An IAQ Approach That Pays For Itself with Energy Savings

By Mike Schell, Engelhard Corporation (reprinted with permission)

Higher productivity, healthier building occupants, and better maintenance all sound good, yet the math required to weigh an IAQ investment against other financial decisions is often elusive to traditional payback analysis. For example, a lighting retrofit will pay for itself in two years with documented energy savings compared to previous energy bills. Investments to improve IAQ do not have such a direct measure. How do you determine the economic impact of improved air quality so that the number crunchers are satisfied with the investment?

IAQ initiatives offer both improved air quality and energy savings. CO2 demand controlled ventilation (DCV) has demonstrated the ability to obtain significant and tangible energy reductions that rival lighting retrofits in economic attractiveness. In some cases, the economic benefit is so significant that additional air quality upgrades can be financed from the energy savings. LaSalle Plaza, a 25-story office building in downtown Minneapolis, saved approximately $200,000 in 1997 by switching to CO2 control. Payback for the installation was reported in less than two months. The savings resulted from controlling ventilation based on actual occupancy (as measured by CO2 levels) rather than assuming the building was always at full occupancy. LaSalle Plaza also reduced heating and cooling cost and lowering fan and chiller pump energy usage by using variable speed drives. According to Dick Nicoski, Chief Engineer for LaSalle Plaza, “Even with large gains in energy efficiency, tenant comfort remained steady and we continue to meet ASHRAE air quality standards.”

Over-Ventilation Is A Common Problem

LaSalle Plaza is typical of many large buildings in that the outside air ventilation rate was originally set based on design occupancy assumptions that had little to do with how the building was actually operated. For example, some building air intakes are set arbitrarily, assuming that a 20% open damper equals 20% of the system capacity. The combination of damper, fan and overall system performance may in fact mean that the physical setting on the damper has little to do with the actual air delivered. As a result, buildings can be significantly over-ventilated even before variable occupancy is considered.

When CO2 DCV is applied, CO2 is not considered a contaminant. Rather, it is simply an indicator of occupancy, as people exhale CO2 at a constant and predictable rate. An indoor CO2 measurement is a good indicator of the number of people breathing in the space tempered by the amount of CO2 that is in the fresh air introduced by ventilation or infiltration. A well-designed CO2 DCV strategy can maintain a target per-person ventilation rate (e.g., the ASHRAE minimum of 15 cfm/person) based on actual occupancy. Excessive ventilation based on assuming a maximum capacity at all times is thus avoided.

In large buildings, sensors (mounted in a manner similar to thermostats) can modulate ventilation based on zone or floor occupancy. In some cases, outside air can be diverted from other lower-occupancy parts of the building. It is important to note that this process has been recognized as a valid approach to modulating ventilation based on occupancy using ANSI/ASHRAE 62-1989.

Predicting CO2 Energy Savings

The viability of any energy savings initiative is highly dependent on the predictability of the energy savings. The entire performance-contracting industry has developed based on the ability to predict and guarantee specific energy reductions from certain building upgrade activities. The balance of this article will discuss how a building owner, engineer or performance based contractor can assess the energy savings potential for CO2 DCV in an existing building. Outlined below is a simple three-step process to identify the potential for energy savings.

Step 1: Initial Assessment

A simple first step is to check CO2 levels with a simple portable hand held monitor. Reliable hand held CO2 sensors are now available for under $500 making it affordable for every facility manager’s and contractor’s tool box.


  • Measure outside levels of CO2 near building air intakes. This provides a reference or baseline point for the outside air that will be used for ventilation.
  • Measure inside levels of CO2 in various areas of the building two to three hours after initial occupancy has occurred. This allows CO2 levels the time to rise and stabilize, particularly in lower occupant density spaces like offices.
  • Make sure to note whether the ventilation equipment is being operated in economizer mode, (use of 100% outside air for free cooling when outside conditions allow), or if other exhaust or ventilation equipment is operating. For the best assessment, the economizer and other equipment should be off.
  • Observe levels for 3 to 5 minutes and wait until you have a stable measurement. If levels continue to rise, it can be an indication that ventilation and CO2 levels have not yet stabilized. Try measuring again in 20 to 30 minutes.
  • Make sure that the sensor is held away from CO2 that may be generated by your own (or others’) breathing.

Typically, a 500 ppm differential between inside and outside concentrations will be indicative of 20 cfm per person ventilation rate and a 700 ppm differential will indicate a 15 cfm per person ventilation rate. If levels are 100 ppm or more lower than these inside/outside CO2 differentials, further investigation is warranted. Concentrations significantly over 700 ppm above outside levels indicate ventilation less than 15 cfm per person. The higher the CO2 level, the lower the ventilation rate.

Step 2: Trend Log CO2 Concentrations

A measurement of CO2 levels with a hand held device only gives you a short-term snapshot of ventilation in what is typically a very dynamic environment. Longer term monitoring of CO2 levels over a typical day or week is essential to determine economic benefit. There are a number of CO2 sensors on the market today with data-logging capability that can record and graph concentrations over the course of a day or week. While slightly more expensive than hand held devices, these units often have the ability to log temperature, humidity and other gases along with CO2. It is a good idea use a sensor/data-logger package that is theft and tamper resistant, so it may be safely left for extended periods of time.

While airflow measurements can be made at building air intakes, it can only indicate the total volume of outside air delivered by an air handler. Airflow measurements are not useful for delivering the amount of fresh air needed to a particular zone because return and supply air streams are mixed. Carbon dioxide or other types of tracer gas measurements are the only way to determine the actual amount of outside air delivered on a zone by zone basis. This is why CO2 demand controlled ventilation is currently the only effective way of providing outside air to a zone, based in proportion to its actual occupancy.


  • Choose a time to monitor over a day or a week where the mechanical system is unlikely to operate on economizer mode. Set the data-logger to sample CO2 levels every 5 or 10 minutes.
  • Select monitoring locations that are representative of occupant activity levels that occur in the major zones of the building. Generally, it is not a good idea to measure in return air ducts because air may be drawn from a number of spaces with diverse occupancy and therefore your measurement may not be representative of an actual space.
  • Check outside concentrations with a hand held sensor periodically (daily) to ensure outside levels are stable during the test.
  • Note typical peak occupancy levels and generally determine the variation in occupancy that occurs through the course of the day.
  • Use software provided with the sensor/data-logger to graph the CO2 levels recorded.

Once you have trend data and a general knowledge of occupancy patterns, you can better assess if CO2 levels are consistently low enough to indicate excessive or inadequate ventilation.

To more accurately interpret the graphed data, use a computer program that allows the simulation of CO2 concentrations in a space. This type of program considers the volume of the space, assumed outside air infiltration and changing occupancy levels throughout the day. The program can generate a series of CO2 trend lines that represent CO2 levels in a space. Figure 1 shows the output of one such program used to simulate CO2 levels over the course of a day. The stair-step line in the graph indicates the assumed occupancy for the space and serves as the basis for estimating CO2 generation. While not shown in this illustration, the program also calculates the actual cfm of constant air delivery that each line represents. The actual outside air ventilation rate of the space can be estimated by identifying the line that most closely approximates the actual trend data recorded. This approach allows a more accurate estimation of current ventilation rates.

Figure 1
Simulation Of CO2 At Various Ventilation Rates For Comparison With
Field Recorded Trend CO2 Concentrations

Step 3: Energy Analysis

Once the actual ventilation rate of the space or zone has been determined using CO2 trend data, it is possible to predict potential energy savings through a computer energy analysis.

The program takes into consideration: estimated occupancy variation, volume of the space and the type and cost of energy used for heating and cooling. The program then calculates actual energy using a CO2 DCV approach versus a constant ventilation approach that assumes maximum occupancy. The reliability of the analysis is dependent on the accuracy of the trend data. In the event that trend logged data is not available the energy analysis can still be performed using assumed or measured outside air ventilation rates. In the case of the LaSalle Plaza building, an energy analysis based on assumed ventilation rates yielded a savings estimate of about 60% of what actually happened. If actual monitored data had been used, the analysis would have been more accurate. In this case, the difference equated to a calculated four-month payback versus an actual two-month payback

A key benefit of CO2 demand controlled ventilation is the ability to eliminate unnecessary over-ventilation while still ensuring ventilation for acceptable air quality by maintaining ASHRAE recommended cfm per person ventilation rates.