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HVAC engineers have long known that monitoring carbon dioxide (CO2) is helpful in controlling indoor air quality; however, until recently, the practice was discouraged by the cost of the sensors and the efforts required to install, monitor, and calibrate them.
Today, CO2 monitoring is growing. With the advent of LEED, the increase in digital controls in buildings, and the wider availability of the sensors, CO2 monitoring is becoming standard practice. In part, this is because the control manufacturers can now link to sophisticated computer networks capable of managing every device in a building; in part, it’s because CO2 monitoring can also play a role in reducing energy costs. In some cases, it can even reduce equipment capital costs by reducing peak heating and cooling loads.
For new projects and retrofits, project teams now see CO2 sensors as part of the job for three prime reasons: they ensure good air quality, they form a key part of an energy-efficiency strategy, and they earn LEED points.Why Monitor CO2?
Buildings can have many different indoor pollutants, ranging from volatile organic compounds emitted by paints and new furnishings to pollutants from chemical uses, equipment, and cleaning processes. In contrast, CO2 is benign – it is part of every breath we take. But because we emit it, CO2 becomes a proxy for other pollutants and indicates whether a ventilation system is operating properly.
While CO2 is not a pollutant, it does have an effect on humans. When CO2 levels rise, occupants feel sleepy and sluggish, have difficulty focusing, may become less productive, and may even get a headache. Pity the presenter in a room with high CO2 levels, because no matter how interesting the material, heads will start to hit the desks.
But CO2 sensing can do more than just monitor room conditions. Engineers can also use CO2 sensing in demand control ventilation, in which the volume of ventilation supplied is based on the actual number of occupants and their activity. Ventilating a building requires heating and cooling outside air. There is always tension in this process. From an energy-efficiency perspective, bringing in as little outside air as possible is optimal; from a ventilation perspective, maximizing outside air intake is desirable. Demand control ventilation using CO2 sensors allows you to adjust the ventilation rate to suit immediate and changing needs.
For instance, in a theater on the night of a performance, there might be 2,000 people in one room, requiring the ventilation system to run at full volume. Earlier in the day during rehearsal, however, there might be 20 people in the same space. In a conventional system the theater would likely be either underventilated during the performance or overventilated during rehearsal because it could not react to a change in occupancy. In the process, a lot of energy might be wasted. But with a demand control scheme, the system would modulate the outside air intake in response to need, saving energy during times of partial occupancy. CO2 sensors in the occupied space would monitor continuously, and in partial occupancy conditions, would recognize that large amounts of fresh air are not required.
LEED rewards CO2 monitoring in two key credits. The principal credit is EQ (Indoor Environmental Quality) Credit 1 – Outdoor Air Delivery Monitoring. The intent of this credit is “to provide capacity for ventilation system monitoring to help sustain occupant comfort and well-being.” This credit has two components within its requirements. First, it requires that all delivery systems have a direct means of outdoor airflow measurement. This is typically accomplished with airflow measuring stations located in air-handling units. Second, it requires room monitoring of carbon dioxide concentrations in all “densely occupied” spaces – and here is where the sensors come in.
It is important to understand what LEED means by “densely occupied.” The credit defines a densely occupied space as a room with an occupancy level greater than 25 people per 1,000 square feet (or one person per 40 square feet). This means spaces like classrooms, conference rooms, and other assembly areas are usually considered “densely occupied.” This requirement applies regardless of the size of the room; a small conference room and a large lecture hall have the same basic requirements. In these spaces, the credit requires carbon-dioxide monitoring.
LEED is very specific about the location of sensors, requiring them to be between 3 and 6 feet above the finished floor in what is known as the “breathing zone.” This is the space in a room where people inhale and exhale. Previous standards of practice put sensors in the return air duct, something unacceptable under LEED because this location does not sense actual room conditions that humans will experience.
Figure 1 illustrates the reason. In many buildings, the air supply diffuser and the return air grille in a room are located on the ceiling. In some situations, the ventilation air supplied to the room may short circuit back to the return air without ever interacting with the room occupants. In this situation a sensor in the return duct would indicate - inaccurately - that the ventilation was adequate.
It is also not uncommon for designers to put the sensors in the return main for a number of rooms on a floor. In this situation, the sensor cannot monitor individual room conditions and it reads only an average of the various rooms served. This means assembly rooms served by the main could be underventilated and their lack of outdoor air could go undetected.
Although EQ Credit 1 specifies the installed height of sensors, it does not require the full implementation of a demand control system. Once sensors are installed, however, installing a demand control scheme is usually an easy option, and may potentially be rewarded with additional points under EA (Energy and Atmosphere) Credit 1 – Optimize Energy Performance.
The 46 Blackstone Street renovation at Harvard University in Cambridge, MA, provides an excellent example of how demand control ventilation and carbon-dioxide sensing can be incorporated into a LEED Platinum project to maintain good performance and reduce energy consumption. This renovation of a 19th-century power station into office space for the Harvard Green Campus and other Harvard Facilities Management groups was led by architects at Bruner Cott in Cambridge, with Arup as consulting engineers and Marc Rosenbaum as energy consultant.
Blackstone’s HVAC system is a dedicated outdoor air system (DOAS). Unlike a conventional VAV system, a DOAS separates heating and cooling from the ventilation so that the building receives appropriate ventilation regardless of heating or cooling demand.
In the Blackstone project, heating and cooling are provided by low-energy passive chilled beams connected to a ground-source heat pump loop. Ventilation, which is still variable in volume, is provided by a 100-percent outdoor air unit equipped with energy recovery capability.
How does this work? Figure 2 shows a schematic of the systems. Office areas and circulation spaces in the renovated building receive a constant volume of outdoor air during normal occupied hours. In the building’s many conference and assembly rooms, occupancy sensors and carbon-dioxide sensors control ventilation. Each room has its own VAV box. When the occupancy sensor shows that the space is unoccupied, the VAV box closes and no ventilation is provided. When occupancy is detected, the box modulates to provide 50 percent of peak ventilation. The wall-mounted CO2 sensor then takes over, modulating the VAV box to maintain a constant CO2 setpoint of about 500 parts per million greater than outdoor air conditions.
Because different assembly spaces are used at different times of the day, the design team was able to downsize the air handler by about 15 percent compared to a system sized for full ventilation simultaneously in all spaces. This is quite important in a cold climate like Massachusetts’ because the impact of reducing ventilation rates on energy consumption can be significant. In fact, this demand control scheme helped the Blackstone project earn 7 out of a possible 10 points in Energy Credit 1, paving the way for its LEED Platinum rating.
CO vs. CO2
While CO2 is relatively benign, carbon monoxide (CO) is potentially life threatening. CO sensors are required by law in many building areas, and they are a good idea wherever products of combustion may appear (e.g. from a malfunctioning or improperly vented furnace or boiler). In large commercial buildings with enclosed parking, CO sensing can also be used to control garage ventilation, just like demand control ventilation described earlier.
If you plan to incorporate CO2 sensors into your design, a few considerations are essential. First, the location of the sensors is critical. Make sure you have met all LEED requirements, and that the sensors are in a location that will actually sense room conditions. Second, calibration is important. If the sensors are inaccurate, information provided may be useless or worse. Accuracy to five significant digits is not necessary, but readings need to be consistent over time. When selecting sensors, be sure to compare requirements for calibration, as some types require calibration more often than others. Also be sure to include sensor calibration in commissioning and operating plans.
Finally, the CO2 control level must be calculated. CO2 levels are usually measured in parts per million (ppm). Currently, outdoor levels are around 385 ppm (and rising), but may be higher depending upon local conditions. Many engineers design systems to control or alarm when indoor levels exceed outdoor levels by about 500 ppm, but appropriate differentials depend on many factors, including the design ventilation rates and space usage. Engineers are encouraged to consult the ASHRAE 62.1-2007 User’s Manual for details of calculating CO2 control levels.
As energy costs rise and the sophistication of sensors and building controls increases, CO2 sensors are likely to become more common. The result? Smarter buildings that reduce energy costs and provide better conditions for occupants.
Chris Schaffner is principal at The Green Engineer, a sustainable-design consulting firm. He is also a member of USGBC’s LEED Faculty, its LEED Curriculum Committee, and its Indoor Environmental Quality (IEQ) Technical Advisory Group (TAG).
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