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Operating Principles
with the highest priority attempts cooling first. Once it is
operating at its maximum, and if additional cooling is
necessary, the other economizer enables before
mechanical cooling begins.
Cooling/Waterside Economizer
Waterside economizing enables when the unit's entering
water temperature is below the unit's entering mixed air
temperature by 4°F plus the user adjustable economizer
approach temperature. The approach temperature default
is 4°F.
Waterside economizing disables when the unit's entering
water temperature is not below the unit's entering mixed air
temperature by at least the water economizer approach
temperature (default value of 4°F).The economizer acts as
the first stage of cooling. If the economizer is unable to
maintain the zone (CV units) or supply air (VAV units)
temperature setpoint, the compressor module will bring on
compressors as required to meet the setpoint.
Cooling/Airside Economizer
On units with an airside economizer, a call for cooling will
modulate the fresh air dampers open. The rate of
economizer modulation is based on deviation of the zone
temperature from setpoint; i.e., the further away from
setpoint, the faster the fresh air damper will open. The first
stage of cooling will start after the economizer reaches full
open.
Note: The airside economizer will only function freely if
ambient conditions are below the enthalpy control
settings or below the return air enthalpy if unit has
comparative enthalpy installed. If outside air is not
suitable for "economizing," the fresh air dampers
drive to the minimum open position. A field
adjustable, factory default setting at the HI panel or
Tracer Summit can provide the input to establish the
minimum damper position.
When outdoor air conditions are above the setpoint or
comparative enthalpy control setting, only mechanical
cooling will function and outside air dampers will remain at
their minimum position.
Mechanical Cooling
If the zone temperature cannot be maintained within the
setpoint deadband using the economizer option or if there
is no economizer, the RTM sends a cooling request to the
MCM. The compressor module checks the compressor
protection circuit before closing stage one. After the first
functional stage starts, the compressor module monitors
the saturated refrigerant temperature and closes the
condenser fan output contact when the saturated
refrigerant temperature rises above the lower limit setpoint.
Water-Cooled Units Only
The WSM modulates the condenser coil water valves to
maintain condenser temperature, if applicable. Otherwise,
it will check the entering condenser water temperature to
52
ensure it is greater than 54°F or if not, it will lock out
cooling.
Auto Changeover (Units with Heat Only)
When the system mode is in auto, the mode will change to
cooling or heating as necessary to satisfy the zone cooling
and heating setpoints. The zone cooling and heating
setpoints can be as close as 2°F (1.1°C).
Occupied Zone Temperature—Heating
Relies on input from a sensor directly in the space, while a
system is in occupied heating mode or an unoccupied
period, to stage electric heat on and off or modulate the
hydronic heating valve as required to maintain the zone
temperature within the heating setpoint deadband. The
supply fan will operate when there is a request for heat.
Electric Heat
On units with electric heat, the zone temperature can be
controlled to a heating setpoint during the occupied mode
by cycling a single stage electric heater. An interface can
be provided for field supplied electric heat (up to three
stages).The zone temperature heating setpoint and
deadband are user defined at the HI panel.
Hydronic Heat: Hot Water or Steam
On units with hot water or steam heating, the zone
temperature can be controlled to a heating setpoint during
the occupied mode. The zone temperature heating setpoint
and deadband are user defined at the HI panel or zone
sensor. VAV occupied heating initiates by closing a field-
supplied switch or relay contacts connected to the
changeover input on the RTM. Supply air static pressure is
maintained.
Supply Air Setpoint Reset (VAV Units
Only)
Supply air reset can be used to adjust the supply air
temperature setpoint on the basis of a zone temperature or
outdoor air temperature. Supply air reset adjustment is
available at the HI panel for supply air heating and supply
air cooling control.
Reset Based on Outdoor AirTemperature
Outdoor air cooling reset is sometimes used in applications
where the outdoor temperature has a large effect on
building load. When the outside air temperature is low and
the building cooling load is low, the supply air setpoint can
be raised, thereby preventing subcooling of critical zones.
This reset can lower usage of mechanical cooling, thus
savings in compressor kW, but an increase in supply fan
kW may occur.
Outdoor air heating reset is the inverse of cooling, with the
same principles applied.
SCXF-SVX01T-EN
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