Rooftop variable-air-volume (VAV) systems are used to provide comfort in a wide range of building types and climates.¹ This kind of system consists of a packaged rooftop air conditioner that serves several individually controlled zones. Each zone has a VAV terminal unit that is controlled by a temperature sensor. This article will discuss HVAC-system control strategies that can be used to save energy in rooftop VAV systems.

Optimal Start/Stop

In some buildings, a simple time clock or time-of-day schedule is used to start and stop the HVAC system. When a building is expected to be unoccupied, the system is shut off and the temperature allowed to drift away from the occupied set point. The time at which the system is to restart typically is set to ensure that the indoor temperature reaches the desired occupied set point prior to occupancy on either the coldest or warmest morning of the year. As a result, for most days, the system starts much earlier than needed. In turn, this increases the number of operating hours and system energy use.

An alternative approach is a strategy called "optimal start." This strategy utilizes a building-automation system (BAS) to determine the length of time required to bring each zone from its current temperature to the occupied set-point temperature. The system waits as long as possible before starting, so the temperature in each zone can reach the occupied set point just in time for occupancy (Figure 1).

This optimal starting time is determined using the difference between the actual zone temperature and occupied set point. It compares this difference with the historical performance of the zone warming up or cooling down.

The optimal-start strategy reduces the number of system operating hours and saves energy by avoiding the need to maintain the indoor temperature at the occupied set point even though the building is unoccupied.

A related strategy is called "optimal stop." As mentioned previously, at the end of an occupied period, the HVAC system is shut off and the temperature allowed to drift away from the occupied set point. However, the building occupants may not mind if the indoor temperature drifts just a few degrees before they leave for the day.

Optimal stop uses a 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 the occupied set point (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.

The optimal-stop 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, 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 approximately two-thirds of the distance along the main supply duct. The rooftop unit varies the capacity of the supply fan to maintain the static pressure in this location at a constant set point. With this approach, however, the system usually generates more static pressure at part load than necessary.

When communicating controllers are used on VAV terminals, it is possible to optimize this static-pressure control function to minimize duct pressure and save fan energy. Each VAV-unit controller knows the current position of its air-modulation damper. The BAS continually polls these individual controllers, looking for the VAV terminal with the damper that is open the widest (Figure 2). The supply fan's set point then is reset to provide just enough pressure so that at least one damper is nearly wide open. This results in the supply fan generating only enough static pressure to push the required quantity of air through this "critical" VAV-terminal unit.

This control strategy, which sometimes is called "fan-pressure optimization," has several benefits:

• Reduced supply-fan energy use. At part-load conditions, the supply fan is able to operate at a lower static pressure and consume less energy (Figure 3).
  
  
• Lower sound levels. The supply fan does not generate as much static pressure and typically generates less noise. In addition, with lower pressures in the supply duct, the dampers in the VAV terminals open wider, resulting in less regenerated noise.
  
  
• Reduced risk of fan surge. With the fan operating at a lower pressure when delivering reduced airflow, the fan operating point is kept farther away from the surge region (Figure 3).

Supply-Air-Temperature Reset

In a VAV system, it is tempting to raise the supply-air temperature at part-load conditions to save compressor and/or reheat energy. Increasing supply-air temperature reduces compressor energy because it allows a compressor to operate at a warmer suction temperature. The corresponding higher suction pressure reduces compressor lift, reducing the power required.

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