Ventilation Optimization

In a typical VAV system, the rooftop unit delivers fresh outdoor air to several individually controlled zones. Demand-controlled ventilation (DCV) involves resetting intake airflow in response to variations in zone population. One approach to optimizing ventilation in a multiple-zone VAV system is to combine the various DCV strategies at the zone level (using each where it fits best) with ventilation reset at the system level. Ventilation reset involves resetting intake airflow based on variations in system ventilation efficiency.

For this approach, carbon-dioxide (CO₂) sensors are installed only in zones that are densely occupied and experience widely varying patterns of occupancy. For the example building in Figure 5, CO₂ sensors are installed only in the conference room and lounge. These zones are the best candidates for CO2 sensors and provide the "biggest bang for the buck." These sensors reset the ventilation requirement for their respective zones based on measured CO₂.

While DCV strategies commonly employ CO₂ sensors, occupancy sensors or time-of-day (TOD) schedules also can be used. Zones that are less densely occupied or have a population that varies only a little (such as private offices, open-plan office spaces, or many classrooms) probably are better suited for occupancy sensors. In Figure 5, each of the private offices has an occupancy sensor to indicate when the occupant is present. When unoccupied, the controller lowers the ventilation requirement for the zone. Occupancy sensors are relatively inexpensive, do not need to be calibrated, and already are used in many zones to control lights.

Finally, zones that are sparsely occupied or have predictable occupancy patterns can be controlled efficiently through the use of a TOD schedule. The schedule can indicate when the zone normally will be occupied/unoccupied or be used to vary the zone ventilation requirement based on anticipated population.

These various zone-level DCV strategies can be used to reset the ventilation requirement for their respective zones for any given hour. The zone-level control then is tied together using ventilation reset at the system level (Figure 6).

In addition to resetting a zone's ventilation requirement, the controller on each VAV terminal continuously monitors the primary airflow being delivered to the zone. A BAS periodically gathers this data from all of the VAV terminals and solves the ventilation-reset equations (prescribed by ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality) to determine how much outdoor air must be brought in at the rooftop unit to satisfy all of the zones served. Finally, the BAS sends this outdoor-airflow set point to the rooftop unit, which modulates a flow-measuring outdoor-air damper to maintain the new set point.

In a direct-digital-control/VAV system, this strategy is fairly easy to implement because the necessary real-time information already is available digitally. Combining DCV at the zone level with ventilation reset at the system level has many benefits, including:

• Ensuring that zones are ventilated properly without requiring a CO₂ sensor in each. CO₂ sensors are used only in the zones in which they can have the most benefit. This minimizes installation costs and avoids the periodic calibration and cleaning required to ensure proper sensor operation. For the other zones, occupancy sensors and TOD schedules can be used to reduce ventilation.
  
  
• Enabling documentation of actual ventilation-system performance. The VAV controllers communicate each zone's ventilation airflow to the BAS, even for the zones that do not have CO₂ sensors. The BAS can be used to generate reports showing ventilation airflow in every zone, hour by hour.
  
  
• Using system-level ventilation-reset equations that are defined in an industrywide standard. Using equations from ASHRAE Standard 62 improves the ability to defend the control strategy's use.

Conclusion

The impact of any energy-saving strategy on the operating costs of a specific building depends on climate, building usage, and utility costs. Building-analysis tools can be used to analyze these strategies and convert energy savings to operating-cost savings and help make financial decisions.

Figure 7 shows the potential energy savings of using these various strategies in an office building with a typical rooftop VAV system. The optimized system uses the optimal-start, supply-air-temperature-reset, and ventilation-optimization strategies discussed in this article. In addition, the supply fan is controlled based on fan-pressure optimization, rather than a constant set point in the ductwork. The optimized rooftop VAV system would reduce HVAC energy use by about 30 percent if the building were located in Atlanta and Los Angeles and by 33 percent if it were located in Minneapolis.

There is real potential to save energy in rooftop VAV systems through optimized system-control strategies. This savings can reduce a building owner's operating costs and help earn points toward Leadership in Energy and Environmental Design certification.

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Unoccupied Humidity Control

A VAV system typically dehumidifies effectively over a wide range of operating conditions because it continues to deliver cold, dry air at part-load conditions. As long as supply-air-temperature reset is used with caution and reheat is available for those VAV terminals that have high minimum-airflow settings or experience low cooling loads, a VAV system typically will provide supply air at a dew point that is low enough to prevent elevated indoor humidity levels during occupied periods.

However, controlling humidity levels always is a priority, not just when a building is occupied. When indoor humidity rises too high during unoccupied times, one option is to turn on the rooftop unit and dehumidify the recirculated air to about 55?F. However, there typically is little sensible load in the zones during these periods, so delivering cold air will result in overcooling. Reheat coils in the VAV terminals, and possibly a boiler and hot-water pumps, will need to be activated.

An energy-saving alternative is to equip the rooftop unit with hot-gas reheat. When after-hours dehumidification is needed, the rooftop unit turns on and diverts hot refrigerant vapor from the compressor through a refrigerant-to-air heat exchanger located in the air stream following the evaporator coil. Sensible heat is transferred from the hot refrigerant to reheat the dehumidified air leaving the evaporator.This strategy uses heat recovered from refrigeration circuits to reheat centrally and saves energy by avoiding the use of new energy to reheat remotely at VAV terminals.

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References

1. Murphy, J. (2007). Rooftop VAV systems applications engineering manual (SYS-APM007-EN). La Crosse, WI: Trane.
   
   
2. California Energy Commission. (2003). Advanced variable air volume system design guide. Sacramento, CA: California Energy Commission.

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An applications engineer for Trane, John Murphy, LEED AP, aids engineers in the proper design and application of HVAC systems. He is a member of several American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) technical committees and a contributing author of ASHRAE's recently published "Advanced Energy Design Guide for K-12 School Buildings."

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