the mixed--air
temperature set point. The EconoMi$er IV damper will be
open at maximum position. EconoMi$er IV operation is
limited to a single compressor.
Heating
The sequenc e of operation for the heating is the same as
an electromechanical unit with no economizer. The only
difference is how the economizer acts. The economizer
will stay at the Economizer Minimum Position while the
evapora tor fan is operating. The outdoor--air damper is
closed when the indoor fan is not operating.
558J