Proper BAS monitoring can help juggle pressure, economizer strategy, and occupant needs.
The proper management of outdoor air is an ongoing challenge for commercial buildings, and one of the key functions that needs to be managed by a building automation system. Controlling outdoor air is a matter of balance. Here are the key parameters we need to monitor and control.
Building pressure. Ideally, we want the building to be at a neutral or slightly positive pressure at all times. This helps to minimize air infiltration, avoids “wind tunnels” in the lobby, and makes it easy to open and close doors to the outside. Outdoor air is required to balance the flow of exhaust required for bathrooms, kitchens, and other functions. From a controls perspective, there are several ways to manage building pressure. The best solution is to attempt to measure building pressure differential, and control directly based on this value. Since it can be a challenge to get good sensing locations, an alternate solution is to calculate anticipated exhaust flow and balance with a like amount of supply air.
Free cooling or economizer. Generally, we would like to use outdoor air to cool the building whenever possible. There are many hours of the year, even in hot humid climates, where outdoor air is all that is needed to provide for cooling. An outdoor air economizer can also be used in conjunction with mechanical cooling during other hours of the year. Management of an outdoor air economizer requires special attention to the indoor and outdoor air conditions (temperature and humidity) as well as building pressure. Special care is required in deciding when to change in and out of the air economizer, and in protecting coils when economizing in sub-freezing weather.
Indoor air quality (IAQ) management. The final part of the outdoor air balance is in managing for proper IAQ. Outdoor air can be used to dilute the contaminants (odors, CO2, volatile organic compounds, etc.) that result from the material in the building as well as from the occupants. Note that a portion of the ventilation air needed for dilution is provided as part of the building pressurization, and generally when in economizer mode, there is ample ventilation air provided. Conditioning outdoor air for dilution is fairly energy intense. It has to be heated in the winter and cooled and dehumidified in the summer. There are several approaches to best managing dilution air:
- The amount of ventilation air can be controlled using demand-controlled ventilation (DCV). This strategy allows you to reduce the “people part” of the ventilation equation based on feedback from CO2 sensors or other methods such as occupant counting.
- Utilize ASHRAE standard 62.1, which provides details for how to best manage IAQ and has been accepted as part of code in most states.
- Note that the latest version of standard 62.1 now allows for alternatives to dilution-based ventilation as part of an IAQ plan. For example, sorbent-based systems utilize a chemical that is able to absorb CO2 and VOC’s, and then use a heater and fan to periodically “regenerate” the sorbent. These systems generally include their own packaged controls and may also be able to interface to the BAS. Careful consideration should be used in designing the control sequences for these new systems.
Developing sequences for outdoor air management that balances the items above can be a challenge, but they are critical for the safe and efficient operation of any building. ES
