CO2-based ventilation control is currently one of the most rapidly expanding building control strategies being employed in both new and retrofit building projects. This approach has the potential to provide improved building ventilation control, while simultaneously, helping assure tenant comfort (thus reducing complaints) and lowering routine ventilation operating costs.
This article answers twelve of the most common questions that building owners may have about the effective use of CO2-based ventilation control in buildings.
Is CO2 a Contaminant that must be Controlled in Buildings?
Carbon dioxide is not deemed to be a health threatening contaminant when it exists at the concentrations normally found in buildings (400-2500 ppm). The true value in employing CO2 sensing in buildings is to approximate the cfm per person ventilation rate within the space.
This approach to ventilation measurement (as outlined in ASTM Guide Standard D6248-98) considers the difference in CO2 concentration between inside and outside areas. The inside CO2 levels represent a dynamic measure of the number of people in the space. These people exhale CO2, mixing this with the amount of lower CO2 concentration air that is taken from outside and used for ventilation.
A relatively straightforward calculation permits the calculation of a cfm per person ventilation rate based on the aforementioned CO2 interior/exterior differential. Portable, handheld CO2 monitors are available that can automatically calculate and show the cfm/person based on CO2 concentrations.
A handheld sensor can be employed to complete a spot measurement to ascertain ventilation rates, though this measurement is best performed 2 to 3 hours after occupancy has commenced.
When using permanently mounted sensors, however, control algorithms used with these sensors are able to measure CO2 concentrations at all times, enabling calculation of ventilation rates and therefore proper control of ventilation.
What is new about CO2 based Ventilation Control?
CO2-based ventilation control offers improvements in existing ventilation procedures in buildings, allowing the control of ventilation on a zone-by-zone basis. This is very much in line with current temperature control standards.
Any commercial building with mechanical ventilation is able to provide an amount of outside air to aid in the dilution of odors and contaminants. In the majority of these systems, dampers are adjusted, then set to transport a specific amount of outside air.
The amount of outside air is dependent on a code required ventilation rate (for example 15 cfm/person), multiplied by the number of people in the building (the design ventilation rate).
Unfortunately, this approach does not offer any genuine control. The system is designed with a fixed ventilation rate which is set or controlled by dampers for the whole life of the building. In some instances, airflow is measured and regulated to make sure that the amount of outside air remains fixed and constant as the movement of air in the system varies.
A building designer should be careful to ensure that the duct distribution system is able to deliver the appropriate volume of outside air per zone in the space, as defined by its intended occupancy.
There was no straightforward way to measure the ventilation rate in each zone until recently, so it was not easy to ensure that an appropriate amount of fresh air is reaching each space. Where buildings did incorporate ventilation control with CO2, the system is designed and sized as standard.
The use of CO2-based ventilation affects how the building is operated, not how it is designed. What does differ is that sensors are installed in each occupied zone to measure the ventilation rate being delivered to the space.
Airflow to each individual zone and at the central air intake is regulated to make sure that the code required ventilation rate per person is provided to each zone based on current occupancy.
This approach bears a resemblance to temperature control in the sense that a sensor in installed in each individual space, measuring and regulating the delivery of warm or cool air to ensure comfort. It is now possible to measure and control ventilation in the same fashion.
So, What does Zone Ventilation Control with CO2 mean to a Building Owner, Operator or Tenant?
Ventilation control enables the measurement and control of fresh air to each individual space. Fresh air is routed based on the space’s occupancy, making sure that enough air is delivered to each space to guarantee comfort while minimizing complaints.
A common phrase in quality control is that you cannot control something unless you are able to measure it. As such, this approach to ventilation control involves the active measurement of ventilation rates in a space, adjusting airflow as required.
This approach offers a number of advantages.
Building owners are able to record ventilation levels, monitoring the system to ensure that it is operating according to its design. Potential ventilation issues can be rapidly identified and addressed, even before building occupants become aware of the issue.
Maintaining consistent ventilation throughout a building is a major factor in ensuring the comfort of the occupants.
Where occupancy in specific zones differs throughout the day, ventilation rates can be altered - up or down - to deliver an appropriate level of ventilation per person, based on the zone’s occupancy (e.g. 15 cfm/person). In the majority of buildings, the addition of ventilation control has resulted in energy savings with installation costs paid back in under a year.
In circumstances where density and usage within a space is different from that of the original design assumptions – for example, 3 people using a space that was originally designed for 1 - the system will adapt to provide appropriate ventilation automatically, adjusting for more or fewer people.
Without ventilation control in place, costly mechanical changes may be required to maintain adequate ventilation in these cases.
Should outside air enter a space through an open door, window, or as a result of infiltration, a CO2 sensor will consider this to be ventilation air, reducing the level of mechanical ventilation conditioned from outside sources.
Additionally, should a room be unoccupied and having accumulated up a reservoir of fresh air, the ventilation control system will begin to utilize this air before drawing additional air from outside.
Even where occupancy is consistent during the day, ventilation control with CO2 has the potential to ensure that an ideal amount of outside air is supplied for occupants in a space. By measuring and appropriately controlling ventilation, superfluous over or under ventilation is prevented.
Do Current Ventilation Codes and Standards Permit CO2 based Ventilation Control?
If a specific code necessitates the provision of a space’s ventilation in terms of cfm/person, then CO2-based ventilation measurement may be employed to confirm compliance to the code. It can also be used to control ventilation rates, ensuring these meet code required levels – for example 15 cfm/person.
Every local code or standard bases its recommendations for ventilation on one of two cornerstone references: The International Mechanical Code (IMC) or ASHRAE Standard 62-1999: ‘Ventilation For Acceptable Indoor Air Quality’.
These documents establish target cfm/person ventilation rates which are included in all today’s codes. CO2-based ventilation control is employed according to the Ventilation Rate Procedure, outlined within the ASHRAE Standard 62.
This procedure contains a provision for variable, intermittent occupancy which permits the total ventilation rate to a space to vary, so long as the target cfm/person ventilation rate is preserved for the space’s actual occupancy.
ASHRAE interpretation IC 62-1999-33 offers further guidance that is specific to ventilation control with CO2. It recommends that a base ventilation rate be ascertained to provide a minimum ventilation level in cases where the space has low occupancy.
In practice, this base ventilation rate is generally 20-30 % of the ventilation rate specified in the design, or around 5 % of the total ventilation capacity.
In the International Mechanical Code (2000), section 403.3.1 ‘System Operation’, states that: “The minimum flow rate of outdoor air that the ventilation system must be capable of supplying during its operation shall be permitted to be based on the rate per person indicated in table 403.3 and the actual number of occupants present.”
The code’s commentary includes explanations and examples for the IMC, with CO2-based ventilation control named an ideal technology which is well suited for meeting this section of the code. It should be noted that the IMC specifies this section as ‘System Operation’, implying that this is an operational strategy for ventilation control rather than a design approach.
Overall, these two most crucial ventilation references to local standard and codes clearly establish that ventilation rates ought to be provided in terms of a certain ventilation rate per person.
Traditional practice involved the provision of a fixed rate of ventilation based on an assumption maximum occupancy, but both these references overtly allow systems to be operated in order to provide the target cfm/person ventilation rate – a rate that is based on actual occupancy at a given time.
It is in this way that CO2 ventilation control is correctly applied. Should a local code provide a cfm/person ventilation rate in a space, then CO2-based ventilation control may be used to manage this rate.
Finally, it should be noted that the U.S. Green Buildings Council supports the use of CO2-based ventilation control when working towards its LEEDs certification program for sustainable, green buildings.
I have heard that CO2 is not a contaminant in buildings (as mentioned above) but is linked to other bio-effluents given off by people. I have also heard there is no link between CO2 and other sources of contaminants in a building, such as furnishings and equipment. If this is the case, how can we use CO2 for control of ventilation?
CO2-based ventilation control does not regard CO2 as a contaminant. As such, CO2 sensors installed in buildings enable ventilation measurement and control on a per person basis, based on requirements and recommendations in related codes and standards.
These cfm/person rates – such as those established in ASHRAE Standard 62 - have been developed to include every contaminant within a space. Because of this, ventilation rates vary between applications in numerous codes and standards.
ASHRAE Standard 62 ventilation rates for school classrooms, for example, are generally 15 cfm/per person because it is understood that high densities in a classroom will result in people being the primary source of contaminants requiring ventilation.
Whereas in an office space, the ventilation rate has been recognized as 20 cfm/per person because it is understood that as well as people, other sources like furnishings, carpets and office equipment will be present, and these will also act as a source of indoor air contaminants.
In areas where smoking is permissible, ventilation rates are raised to 30 cfm in order to take into account the need for additional ventilation due to odors and smoke.
CO2 is only used to measure ventilation rate in instances where it is used to control ventilation, though CO2 can be utilized to measure and control ventilation requirements at any cfm/person ratio.
CO2 is Often Referred to as an Air Quality Parameter. In Rooms Where CO2 Levels are High and the Air Quality Appears Low, Occupants are Tired, Lethargic, Some may even have Headaches and the Air Seems Stale. Does CO2 have Anything to do with this?
This example clearly shows the relationship between ventilation and CO2 levels. If CO2 levels are higher, this is a sign of lower ventilation rates. If ventilation rates are low, other contaminants and gases in a space are able to build up alongside CO2 concentrations.
These physical reactions are a result of the low ventilation levels which result in higher exposure to other contaminants. In these cases, CO2 acts as an indicator but is not the cause.
As the Calculation of Ventilation Rates is based on the Difference Between Interior and Exterior CO2 Levels, will Changing Outside Levels affect the Accuracy of the Ventilation Measurement?
Seasonal or daily variations in a location can vary by 20 to 30 ppm, though this is not a wide enough variation to significantly affect ventilation calculations. Exterior concentrations around the world generally range from 380 ppm up to 500 ppm.
System designers are advised to take a series of outside measurements to establish what the exterior CO2 levels are in their area, then factor in these readings when developing and installing a CO2-based ventilation control system.
Another option is the permanent installation of CO2 sensors in outside air, providing real-time calculation of ventilation based on interior/exterior CO2 differences.
However, this approach may be excessive. For example, if a designer assumes exterior levels to be 400 ppm while the actual concentration is 500 ppm, the only impact on the ventilation control strategy would be that ventilation rates are higher by approximately 2 cfm/person than the design specification. This error is actually in the occupant’s favor, and would only slightly adversely affect energy usage.
Should Ventilation Control be Considered a Competitive Technology to other Energy Saving Technologies such as Energy Wheels or Fresh Air Economizers?
All these technologies are complimentary. If outside air can be used for free cooling an economizer control should be designed to override CO2 control in a building.
If outside air is not appropriate for free cooling, then CO2 control can be used to ensure that an appropriate amount of air is heated, cooled or dehumidified in order to meet the building’s ventilation requirements.
When one of the systems is not functioning, the other is, though this does depend on outside conditions. This combined approach is able to work year-round, ensuring provide good energy savings and ventilation. Energy wheels are an ideal means of reducing the cost of preconditioning outside air before cooling or heating.
An alternative, however, is the use of ventilation control via CO2. This approach can be utilized to adjust ventilation rates based on the space’s demands. For example, should less air be required at specific times of the day, the amount of energy used by air conditioning equipment or fans may be reduced.
An energy wheel is capable of reducing heating or cooling costs for air entering a space. Here, ventilation control with CO2 will make sure that fresh air entering the building is appropriately distributed. In this case, as previously, these technologies serve different purposes while their use complements one another and they both save energy.
CO2 Sensors are Thought to be Fragile and Difficult to Maintain, Often Requiring Frequent Calibration. Is this Still True?
The sensor technology which enables the measurement of CO2 for ventilation control has been in use for over a decade. However, when this technology was first used there were some issues with reliability and sensor drift.
Now, every major building control and equipment company is able to provide CO2-based ventilation control, with the majority of sensors being designed to function without maintenance for at least five years. When selecting sensors – as with any product – it is important to choose a trusted brand name to ensure reliable CO2 based ventilation control.
Where Should CO2 Sensors be Placed? Should these be Placed in the Return Air Ducts of a Building or in the Space?
Use of duct sensors for CO2-based ventilation control is not advisable, in the same way that space temperature is not generally controlled using in-duct temperature sensors. Return air ducts generally take air from a variety of different spaces, and these may have varying CO2 concentrations.
A duct concentration is only able to represent average of conditions, not the actual conditions within a given space. As such, in order to ensure that each space has the correct amount of ventilation, sensors must be placed within the space itself. Duct sensing is only acceptable in cases where an air handler serves a single zone.
Many manufacturers now offer combined CO2 and temperature sensors which are wall mounted. These sensors are inexpensive and can provide sensing for both ventilation control and temperature control.
There are a Number of Systems on the Market that Measure and Regulate Outside Airflow as it Enters a Building. Do these Devices Provide the same type of Ventilation Control as a CO2 based System?
Airflow monitoring stations are commonly used within heating and air conditioning systems utilizing Variable Air Volume Systems (VAV) and where the total amount of air delivered to the building fluctuates based on the total need for cooling or heating across all building zones. These airflow measurement and control systems are designed to ensure provision of a fixed rate of outside air, because total airflows within the system will be different.
Additionally, these devices are unable to regulate or measure airflows to each zone specific within the space. In fact, all this equipment actually does is make sure that sufficient air is provided to meet the ventilation requirements for full occupancy are provided at all times. As such, these systems are not generally associated with measurement and control of ventilation in different building zones.
In many cases, these systems are prone to unnecessarily over-ventilating a building, because the system aims to deliver fresh air to every zone under all operating conditions. There is no need for this costly form of air monitoring and flow control equipment in systems where CO2 control is in place.
Can Ventilation Control using CO2 be Retrofitted into Existing Buildings?
Integrating CO2 control is relatively straightforward when equipment is being replaced during a retrofit and a direct digital building control (DDC) system is being installed.
Control algorithms may even be incorporated into their building control system, depending on the system’s manufacturer. However, some cases will likely require a degree of custom programming if provisions for CO2 control are not included in the package as standard.
It should be noted that cost savings related to the integration of CO2 control may help contribute to other building upgrades. If a building already has a DDC system in place, the addition of CO2 may only actually involve placing the sensors in the space before wiring them into the existing system.
It is possible to use ventilation signals from the CO2 sensor to regulate individual VAV boxes or manage the amount of fresh air introduced at each floor. Some custom programming may be required to guarantee that proper ventilation control algorithms are applied, so it is advisable to check specific controls or speak to a building contractor to find out more about this type of upgrade.
When considering an upgrade to CO2-based ventilation control, the key consideration is how is the delivery of fresh air is currently regulated to the space. If, for example, the space is served by a rooftop air handler fitted with an economizer control, then the installation may only involve running wires from the CO2 sensors to the rooftop unit.
The signal from the CO2 sensor can act as a direct control signal for the adjustment of air intake dampers. If several zones are involved, one CO2 sensor must be placed in every major zone. The output of each zone sensors should then be passed through a transducer that examines all the signals before passing through the highest value.
When this approach is used, the system is able to control the space which needs the most amount of ventilation air. This method may be applied to schools, retail space, and other applications in common place low rise buildings.
For a CO2 ventilation retrofit in large buildings, however, it is important to understand the way in which air is delivered to the building and to each individual floor. Installed CO2 sensors must be able to control something, and in these types of application, the ability to modulate ventilation at the air intake as well as at each individual floor plate is vital.
If this capability is not available, it must be installed during the ventilation control upgrade, with sensors installed in each major occupancy zone on each floor. Like the rooftop example outlined earlier, fresh air introduced at each floor must be regulated by the sensor supplying the highest reading on the floor.
Additionally, a degree of control feedback from each floor must be provided to the central air intake, prompting this to open or close depending on the overall demand for fresh air within the building. A good building controls contractor will be able to design a ventilation control system which can accommodate a building’s existing control system.
This information has been sourced, reviewed and adapted from materials provided by Amphenol Advanced Sensors.
For more information on this source, please visit Amphenol Advanced Sensors.