How to Design a VAV System
A VAV system enables a single RTU to supply air to different zones with different cooling demands. The RTU provides air at a constant temperature. But the airflow will vary depending on the air pressure in the supply air duct. VAV terminal boxes placed downstream of RTU in the corresponding zones will control the zone temperature by modulating the airflow based on the input from a zone temperature sensor. A VAV system comprises the following:- Rooftop unit with an RTU controller controlling supply air temperature and supply duct static pressure. A VAV terminal box with direct digital control(DDC). SELECTING ROOF-TOP UNIT (RTU) RTU should have external static pressure (ESP) of atleast 2 in. wg. This ensures that each VAV terminal box receives sufficient airflow at the right air pressure to deliver the maximum design airflow to the zone. The supply air temperature/leaving air temperature should be constant at 55°F. Entering air temperature for cooling should be mix-air temperature for sy