Room Sensor Placement; Pressure Independence; Vma Airflow Rate - Johnson Controls VMA 1410 Overview And Engineering Manuallines

10 VMA1400 Series Overview and Engineering Guidelines Technical Bulletin

Room Sensor Placement

Pressure Independence

VMA Airflow Rate

When considering room sensor placement for each zone:
• Verify that the room sensor is the correct one for the
application.
• Review architectural requirements such as furniture height and
location, aesthetics, and type of mounting. Wall plates are
required if mounting on a conduit handibox.
• Review room sensor location. The best room sensor location is
on an interior wall, about 1.5 m (5 feet) above the floor, out of
direct sunlight, out of the direct path of supply air from the
diffuser, away from heat sources, such as equipment, machines,
and perimeter radiation, and away from doors and other draft
sources. Local codes or disabilities act requirements
occasionally influence the actual mounting height.
• Do not locate a sensor near zone boundaries, where primary
influence is from an adjacent zone.
The pressure independent VMA employs patented self-tuning,
cascaded proportional/integral control loops. The zone temperature
loop samples space temperature and resets the airflow setpoint
between the minimum and maximum flow settings. Since inlet duct
static pressure influences the amount of air passing through the VAV
box, the VMA airflow loop samples airflow via a flow pickup in the
box inlet. It modulates the damper to control the flow. Thus, the
VAV box flow is independent of duct static pressure.
The engineering basis for this method of control is that the temperature
of a space with a constant load is linearly proportional to the flow of
conditioned air into the space. The engineer must accurately determine
the required maximum and minimum flow for each space based on
heating, cooling, and ventilation loads.
The VMA determines airflow rate by dynamic pressure measurement.
The VMA contains a Differential Pressure Transducer (DPT) to sense
velocity pressure in pressure independent VAV applications.
The DPT is connected to the VAV box airflow pickups. It measures
velocity pressure and generates a proportional voltage signal. The
VMA reads voltage signal from the DPT and converts it to airflow in
cubic feet per minute (cfm), liters/second, or cubic meters/hour.
Calibration is not required, with the exception of zero calibration,
which the controller performs automatically as set in the configuration.