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Demand-Controlled Ventilation: A Balancing Act

Key Points
  • Carbon dioxide (CO2) sensors are designed to measure CO2 concentrations, typically as a result of human respiration or combustion.
  • They can be used with demand controlled ventilation (DCV) systems to modulate outdoor air intake rates in response to space population changes.
  • DCV using CO2 sensors is more effective in places of worship, sports arenas, school gymnasiums, large conference rooms, theaters, and lecture halls.

Source: www.nrel.gov
air quality testing
Demand controlled ventilation (DCV) can not only help you save energy and comply with indoor air quality standards, but also keep your occupants healthy, safe, and productive when it is properly applied. ASHRAE's definition of acceptable indoor air quality is defined as, "air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction[2].”

ASHRAE 62.1-2007 Ventilation for Acceptable Indoor Air Quality (IAQ) offers the choice of either the Ventilation Rate Procedure (VRP) or the Indoor Air Quality Procedure (IAQP) to meet the proposed standard. The VRP prescribes ventilation rates per person and per floor area intended to provide sufficient dilution air to satisfy acceptable IAQ from a comfort perspective. The IAQ Procedure allows the user to reduce ventilation rates from the table by an analysis of contaminant sources, contaminant concentration targets, and perceived acceptability targets rather than the combination of occupant density and conditioned area (sq ft) factors.

Carbon dioxide (CO2) sensors are designed to measure concentrations and provide a control signal that with appropriate mathematics can approximate space population or population changes to the automated controls within a building's HVAC system. From this signal, intake rates per person can be estimated and the rates can be adjusted. If the differential CO2 levels exceed a predetermined setpoint, the ventilation is increased until they return to the proper range. The sensors are generally compatible with the controls used by most individual packaged rooftop or built-up air handling units. When the CO2 levels are within the needed range, the control system reduces the flow of outside air into the building, thereby minimizing ventilation during air conditioning and heating modes to lower energy consumption.

During periods of non-occupancy or low levels of human activity in commercial facilities, ventilation rates can be reduced. Sensors can monitor and control ventilation rates, meeting occupant safety requirements and minimizing energy consumption. In the summer cooling season for some geographical areas that do not have excessive humidity (mostly west of the Rockies), a great amount of free cooling can be realized by using a DCV scheme that takes advantage of cooler outdoor temperatures. Free cooling or air-side economizer operation is effectively allowing the air handler to supply 100% outdoor air. This assumes that the outdoor air is acceptable for use in this way and that the proper controls are in place to avoid humidity and pressurization problems.

A single setpoint CO2-DCV control system is not recommended for most applications. It is more effective in high occupant density spaces that are used intermittently or whose total population varies dramatically, such as would be the case in places of worship, sports arenas, school gymnasiums, large conference rooms, theaters, and lecture halls. On the other hand, offices, elementary school classrooms and other more constant (or less dense) occupancy areas are not likely to provide the energy savings expected and more likely to introduce multiple sources of control error with a high potential for over ventilation.

Where Is CO2 Generated Within a Building?

CO2 can come from the construction materials used in the building itself, but is primarily produced by the occupants as a result of their activities. This latter source is more dominant and has provided many designers with the opportunity to reset outside airflow rates in spaces with variable occupancy. Based on scientific chamber studies, the amount of differential CO2 is mathematically related to the intake air rate in CFM/person. It is called the steady-state equation, but is predicated on the existence of five conditions and knowing or assuming the value of five variables. These variables are occupant breathing rate, CO2 generation rate per person, CO2 concentration in exhaled breath, CO2 concentration in the space, and CO2 concentration in outdoor air.

By "maintaining a steady-state CO2 concentration in a space no greater than about 700 ppm above outdoor air levels will equate to about 15 CFM/person and indicate that a substantial majority of visitors entering a space will be satisfied with respect to human bioeffluents (body odor)[2]." This level varies not only with population but also with occupant activity level, diet, outdoor CO2 concentration changes, and so on.

The following table looks at both the CO2 generation and the outdoor airflow rate for individuals at various activities:

Activity

CO2 Generation Rate
(L/min per person)

Outdoor Airflow Rate
(CFM per person)

Sleeping

0.20

10

Office Work

0.30

15

Walking

0.55

28

Light Machine Work

0.60

30

Heavy Work

1.05

53

Required CO2 Settings

ASHRAE Standard 62.1 specifies minimum dilution ventilation rates for acceptable indoor air quality, not CO2 levels. In many cases, differential CO2 control methods may result in more ventilation (and expense) than is really required to comply with the standard for acceptable IAQ. CO2-based DCV is often implemented with little regard to the actual relationship between ventilation rates and CO2 levels. As a general practice, the actual settings for the CO2 levels are based on the equivalent dilution ventilation rates required by the IAQ standards for a given application. In the case of office buildings, the interior CO2 control level might be in the range of 1,000 ppm to about 1,500 ppm.

Sensor accuracy is another source of ventilation control error, with the better performing sensors publishing accuracies of ±75ppm. Zero drift, typically adds another ±20 to 50 ppm. Sensors are also sensitive to changes in temperature, humidity, and barometric pressure.[3]  Total uncertainty in control can be substantial. A deviation of ±75ppm under steady-state conditions can equate to ventilation control errors of -13% to +15% of setpoint, without consideration of any other source of ventilation rate error. There certainly is some variation as to what is an acceptable level of CO2 required for a specific intake rate.

Ebtron[2] presents useful information, and does not recommend CO2-based DCV for most applications. It illustrates the variability in determining CO2 levels for a typical office space in the following table:

Required CO2 Level at Various Population Densities in an Office Space
(Area = 1,000 ft²)
# People *Total OA CFM Required prior to 2004 Ci-Co Setpoint prior to 2004 *Total OA CFM Required after 2004 *CFM/person Required after 2004 Ci-Co Setpoint after 2004

7

140

548ppm

95

13.5

811ppm

6

120

548ppm

90

15

700ppm

5

100

548ppm

85

17

644ppm

3

60

548ppm

75

26

438ppm

* Per ASHRAE 62.1 (Table 6-1 Minimum Ventilation Rates in Breathing Zone)—In an office setting, the total outside air required = 20 CFM/person prior to 2004 and 0.06 CFM/ft² + 5 CFM/person after 2004.
** Ci = Indoor CO2 level, and Co = Outside CO2 level—related to the Steady-State formula C-1 (ASHRAE 62.1-2007, Informative Appendix C, “Rationale for Minimum Physiological Requirements for Respiration Air Based on CO2 Concentration”)

The table above shows the difficulty in correlating CFM requirements to a single, fixed CO2 setpoint in an office setting that has variable building occupancy. After Standard 62-2001’s Addendum N was adopted in 2003, any reduction in outside airflow rates due to decreasing occupancy became non-linear to CO2 concentrations and required multiple setpoints or intentional over ventilation to provide compliance with the minimum rates in the standard. A 57% reduction in the population (7 to 3 people per 1000 ft²) only results in a 21% decrease (95 to 75) in the required outside air.

Building Pressure

Negatively-pressurized buildings are vulnerable to mold growth, particularly in areas of high humidity. Positive building pressure must be maintained in most climates and regions in order to minimize the impact of molds that may be toxic to humans or damaging to the building structure. Positive pressurization flow within a building is maintained when the outside air intake flow rates exceed the exhaust flow rates. Therefore, designers must carefully consider building pressurization when using demand controlled strategies, because the amount of outside air introduced into a building can be limited by DCV, allowing the mechanical system to stimulate excessive infiltration without anyone knowing it.

Building pressurization control is also essential to maximize the efficiency of conditioning spaces for occupant comfort. This is an objective that generally gets more attention on a daily basis than mold growth in exterior walls. Providing reliable pressurization flow at minimum fan energy levels is a worthy goal of any air system design and its operation.

Even though the standard clearly does not specify acceptable CO2 levels for compliance, many believe that maintenance of space CO2 levels can result in acceptable indoor air quality. However, building designers must fully understand the parameters and assumptions regarding dilution ventilation rates, occupancy variations, and building pressurization when implementing any DCV strategy.


Sources:

[1] "CO2-based Demand Controlled Ventilation: Do Risks Outweigh Potential Rewards?", Dougan, D.S. and L.A. Damiano, Ebtron Inc, 2004. ASHRAE Journal, October 2004.

[2] "Ventilation for Acceptable Indoor Air Quality." ANSI/ASHRAE Standard 62.1-2007, and Informative Appendix C, “Rationale for Minimum Physiological Requirements for Respiration Air Based on CO2 Concentration,” p.31-32.

[3] National Building Controls Information Program (NBCIP), 2009. Product Testing Report, Wall Mounted Carbon Dioxide (CO2) Transmitters, Iowa Energy Center, Energy Resource Station, Ankeny, IA. Research funded by California Energy Commission and Iowa Energy Center, June 2009.
 

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