Ingersoll-Rand ReliaTel, Manual, Page: 129 / 176

RT-SVD03L-EN 129
Barometric Relief
Units with economizers bring in outside air for
ventilation and/or economizer cooling. Because the
economizer contains a return air damper that operates
inversely to the outdoor air damper, the outdoor air will
tend to pressurize the conditioned space. This can
cause exterior doors to open or audible noise from air
escaping through various building openings.
Units without exhaust fans rely on barometric dampers
to vent the pressure that builds up in the space due to
outdoor airflow.
Power Exhaust
Exhaust fans offer improved performance since they
can be sized to overcome the pressure drops
associated with the return duct that would otherwise
add to the space pressure. This function allows the
exhaust fan to operate when appropriate to maintain
space pressure.
Units without RTOM - The power exhaust is on
whenever the economizer damper is at 25% outside air
or greater. This is not adjustable. The power exhaust is
off whenever the economizer damper is below 15%.
This is not adjustable.
Units with RTOM - The power exhaust set point (point
at which the power exhaust is turned on) is adjustable
from 0% to 100% at the RTOM “exhaust setpoint”
potentiometer. The power exhaust will be off when the
economizer damper falls below the Power Exhaust
Setpoint - 10%.
Economizer Control Methods
Three different methods can be used to determine if
outdoor air contains more cooling capacity than the
return air and are described below. The different
methods are suited for different applications and
environments.
• Comparative Enthalpy — Outdoor Air Enthalpy is
compared with Return Air Enthalpy. This method is
best suited for high humidity climates and
applications in which humidity can affect the
cooling capacity of the outdoor air or return air.
• Reference Enthalpy — Outdoor Air Enthalpy is
compared with a reference enthalpy point. This
method is best suited for high humidity climates in
which humidity can affect the cooling capacity of
the outdoor air, but not necessarily the return air.
• Reference Dry Bulb— Outdoor Air Temperature is
compared with a user set reference temperature.
This method is best suited for low humidity
climates and applications in which humidity does
not strongly affect cooling capacity of the outdoor
air or return air.
Dry bulb temperature and relative humidity data are
used to determine enthalpy.
Economizer-based cooling is enabled only when
outdoor air is determined to have more cooling
capacity than the return air. The method used is
according to the available data. When temperature and
humidity data are available for the outdoor air and
return air, the Comparative Enthalpy method is used.
One of the other methods are used if data is invalid or
unavailable. Ultimately, when there is insufficient data
to use any of the three methods, economizer-based
cooling is disabled.
Thermostat Notes
When the active unit mode is Cool, one of the enthalpy
or temperature methods are used to determine if
economizer-based cooling should be enabled or
disabled.
If the unit is applied with a thermostat, the algorithms
use a fixed mixed air temperature setpoint of 53° ±3°
when the Y1 input is closed. If the unit is applied with a
zone sensor, the algorithms use a dynamically
calculated mixed air temperature setpoint, which is
calculated by other algorithms, when cooling is
requested.
The damper can be in four different states.
• Closed — — The damper is held at 0%.
• Minimum Position — — The damper is held at the
Minimum Position as determined by the Minimum
Position Potentiometer on the ECA or by an edited
input from ICS. This position is between 0% and
50%.
N No otte e:: When making minimum position
adjustments, allow 30-45 seconds for
actuator to respond.
• Modulating — Algorithms control the damper to
meet cooling demand. When modulating, the
damper’s range of motion is between Active
Minimum Position and 100%.
• Night Setback — During NSTS, the damper drives
to full closed position. If there is a call for cooling,
the damper opens to satisfy the cooling
requirement. The damper stays closed during
heating mode.
During unoccupied mode, the minimum position is 0%.
The following Inputs Are Used
Mixed Air Sensor (MAS)
MAS measures the dry bulb temperature of the air
leaving the evaporator coil while economizing. Return
air, outdoor air and cooling caused by any compressor
cooling make up the mixed air input. The MAS is
plugged into the economizer actuator module (ECA).
Outdoor Air Sensor (OAS)
The OAS measures the ambient air surrounding the
unit. It is located in the compressor section on the
lower or upper left side for Precedent
™ units
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