High-Performance VAV Systems

Optimized VAV System Controls The first key ingredient to make a VAV system truly “high-performance” is the use of optimized system control strategies.1 Optimal start/stop. Optimal start is a control strategy that uses a building automation system (BAS) to determine the length of time required to bring each zone from current temperature to the occupied setpoint temperature. Then the system waits as long as possible before starting, so that the temperature in each zone reaches occupied setpoint just in time for occupancy (Figure 1). This strategy reduces the number of system operating hours and saves energy by avoiding the need to maintain the indoor temperature at occupied setpoint even though the building is unoccupied. Optimal stop is a control strategy that uses the BAS to determine how early heating and cooling can be shut off for each zone so that the indoor temperature drifts only a few degrees from occupied setpoint before the end of scheduled occupancy (Figure 1). In this case, only cooling and heating are shut off; the supply fan continues to operate and the outdoor-air damper remains open to continue ventilating the building. This strategy also reduces the number of system operating hours, saving energy by allowing indoor temperatures to drift early. Fan-pressure optimization. As cooling loads change, the VAV terminals modulate to vary airflow supplied to the zones. This causes the pressure inside the supply ductwork to change. In many systems, a pressure sensor is located