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VAV-4020:
8/15/96
Subject: Damper Sequence of Operation for the VAV-4020.
-------VAV Mode, airflow
sensor is active: -------
Cooling: In this
control mode, the damper controls
to a CFM setpoint that is between the minimum and maximum cooling CFM
setpoints that are defined on the Control Settings and Parameters screen. The CFM setpoint that it attempts to
maintain is reset based on the space temperature. As the space temperature rises, the CFM setpoint increases,
but not more than the maximum CFM specified. As the space temperature falls, the CFM setpoint decreases,
but not less than the minimum CFM specified.
Heating: In the
VAV heating mode, the box will maintain the heating minimum CFM that is
specified on the Control Settings and Parameters screen. The reheat valve (or heating stages)
will then modulate to maintain space temperature.
------- VVT mode, airflow
sensor is active: -------
In the VVT mode,
the damper will modulate to try and maintain the heating and cooling space
temperature setpoints. The CFM will
be limited by the minimum and maximum values listed in the Control Settings
and Parameters. The action of the
damper will change based on whether the associated rooftop unit is in the
heating or the cooling mode of operation.
------- VVT Mode, airflow
sensor is inactive: -------
The damper will
modulate to try and maintain space temperature setpoints. The action of the damper will change
based on whether the associated rooftop unit is in the heating or the
cooling mode of operation.
____________________________________________
8/15/96
Subject: Inaccurate airflow
readings.
The airflow
sensor of a VAV-4020 does not use a pressure difference to calculate
airflow. It actually uses a flow of air.
Therefore, any excess restriction in the tubing will cause the
airflow readings to be inaccurate, especially under low airflow
conditions. If airflow readings
cannot be calibrated to be accurate across the full range air flow, make
sure you are using 3/8” nominal tubing (1/4” inside diameter) between the
pitot tubes and the airflow sensor of the VAV module. If smaller tubing is needed for
connection to the pitot tubes, adapt it to 3/8” as soon as it leaves the
pitot tubes. Keep the VAV-4020
within 3 feet of the pitot tubes to minimize the length.
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