Technical Brief

Measuring carbon dioxide inside buildings

Workers and students spend about half of their waking hours at work or school. Therefore, maintaining adequate indoor air quality (IAQ) in their buildings is becoming a top priority of facility managers and building operators. To maintain adequate indoor air quality it is essential to provide outside air to dilute indoor air pollutants and exhaust these contaminants along with moisture and odors.

Why measure carbon dioxide?

Most heating, ventilating, and air conditioning systems (HVAC) re-circulate a significant portion of the indoor air to maintain comfort and reduce energy costs associated with heating or cooling outside air. It's virtually impossible for occupants and building operators to judge how much of the air coming out of an air supply duct is simply re-circulated air and how much is outside air. Current technology now allows easy and relatively inexpensive measurement of carbon dioxide (CO2 ) as an "indicator" to help ensure that ventilation systems are delivering the recommended minimum quantities of outside air to the building's occupants.

What is carbon dioxide?

Carbon dioxide is a natural component of the air on this planet. The amount of CO2 in a given air sample is commonly expressed as parts-per-million (ppm)—the number of molecules of carbon dioxide per million molecules. The outdoor air in most locations contains about 350 ppm carbon dioxide. Higher outdoor CO2 concentrations can be found near vehicle traffic areas, industry, and sources of combustion.

Where indoor concentrations are elevated (compared to the outside air) the source is usually the building's occupants. People exhale carbon dioxide—the average adult's breath contains about 35,000 to 50,000 ppm of CO2 (100 times higher than outdoor air). Without adequate ventilation to dilute and remove the CO2 continuously generated by the occupants, CO2 can accumulate.

How much CO2 is too much?

The concentrations of CO2 found in most schools and offices are usually well below the 5,000 ppm occupational safety standard (time-weighted average for a 10-hour workday within a 40-hour workweek) for an industrial workplace. While levels below 5,000 ppm are considered to pose no serious health impacts, experience indicates that individuals in schools and offices with elevated CO2 concentrations tend to report drowsiness, lethargy, and a general sense of stale air. Researchers are looking for linkages between elevated CO2 concentrations and reduced productivity and achievement.

What are the guidelines and standards for ventilation?

Various codes and standards define ventilation rates for schools and office spaces. The most widely accepted standard is the American Society of Heating, Refrigeration, and Air Conditioning Engineers(ASHRAE) Standard 62–1989. Some state and local codes have adopted the ASHRAE ventilation requirements.

According to ASHRAE Standard 62-1989, classrooms should be provided with 15 cubic feet per minute (cfm) of outside air per person, and offices with 20 cfm outside air per person. Ventilation rates for other indoor spaces are also specified. Standard 62 is currently being revised, so future rates may be different.

Using CO2 as an indicator of ventilation, ASHRAE has recommended indoor CO2 concentrations be maintained at—or below—1,000 ppm in schools and 800 ppm in offices. Clearly, the outdoor CO2 concentration directly impacts the indoor concentration. Therefore, it is critical to measure outdoor CO2 levels when assessing indoor concentrations. ASHRAE recommends indoor CO2 levels not exceed the outdoor concentration by more than about 600 ppm.

The following table illustrates the relationship between outside air ventilation rates and the resultant indoor CO2 levels, assuming an outdoor CO2 of 350 ppm.

The CO2 values in this table are approximate, and are based on a constant number of occupants (sedentary adults), a constant ventilation rate, an outdoor air CO2 concentration of about 350 ppm, and good mixing of the indoor air.

Is it that simple?

Unfortunately, the interpretation of CO2 data is often a more significant source of error than instrument accuracy. Meaningful assumptions of ventilation rate based on CO2 values require the building or zone to be occupied for a duration long enough to allow CO2 levels to reach a balance with the ventilation rate. This balance is known variously as equilibrium, unity, or steady-state. In an occupied building with a very low ventilation rate, the CO2 levels will likely continue to increase throughout the day, never reaching a steady-state concentration. On the other hand, buildings with an aggressive ventilation rate and good mixing of the outside air may prevent CO2 from accumulating much beyond outdoor levels, resulting in low CO2 concentrations throughout the day.

Unless the steady-state or equilibrium has been reached, low CO2 readings don't necessarily mean adequate ventilation. For example, consider a CO2 measurement taken in a school classroom during the first class period of the day—it is unlikely that the CO2 concentration will have accumulated to the point where an equilibrium condition has been reached. Therefore, assumptions based on this CO2 measurement may lead to an overestimation of the ventilation rate.

On the other hand, consider a CO2 measurement taken in the same classroom during the last class period of the day. Assuming the ventilation rate and occupancy of the classroom have remained fairly consistent throughout the day, it is reasonable to assume that a CO2 concentration below about 1,000 ppm indicates 15 cfm per person (assuming also that the outside air CO2 is in the 350 ppm range, see table above).

Sources of error in interpreting CO2 data include:

• Ventilation systems that modulate the amount of outside air allowed into the building over the course of a day
  
• Occupancy rates that fluctuate significantly
  
• Measurement errors (instrument or calibration problems, measurement location, and/or poor mixing of the air within the space)

How can I calculate percent outside air?

It can be difficult and unreliable to directly measure the amount of outside air entering large air handling units. An effective method is to measure the concentration of carbon dioxide in the outside air, return air, and mixed air streams. The values obtained are used in the following formula to determine the percentage of outside air for a particular air-handling unit:

% Outside Air (OSA) = (Cr - Cs / Cr - Co ) X 100

Where:

Co is the carbon dioxide concentration (ppm) in the outside air
Cr is the carbon dioxide concentration (ppm) in the return air
Cs is the carbon dioxide concentration (ppm) in the supply air (or mixed air)

The total supply air volume is required in order to calculate the approximate cfm of outside air supplied to the building using the percentage:

Outside Air (cfm) = % Outside Air X Total Supply Air (cfm)

What technologies can help with IAQ?

The measurement of carbon dioxide is an important "tool" to help ensure adequate outside air ventilation and save energy by reducing over-ventilated buildings. Technological breakthroughs have made it possible to use relatively inexpensive CO2 sensors to continuously monitor the CO2 in buildings. HVAC control systems can use these CO2 values to automatically modulate the volume of outside air in order to maintain indoor CO2 at or below a preset target concentration. This strategy is known as Demand Controlled Ventilation (DCV). DCV systems are especially useful for spaces or zones that experience variable occupancy rates; the ventilation rate responds proportionally to changes in the occupancy density.

(click image for 38k .jpg enlargement)
A variety of tools for measuring indoor air quality

What do these instruments cost...and how accurate are they?

The accuracy of most of the CO2 measurement instruments available today is more than adequate for use as an indicator of ventilation in offices and schools. You should occasionally check their accuracy to ensure they continue to respond correctly. Some of these instruments measure only CO2 while others will simultaneously measure temperature, relative humidity, and other gases such as carbon monoxide. Instruments that either record internally or have output capability to an external data logger are valuable for trending and troubleshooting.

The cost of these instruments range from about $500 to over $5,000, depending upon features. Installed costs of complete DCV systems have a much broader price range.

What about other indoor pollutants?

Clearly, elevated indoor CO2 levels suggest inadequate outside ventilation air, and it follows that inadequate ventilation permits other potentially harmful air pollutants to build up and create health, productivity, and comfort problems.

Keep in mind that many indoor air pollutants are generated independent of occupancy (carpet off-gassing, stored materials, air entry from contaminated utility tunnels, etc.). In these cases, CO2 may not be a good "yardstick" to use in evaluating the quality of indoor air.

Unusually strong indoor air pollutants can easily overwhelm the ventilation rates suggested by ASHRAE Standard 62-1989. A general rule of thumb is that a pollutant concentration is reduced by about 50% for each doubling of the ventilation rate (for those sources that have a fairly constant generation rate). Not only does more and more ventilation result in less and less dilution, but high ventilation rates can have huge impacts on energy costs and comfort. Therefore, many indoor air pollutant sources are best controlled with methods other than dilution with outside air.

Not surprisingly, the energy-efficient approach is also the wise approach: Ventilation only reduces exposure, while removing the source can eliminate exposure. Therefore, methods of source control include keeping pollutant sources out of the building through wise choice of furnishings and finish materials, using exhaust fans to capture and remove pollutants, and controlling pressures between zones to keep pollutants from migrating to populated or sensitive areas. Good practice suggests that we exclude, remove, and otherwise minimize pollutants suspected of having the potential to cause health impacts and/or affect performance and comfort.

Richard Brisbois and Greg Jourdan discovered a painted-over outside air intake grill that blocked air flow

References

ASHRAE Standard 62-1989: Ventilation for Acceptable Indoor Air Quality. American Society of Heating Refrigerating, and Air Conditioning Engineers (ASHRAE). Atlanta, GA, 1992.

ASTM Standard D-6245–98 Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation.

IAQ Diagnostics Reference Manual: Hands-On Assessment of Building Ventilation and Pollutant Transport. University of Tulsa, College of Engineering and Applied Sciences, Department of Chemical Engineering.

1994 Manual for Ventilation Assessment in Mechanically Ventilated Buildings. NISTR #5329-1994, National Institute for Standards and Technology.

Where can I get more information?

Western's Energy Services:

• The Power Line Hotline at (800) 769-3756
• Energy Services Web site

This Technical Brief, and others, are available on-line at this Energy Services website.

• E-Mail your question to Western's Power Line
• Fax your question to Western's Power Line at (360) 586-8303
• The Energy Ideas Clearinghouse Web site
• "An Office Building Occupant's Guide to Indoor Air Quality," U.S. Environmental Protection Agency, October 1997.
• ASHRAE website

Western's Energy Services

Western's Energy Services offers customers information, resources and solutions to improve their energy efficiency, use of renewable energy, and competitive positions. For additional information about any commercial, industrial, agricultural or residential technologies, programs or products, use the Western contacts listed above.

Acknowledgment: The Washington State University Cooperative Extension Energy Program produced this technical brief.