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AUTOMATIC FAN SPEED CONTROL
The control is capable of controlling up to three fan speeds. The
motor will operate at the lowest speed possible to provide quiet
and efficient fan operation. The motor will increase speed if addi-
tional cooling or heating is required to maintain the desired space
temperature set point. The control increases the motor's speed by
one step for each 2°F above the cooling or below the heating set
point. Also, the control will increase the fan speed as the Supply
Air Temperature approaches the configured minimum or maxi-
mum limits. Fan speed for PSC motors is controlled by energizing
and de-energizing low, medium and high speed relays. Fan speed
for ECMs is controlled by first energizing the low speed tap. If a
higher speed is required, the low speed tap remains energized
while the medium speed tap is also energized. If high motor speed
is required, all three speed taps are energized. If more than one
speed tap is energized for an ECM, the ECM will default to the
higher speed.
FAN SPEED CONTROL - DURING HEATING
Whenever heat is required and active, the control continuously
monitors the supply air temperature to verify it does not rise above
the configured Maximum Heating SAT Limit (110°F Default). As
the SAT approaches this value, the control will increase the fan
speed as required to ensure the SAT will remain within the limit.
This feature provides the most quiet and efficient operation by op-
erating the fan at the lowest speed possible.
FAN SPEED CONTROL - DURING COOLING
Whenever mechanical cooling is required and active, the control
continuously monitors the supply air temperature to verify it does
not fall below the configured Minimum Cooling SAT Limit (50°F
Default). As the SAT approaches this value, the control will in-
crease the fan speed as required to insure the SAT will remain
within the limit.
MODULATING WATER ECONOMIZER CONTROL
The control has the capability to modulate a water valve to control
condenser water flowing through a coil on the entering air side of
the unit.
Cooling
The purpose is to provide an economizer cooling function by using
the water loop when the entering water loop temperature is suitable
(at least 5°F below space temperature). If the water loop conditions
are suitable, then the valve will modulate open as required to main-
tain a Supply Air Temperature that meets the load conditions.
Should the economizer coil capacity be insufficient for a period
greater than 5 minutes, or should a high humidity condition occur,
then the compressor will be started to satisfy the load. As the SAT
approaches the Minimum Cooling SAT limit, the economizer valve
will modulate closed during compressor operation.
Heating
Additionally, the control will modulate the water valve should the
entering water loop temperature be suitable for heating (at least
5°F above space temperature) and heat is required. The valve will
be controlled in a similar manner except to satisfy the heating re-
quirement. Should the coil capacity be insufficient to satisfy the
space load conditions for more than 5 minutes, then the compres-
sor will be started to satisfy the load. As the SAT approaches the
Maximum Heating SAT limit, the economizer valve will modulate
closed during compressor operation.
2-POSITION WATER ECONOMIZER CONTROL
The control has the capability to open a NO or NC, two position,
water valve to control condenser water flow through a coil on the
entering air side of the unit.
Cooling
The purpose is to provide a cooling economizer function directly
from the condenser water loop when the entering water loop tem-
perature is suitable (at least 5°F below space temperature). If the
optional coil is provided and the water loop conditions are suit-
able, then the valve will open to provide cooling to the space when
required. Should the capacity be insufficient for a period greater
than 5 minutes, or should a high humidity condition occur, then
the compressor will be started to satisfy the load. Should the SAT
reach the Minimum Cooling SAT limit, the economizer valve will
close during compressor operation.
Heating
Additionally, the economizer control will open the water valve
should the entering water loop temperature be suitable for heating
(at least 5°F above space temperature) and heat is required. The
valve will be controlled in a similar manner except to satisfy the
heating requirement. Should the coil capacity be insufficient to
satisfy the space load for more than 5 minutes, then the compres-
sor will be started to satisfy the load. Should the SAT reach the
Maximum Heating SAT limit, the economizer valve will close
during compressor operation.
POWER FAIL RESTART DELAY
The control provides a delay when recovering from a power fail-
ure in order to ensure stable utility power and to prevent excessive
demand when many units start simultaneously. Each unit can be
user configured for a unique delay between 0 and 600 seconds.
The factory programmed default delay is 60 seconds. (Note that
the onboard control will not start the compressor on any call for
heating, cooling or dehumidification until 5 minutes has elapsed
from the power restoration. If a lower restart time delay is config-
ured, only the fan start will be affected as the internal logic boards
will prevent compressor operation for more than 300 seconds).
SUPPLY
AIR
TEMPERATURE
CONTROL / ALARM
The control has 2 configurable control limits for supply air tem-
perature. The control will monitor the supply air temperature
(SAT) and verify it is within limits. During cooling, the control
will increase fan speed and reduce compressor stages should the
SAT approach the Maximum Cooling SAT limit. Likewise, during
heating, should the SAT approach the Maximum Heating SAT
limit, the fan speed will be increased, followed by reducing com-
pressor stages. Auxiliary heating coils are controlled so as not to
exceed the Maximum Heating SAT limit. Additionally, a separate
High SAT Alarm Limit and Low SAT Alarm Limit are provided
so that an alarm can be generated to indicate an abnormal SAT
condition should the SAT exceeds the alarm limit for more than
1 minute.
DEHUMIDIFICATION
The control can monitor the space relative humidity and if the unit
is equipped with the factory installed hot gas reheat, whenever the
humidity exceeds the appropriate (occupied or unoccupied) hu-
midity set point and if the unit is not heating or cooling, the control
will activate cooling (compressor and reversing valve) and the hot
gas reheat outputs to start dehumidification. The fan will operate
at medium speed if equipped with a three speed fan.
SPACE TEMPERATURE ALARMS
The control provides the ability to generate an alarm whenever
the space temperature exceeds the alarm set point. A separate oc-
cupied hysteresis and unoccupied high and low alarm set points
are provided. The control provides a 5 minute alarm delay
during unoccupied periods. During occupied periods, the control
uses the occupied temperature set point and applies the hystere-
sis value to determine the alarm set points. Whenever an occu-
pancy transition from unoccupied to occupied occurs or the oc-
cupied temperature set points are changed causing an alarm con-
dition to occur, the control will automatically calculate an alarm
delay (equivalent to the configured delay time in minutes / deg F
times the temperature error that occurred). This will prevent nui-
sance alarms whenever an occupancy change occurs and allows
time for the unit to correct an alarming temperature condition.
52
MONITORING
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