Application Guide

63-7062 34

Variable Air Volume (VAV) Primary Plant Control (See Fig. 40)

Honeywell

EMC

Controller

RST

MON PAR REF BTNS

RUN READY FAULT

Honeyw ell

RST

MON PAR REF BT NS

RUN REA DY FAU L T

Honeyw ell

Fig. 40. Application Diagram.

Application Description

A Variable Air Volume (VAV) system controls the space temperature by varying the volume of supply air, rather than the supply

air temperature. The interior zones of most large buildings normally require cooling only, because of occupancy and lighting

loads. Air terminal units serve these zones and operate under thermostatic control to vary the airflow into the space to maintain

the required temperature. The perimeter zones often have a variable load, dependent on the season and the losses through

the building fabric. Heating in these areas may be supplied via reheat coils while the air terminal units maintain minimum

airflow.

Airflow in the supply duct varies as the sum of the airflow through each unit varies. In light load conditions the air terminal units

reduce airflow; as more cooling is required, the units increase airflow. When the VAV terminal unit dampers open, the static

pressure drops in the supply duct; the sensor detects the pressure drop and the controller increases the speed of the supply

fan. The opposite occurs when the VAV terminal unit dampers close.

A feature of VAV systems is that the minimum outside air delivered by the system is determined by the difference in air flow

between the supply and return fans (and not the position of the outdoor air damper). To increase the volume of outside air, the

return air damper is regulated. This airflow control provides a slightly positive building static pressure with respect to outdoor

air, in a properly designed system. As supply air volume is reduced, so is return air volume. The return air fan is normally sized

smaller than the supply air fan. Air velocity sensors are located in both the supply and return ducts, so the control of the return

air fan mimics the supply air fan; the airflow in both ducts are controlled with a constant differential.

Benefits

– Reduced electrical running costs; especially when compared with the traditional guide vane control—with no energy

savings—where the motor is running at full speed all the time.

– Maintenance savings arise from running the motor at reduced speed which lowers the load torque; lessens mechanical

stress on belts, bearings.