87
SEQUENCE OF OPERATION
General
The sequence below describes the sequence of operation
for an electro--mechanical unit with and without a fact ory
installed EconoMi$ert IV (called “economizer” in this
sequence ). For information regarding a direc t di gital
controller, see the start--up, operations, and
troubleshooting manual for the applicable controlle r.
Electr o--mechanical units with no economizer
Cooling —
When the thermostat calls for cooling, terminals G and Y1
are energized. As a result, the indoor--fan contactor (IFC)
and the compressor contactor (C1) are energized, causing
the indoor--an motor (IFM), compressor #1, and out door
fan to start. If the unit has 2 stages of cooling, the
thermostat will additionally energize Y2. The Y2 signal
will energize compressor contactor #2 (C2), causing
compressor #2 t o start. Regardless of the number of
stages, the outdoor--fan motor runs continuously while
unit is cooling.
Heating
NOTE: The 50TC is sold as cooling onl y. If electric
heaters are required, use only factory--approved electric
heaters. They will operate as described below.
Units have either 1 or 2 stages of electric heat. When the
thermostat calls for heating, power is applied to the W1
terminal at the unit. The unit control will energize the
indoor fan contactor and the first stage of electric heat. On
units with two--stage heating, when additional heating is
required, the second stage of electric heat (if equipped)
will be energized when power is applied at the W2
terminal on the unit.
Electr o--mechanical units with an economizer
Cooling —
When free cool ing is not available, the c ompressors will
be controlled by the zone thermosta t. When free cooling is
avai lable , the outdoor--air damper is modulated by t he
EconoMi$er IV control to provide a 50_F(10_C) to 55_F
(13_C) mixed--air temperature into the zone. As the mixed
air tem perature fluctuates above 55_F(13_ C) or below
50_F(10_C) dampe rs 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(9_C), then the outdoor--air damper position
will be decreased to the minimum position. If the
mixed--air temperature continues to fall, t he outdoor--air
dampe r will close. Control returns to normal once the
mixed--air t empera ture ri ses above 48_F(9_C). The
power exhaust fans will be energized and de --energized, if
installed, as the outdoor--air dam per opens and closes.
If field --installed accessory CO
2
sensors are connected to
the EconoMi$er IV control, a demand controlled
ventilation strategy will begin to operate. As the CO
2
leve l i n the zone increases above the CO
2
setpoint, the
minimum position of the damper will be increased
proportionally. As the CO
2
level decreases because of the
increase in fresh air, the outdoor--air damper will be
proportionally closed. For EconoMi$er IV operation, there
must be a thermostat call for the fan (G). If the unit is
occupied and the fan is on, the damper wil l ope rate at
minimum position. Otherwise, the damper will be closed.
When t he EconoMi$e r IV c ontrol is in the oc cupied m ode
and a call for c ooling exists (Y1 on the thermostat), the
control will first check for indoor fan operation. If the fan
is not on, the n cooling will not be act ivated. If the fan is
on, then the control will open the EconoMi$er IV damper
to the minimum position.
On the initial power to the EconoMi$er IV control, it will
take the dam per up to 2 1/2 minutes before i t begins to
position itself. After the initial power--up, further changes
in damper position can take up to 30 seconds to initiate.
Damper movement from full closed to full open (or vice
versa) will ta ke between 1 1/2 and 2 1/2 minutes. If free
cooling c an be used as determined from the appropriate
change over comma nd (switc h, dry bulb, enthalpy curve,
differential dry bulb, or differential enthalpy), then the
control wil l modulate the dampers ope n t o maintain the
mixed--air temperature setpoint a t 50_F(10_C) to 55_F
(13_C). If there is a further demand for cooling (cooling
second stage -- Y2 is energized), then the control will
bring on compressor stage 1 to maintain the mixed--air
temperature setpoint. The EconoMi$er IV damper will be
open at maximum position. EconoMi$er IV operation is
limited to a single compressor.
Heating
The sequence 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 opera ting. The outdoor--air damper i s
closed when the indoor fan i s not operati ng.
Optional Humidi--MiZer Dehumidification System
Units with the factory--equipped Humidi --MiZer option
are capable of providing multiple modes of improved
dehumidification as a variation of the normal cooling
cycl e. The Humidi--MiZer option includes additional
valves in the liquid line and discharge line of each
refrigerant circuit, a small reheat condenser c oil
downstream of the evaporator, and Motormaster
variable--speed control of some or all outdoor fans.
Operation of the revised refrigerant circuit for each mode
is described below.
NOTE: x = refrigerant circuit A, B, or C.
50TC
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