Application

Room Segment Functions
Device Control Functions HVAC
59
Siemens Industry, Inc.
Application Note
A6V11692042
Smart Infrastructure – Building Products
2019-04-01
Where:
⚫ Area = area of duct
⚫ FlCnv = conversion constant for converting between box dimensional
engineering units and desired flow engineering units
Air Volume Flow as a percentage of the nominal airflow (Relative Air Flow
[VavSuAirFlRel]) is calculated by the formula:
Where:
⚫ AirFlNom is the nominal air volume flow of the VAV terminal box.
Pressure Dependent Control - Low airflow
If the box cannot satisfy airflow demands due to not having enough available
airflow (or the airflow sensor object is invalid), then open loop operation begins. In
open loop operation, damper position is controlled without using the measured air
volume flow value from the sensor; instead, damper position = airflow setpoint in %
of nominal airflow.
When the air volume flow setpoint falls below 5% of nominal or the measured air
volume flow value rises above 7% of nominal, closed loop operation resumes.
Airflow Sequencing and Airflow Limits
The VAV airflow controller adjusts airflow to accomplish cooling and ventilation,
and to support heating from the heating coil.
Each function has an operating range selected by the engineer to satisfy particular
physical objectives. This design separates minimum cooling flow and minimum
heating flow from the flow rate needed for ventilation.
The ventilation rate can be constant, vary with room occupancy mode, or vary
according to IAQ measurements. These factors have no effect on the flow rate
needed for the heating and cooling processes and devices (airflow through heating
coil, airflow needed for ceiling louver, etc.).
The following table provides suggestions for setting parameters affecting airflow
when the control system is in the cooling, ventilation and heating part of the control
sequence. See Operating Diagrams in Section 1 for a pictorial of the relationship of
these parameters.
Object
Description
Most common use
VavSuAirFlMaxC
Supply air VAV maximum air volume
flow for cooling
The highest flow rate for the terminal or space that corresponds
to cooling load.
VavSuAirFlMinC
Supply air VAV minimum air volume
flow for cooling
The lowest flow rate for the terminal or space that allows cooling
to work properly.
VavSuAirFlMaxH
Supply air VAV maximum air volume
flow for heating
The highest flow rate for the terminal or space and should be half
of the cooling max to comply with standard codes.
VavSuAirFlMinH
Supply air VAV minimum air volume
flow for heating
The lowest flow rate for the terminal or space that allows heating
to work properly.
VavSuAflMaxVnt
Supply air VAV maximum air volume
flow for ventilation
The highest flow rate that the DCV loop can request (can be equal
to max cooling).