Reducing the amount of outdoor air entering a space when an HVAC system is mechanically cooling or heating and an air-side economizer is inactive has distinct advantages. For some spaces, demand-controlled ventilation (DCV) based on changes in occupancy may be desirable. According to one method of DCV, ventilation rate is adjusted based on carbon-dioxide (CO₂) level. Unfortunately, many designers do not have a clear understanding of the relationship between CO₂ and ventilation and the requirements for proper ventilation control. This article will clarify the relationship between CO₂ level and ventilation rate and offer suggestions for improving traditional single-setpoint CO₂ DCV through the addition of a direct outdoor-airflow-measuring device.

Application of DCV
Generally, DCV should be considered only for high-occupant-density spaces with potential for significant changes in population density. Most standards and rating systems (American Society of Heating, Refrigerating and Air-Conditioning Engineers [ASHRAE]; Title 24, Part 6, of the California Code of Regulations; Leadership in Energy and Environmental Design [LEED]) consider high occupant density to be 25 or more people per 1,000 sq ft. DCV should not be considered for low-occupant-density spaces because the uncertainty of most DCV techniques is too great and the potential benefit too meager for practical application.

CO₂ Level and Ventilation
Indoor CO₂ level is used to estimate outdoor-airflow rate per person based on a steady-state relationship (Figure 1), which can be applied to single-zone systems.

Mass-balance equation:

(In = Out)

Co × Vo + N × P = Ci × Vo

where:
Co = Outdoor-air CO₂ concentration, standard cubic feet per minute of CO₂ per cubic feet per minute of air
Vo = Outdoor-airflow rate, standard cubic feet per minute of air
N = CO₂-generation rate, standard cubic feet per minute of CO₂ per person
P = Number of people
Ci = Inside CO₂ concentration, standard cubic feet per minute of CO₂ per cubic feet per minute of air

This simplifies to:

N ÷ (Ci − Co) = Vo ÷ P = R

where:
R = Rate, standard cubic feet per minute of air per person

The model's ability to predict source airflow rate per person is dependent on CO₂-production rate, which can vary significantly with activity and age. A typical adult produces 0.0084 scfm of CO₂ per met; for office work, the activity level of an adult is approximately 1.2.¹ Thus, for an average adult with an activity level associated with office work:

R = 0.010080 ÷ (Ci − Co)

Because CO₂ sensors report CO₂-concentration levels in parts per million, the equation is more useful written as follows:

R = 10,080 ÷ (Ci* − Co*)

where:
Ci* = Inside CO₂ concentration, parts per million
Co* = Outdoor-air CO₂ concentration, parts per million

With single-zone systems, a 700-ppm rise in indoor CO₂ concentration results in an outdoor-airflow rate of approximately 15 cfm per person. This approximates the minimum ventilation rate established to dilute body odor. Also, it relates to historical references (including ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality) to a 700-ppm differential as an indicator of acceptable ventilation.

Because outdoor CO₂ level typically hovers around 400 ppm, an indoor CO₂ level of 1,100 ppm would suggest the body odor generated in a space would be maintained at acceptable levels.

ANSI/ASHRAE Standard 62.1-2010-Compliant CO₂-Based DCV
The ventilation-rate procedure in Section 6 of ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, has not changed significantly since 2003, when breathing-zone outdoor-air requirements (Section 6.2.2.1) were adjusted to include a floor-area component reflecting the dilution needed for building-related contaminant sources. Today, breathing-zone outdoor-airflow rate, Vbz, is calculated based on table values, as follows:

Vbz = Rp × Pz + Ra × Az

where:
Rp = Outdoor-air rate per person
Pz = Number of persons
Ra = Outdoor-air rate per floor area
Az = Floor area

Traditional fixed-CO2-setpoint DCV. As a result of changes to ANSI/ASHRAE Standard 62.1 in 2003, a fixed CO₂ setpoint no longer will result in specified ventilation rates, unless the setpoint is calculated at the lowest expected occupancy (i.e., when ventilation rate is highest).

The relationship between space CO₂ level and outdoor-airflow rate per person for a 1,000-sq-ft classroom with a variable occupancy of up to 35 students is illustrated in Figure 2.

The relationship between outdoor airflow and the CO₂ level required for compliance with ANSI/ASHRAE Standard 62.1-2010 is a variable and illustrated in Figure 3.

A single CO₂ setpoint provides required ventilation for a single population. Excessive population results in a ventilation rate greater than that required by ANSI/ASHRAE Standard 62.1-2010, while deficient population results in a ventilation rate less than that required by ANSI/ASHRAE Standard 62.1-2010. Consider, for example, a CO₂ setpoint of 1,000 ppm, which equates to a ventilation rate of approximately 17 cfm per person under the assumptions of steady-state, office-level activity and an outdoor CO2 level of 400 ppm. Figure 4 shows the relationship between ventilation and the requirements of ANSI/ASHRAE Standard 62.1-2010 when a 1,000-ppm CO₂ setpoint is maintained. Note that this method will overventilate when population exceeds 15 people and underventilate when population falls below 15 people. Additionally, the CO₂ response line will rotate around the origin (0,0) if activity level differs from that assumed. Activity-level variations can affect the outdoor-airflow-intake rate of CO₂-based DCV systems significantly (typically, resulting in overventilation). The response also will shift if indoor or outdoor CO₂ level is inaccurate.

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