BUILDING AUTOMATION
Demand-Controlled Ventilation
The right design strategy can lead to safe, more efficient environments.
One of the challenges we have with commercial buildings is the fine line between providing a safe and healthy indoor environment and maximizing energy efficiency. This challenge is exacerbated when it comes to the topic of the “V” in HVAC — ventilation. Ventilation air (or minimum outside air) is essential for balancing building pressurization and for removing, or diluting, the impact of indoor pollutants including various chemicals such as VOCs as well as CO2.
Of course, during part of the year, we can also use ventilation air for free cooling. Bringing in too little outdoor air can result in building pressure issues and can jeopardize the health of the building occupants. On the other hand, bringing in too much ventilation air can have a significant impact on the amount of energy required to heat, or cool and dry, the makeup air.
Ideally, we want to bring in just enough outdoor air to keep the building at a net positive pressure and to maintain healthy indoor environmental quality. Of course, this is not as easy as it sounds since the distribution of contaminants is not uniform. Fortunately, there has been extensive research and discussion conducted by the industry over the last several decades, much of which is reflected in ASHRAE Standard 62 and eventually adopted into state and local mechanical codes. While you should consult your local code for more details, the general trend for ventilation management is as follows.
Ventilation Design
The codes and standards for ventilation design have changed significantly over the last 20 years. We often find existing buildings that may still be operating as originally designed (back when indoor smoking was common) or those that over the years have gone too far in reducing ventilation levels. For any new or existing building, the first step in evaluating ventilation is to look closely at the building pressurization, occupancy, code when constructed, and the current code requirements. Often, we find that just rebalancing to the current code can have a significant impact on energy efficiency.
Demand-Controlled Ventilation
Many state codes now allow (or require) for the use of demand-controlled ventilation or DCV. The concept of DCV is to measure and control the amount of ventilation air based on indicators of occupancy. ASHRAE Standard 62 has a series of tables that outline the required levels of building area and occupant ventilation for various building types. The area rates are intended to deal with the impact of chemicals and odors and must be provided at all times when the building is occupied.
The people portion of ventilation can be varied based on occupancy using counts or an indicator such as CO2. In order to properly apply DCV, you need a method of measuring and controlling ventilation air as well as a way of measuring occupancy. The use of DCV is a natural for buildings, or areas of buildings, that have highly variable occupancy. For example, a church or an auditorium is a perfect application. Properly applying DCV in facilities with VAV systems is more complicated to do properly.
Proper management of ventilation is an essential task for any BAS, and when properly applied, it can result in a building that is both healthy and efficient.
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