Ventilation
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4 |
Improves IAQ, protects occupant health and can reduce HVAC energy consumption |
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3 |
One to three months for ventilation system maintenance and adjustments, three to six months to select, install and configure CO2 sensors, and six to 12 months for implementation of demand-controlled ventilation |
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2 |
Budget for testing and balancing, replacement filters, and CO2 sensors for high-occupancy spaces |
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FAC, TAB, BPO |
Ventilation supplies outdoor air to dilute pollutants of indoor origin, increase oxygen levels, and remove stale air. Ventilation may be accomplished by a mechanical AHU, operable windows, or both. Either too little or too much ventilation can be a problem. A building with insufficient outside air may smell, feel stuffy, and affect staff productivity. Over-ventilation results in high energy costs and may make humidity control difficult. ASHRAE maintains standards for minimum outside air ventilation rates in buildings. The current ASHRAE requirement for office space is 8.5 liters per second (L/s) (17 cubic feet per minute (CFM)) per person, given typical occupant density of five people per 100 square meters (m2) (1,000 square feet (ft2)).
Research by the Department of Energy’s Lawrence Berkeley National Laboratory suggests that decision-making performance declines with indoor CO2 levels as low as 1000 parts per million (ppm), far below OSHA’s current exposure limit of 5000 ppm.12
IAQ problems often result from improper pressurization, which causes unwanted airflow from outdoors to indoors or between areas within the building. To reduce the introduction of unconditioned outdoor air and pollutants, the building should operate with slight positive pressure to the outdoors.
If the building is designed with a tiered pressurization scheme, the delta between each tier should be a minimum of 2.5 Pascals (Pa) and the lowest tier should be a minimum of 2.5 Pa above the outdoors (Resources: Building Air Quality Guide: A Guide for Building Owners and Facility Managers | US EPA).
Even though CO2 occurs naturally in the atmosphere at low concentrations, elevated levels correlate to human activity (breathing) and can indicate inadequate dilution of indoor contaminants. As a result, CO2 sensors can be used as a means of monitoring IAQ. ASHRAE recommends that indoor CO2 levels be maintained at a maximum of 700 ppm above outdoor levels;13 OBO criteria state 900 ppm, not relative to outdoor conditions.
Figure 5: Balancing pollutant concentration and energy use
Image Source: Air Infiltration and Ventilation Centre
Best practices for monitoring and reducing CO2 levels include:
- Installation of CO2 sensors in occupied areas, especially in spaces where a high number of occupants may congregate.
- Selection of sensors with an alarm function and programming the alarm to notify occupants or the building engineer when CO2 levels rise above 900 ppm.
- Development of a plan to recalibrate CO2 sensors every five years, or at the manufacturer’s recommended interval.
- Research of sensor options for automated demand control ventilation (DCV). DCV regulates the amount of outside air based on number of occupants in the building and is particularly appropriate for spaces where occupancy levels vary, such as multi-purpose rooms. DCV control systems vary based on the existing equipment and control system, and are likely to require OBO engineering services (see Resources: Demand Controlled Ventilation Using CO2 Sensors for guidance).