Using CO2 To Know Your Building Better

By Mike Schell, Engelhard Corporation (reprinted with permission)

Most facility managers already know their buildings intimately. With the increased availability and reliability of carbon dioxide (CO2) measurement devices, a new tool is now available to fine tune buildings to save energy and optimize air quality. There are also a number of ways that CO2 can allow building owners to quickly identify potential problems before they become complaints. This article summarizes a number of simple approaches that will allow facility managers to use CO2 sensors to understand and control their buildings in ways that were never possible before.

Besides giving us the fizz in our favorite soft drink or brew, CO2 is one of the most common gases on the planet. Plants give off CO2. People exhale CO2. And CO2 is one of the most plentiful by-products of the combustion process in devices ranging from furnaces to lawn mowers to coal fired electrical power plants.

CO2 And Ventilation

People exhale CO2 at a very constant rate given their activity level. Outside levels are very low, typically at 350 to 450 parts per million (ppm). As a result it can be a good indicator of occupancy. The more people there are the more CO2 is generated. An indoor CO2 measurement is a dynamic indicator of indoor ventilation that considers both the amount of low-concentration CO2 laden outside air brought in by a ventilation system and the number of people in a space. Using the right method, the resulting CO2 concentration can be related to the actual amount of fresh delivered in terms of cubic feet per minute (cfm) per person.

Over Or Under Ventilated?

Is your building getting enough fresh air for the number of occupants it holds? This can be a scary question for even the most seasoned manager. Until recently there has been no reliable method of estimating indoor ventilation rates particularly from zone to zone in a building. As a result many buildings have been designed or are operated to provide much more ventilation than the building needs in the interest of avoiding tenant complaints. In some buildings, ventilation rates may be too low in one area and too high in another.

A problem faced by many managers is that buildings are occupied at densities that are far different from when the original building was designed. In many cases buildings have been over-designed and actually provide much more fresh air than current standards recommend. Fresh air intakes can be manually set by the manager with little ability to determine if ventilation is being optimized for the space. This over-ventilation can be costly and in a humid climate can result in excessive indoor humidity levels that can result in microbial contamination. Under ventilation and potential air quality complaints can result due to broken damper linkages, lack of maintenance or overzealous energy cost reduction measures.

It is important to note that CO2 is only an indicator of the amount of fresh air getting to the space relative to the number of people in the space. It is not really a contaminant itself and cannot directly indicate if other contaminants exist in the space. Some individuals have reported symptoms of drowsiness related to elevated indoor CO2 levels. Medical research has failed to confirm this relationship between CO2 and drowsiness. More than likely individuals experiencing drowsiness are reacting to the general build-up of a variety of contaminants that occurs when ventilation is reduced. In this way CO2 may be a surrogate for air quality in occupied spaces.

Fast Ventilation Checks

Ventilation rates and CO2 levels will vary greatly in a building over the course of day. Dampers open, people come and go, systems cycle on and off. A manager must understand that a measurement of less than five minutes is only a snapshot of a very dynamic process. Still, it can help immensely in understanding building operation.

As a rule of thumb, measurements should be made a few hours after the building or space has been occupied. While making the measurement note the mode of the mechanical system, particularly if it is in heating, cooling or economizer mode. Outside climatic conditions may also have an impact on infiltration that will also affect ventilation rates.

Under ventilation may be an issue if a spot measurement indicates high CO2 levels (e.g. levels over 1100 ppm which corresponds to an outside air ventilation rate of 15 cfm per person) or significant concentration differences between spaces. If levels are low during a single measurement it may not mean much. If levels are consistently low throughout the day, over ventilation may be a suspect. In both cases further investigation of the system is warranted.

Saving Energy With CO2 Demand Controlled Ventilation

In the next 5 years, CO2 sensors that control ventilation in a building will be as common as temperature control is today. Called demand-controlled ventilation, this approach offers significant energy savings by eliminating over ventilation of spaces. The guesswork is taken out of damper adjustments and settings by regulating outside air ventilation based on the actual number of people in the space at any particular time (as indicated by the Change in CO2 concentrations).

This approach was recently recognized by the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) in a recent interpretation to ASHRAE standard 62-1989 (IC 62-1989-27). Rather than ventilating at an arbitrary rate based on the expected occupancy when the building was designed, CO2 control adjusts based on the actual occupancy in the space, which is typically much less than the actual occupancy. Further energy savings result if spaces are partially or intermittently occupied. CO2 control is applicable to all types of space where occupant density exceeds 7 people per 1,000 square feet.

The energy savings that can result are significant. Energy savings payback for the sensors themselves is often a few months, even for office type spaces. In fact, many building owners have used the energy savings from CO2 control to pay for other building upgrades such as digital building control systems, variable speed drives for fans and zone by zone airflow regulation. In these cases paybacks are still less than 2 years.

In addition to energy savings, occupants end up being the big winner because ventilation for air quality as recommended by ASHRAE is maintained at all times with the CO2 control. This also means fewer complaints from occupants to the facility manager.

In humid climates, where bringing in too much humid outside air can lead to mold and mildew problems, CO2 can ensure that only the necessary amount of outside air has to be brought in from outside. This helps ensure that the dehumidification capacity of the cooling system will not be overwhelmed.

An additional important benefit of using CO2 is that a sensor used for ventilation control is not discriminating about where fresh air comes from. If there is an open window, outside air used for building pressurization, a leaky building or just an area of the building with lower occupancy, a CO2 control approach will recognize these sources before they bring air from outside.

Calibration of CO2 Sensors

There are a wide variety of CO2 sensors currently available on the market. Calibration requirements can range from a few months to units that have a built in self-calibration feature that requires a sensor check once every five years. The calibration interval of a sensor is an important consideration in purchasing a CO2 demand controlled ventilation system. Significant energy savings can be eaten up very quickly if a maintenance technician has to calibrate a unit on a frequent basis. When evaluating sensors look for sensors that have documented proof of stability. Look for referrals or recommendations from other facility managers.

A good portable CO2 sensor can be used to calibrate most CO2 sensors. This is a far-preferred approach than having to use different gases and regulators to calibrate installed sensors.

Measurement of Percent Outside Air

The percent of outside air delivered by an air handler can be easily calculated by comparing the CO2 concentrations between the outside air, return air, and supply air. A similar approach using temperature has a disadvantage in that it is not very accurate if the difference between the sample points is not very great. Using CO2, differences in concentrations during occupied hours are usually significant enough to ensure an accurate measurement.

This type of measurement allows a quick check of outside air delivery rates as compared to damper settings. One major controls company offers a permanently installed system that actually controls damper position.

Trend Measurement of CO2

Trend measurements are measurements made over time. Using a PC, results are typically displayed in graph format. These measurements can be made by an established building management system and permanently installed sensors or by a portable system that can be left in place for periods of one day to one week. Ideally, a manager should be measuring a number of parameters in the space in addition to CO2. Typical parameters measured include temperature, humidity and in some cases Carbon Monoxide (CO).

Trend measurements can be used in a number of ways to better understand building operation.

They are used to help investigate and understand air quality complaints.
Repeat baseline building conditions can be measured in a number of areas to ensure that mechanical systems consistently deliver similar conditions over time. A change in measurements between locations or in the same location at different times may help identify potential problems before they become serious.
Trend measurement systems can provide important documentary evidence of building operation in case of lawsuits or non-specific occupant complaints.

An important consideration when considering a trend measurement system is its durability and susceptibility to curious tampering and potential theft. These systems must operated for extended periods of time without interference to get the best results.

Identify Supply Air Duct Leakage

For spaces that have a common ceiling return plenum that accommodates supply air ducting, a CO2 measurement can provide an indication of the amount of air leaking from the supply air ducts to the return air. This short-circuiting of air can be costly and can make it difficult to ensure the comfort levels in a space.

In all cases, the level of CO2 in a return air plenum should be close to the concentrations actually measured in the spaces feeding the plenum. If concentrations are significant lower in the ceiling plenum than in the space, duct leakage is likely the cause.

Identify & Control Combustion Fumes In Air Intakes

While CO2 is not a dangerous gas, it does constitute a major component of all combustion exhaust and can be used to indicate the presence of harmful combustion fumes. It is important to note that Diesel vehicles product almost no carbon monoxide but do produce plenty of CO2 along with other (more harmful) contaminants.

CO2 sensors can be used to periodically check or permanently monitor building air intakes. This is especially important if loading docks or vehicle loading zones are in close proximity to building air intakes. As a rule of thumb, if CO2 concentrations at an outside air intake begin to exceed 600 to 800 ppm of CO2 it is likely that combustion sources are nearby.

In one hospital, general complaints of combustion fumes were traced to a helicopter landing pad on the roof after a number of trend logged CO2 measurements were made. The peaks in CO2 concentrations corresponded to the times the helicopter was on the landing pad.

It is ironic that all the ugly stuff on the building gets lumped together in the same out-of-the-way location. Loading docks and building air intakes are a common combination that facility managers must deal with. A permanently installed CO2 sensor in the building air intake can sense the presence of elevated CO2 levels associated with combustion fumes and close the air intake until levels fall to within a acceptable range (e.g. under 600 ppm).

A New And Useful Tool

Portable and permanently installed CO2 sensors are an essential tool now readily available to facility managers to reduce energy costs, track down potential ventilation-related problems, document proper system operation and reduce customer complaints. In the next few years CO2 sensors on the walls and in the ductwork of your building could be paying for your new building control system. The portable CO2 sensor in your toolbox could see as much action as your electric multi-meter!