Review and Extension of CO2-Based Methods to Determine Ventilation Rates with Application to School Classrooms

Abstract

The ventilation rate (VR) is a key parameter affecting indoor environmental quality (IEQ) and the energy consumption of buildings. This paper reviews the use of CO2 as a “natural” tracer gas for estimating VRs, focusing on applications in school classrooms. It provides details and guidance for the steady-state, build-up, decay and transient mass balance methods. An extension to the build-up method and an analysis of the post-exercise recovery period that can increase CO2 generation rates are presented. Measurements in four mechanically-ventilated school buildings demonstrate the methods and highlight issues affecting their applicability. VRs during the school day fell below recommended minimum levels, and VRs during evening and early morning were on the order of 0.1 h−1, reflecting shutdown of the ventilation systems. The transient mass balance method was the most flexible and advantageous method given the low air change rates and dynamic occupancy patterns observed in the classrooms. While the extension to the build-up method improved stability and consistency, the accuracy of this and the steady-state method may be limited. Decay-based methods did not reflect the VR during the school day due to heating, ventilation and air conditioning (HVAC) system shutdown. Since the number of occupants in classrooms changes over the day, the VR expressed on a per person basis (e.g., L·s−1·person−1) depends on the occupancy metric. If occupancy measurements can be obtained, then the transient mass balance method likely will provide the most consistent and accurate results among the CO2-based methods. Improved VR measurements can benefit many applications, including research examining the linkage between ventilation and health.

VOC sources and exposures in nail salons: a pilot study in Michigan, USA

“…Ventilation and air change rates (ACRs) were determined using CO 2 as a “natural” tracer gas, the steady-state mass balance model, field-measured CO 2 concentrations (20-min average from 5-sec measurements), observed occupancy (20-min average), measured salon volume, and CO2 emission rates for adult women (Batterman 2017). While the derived ACRs are approximate due to possible changes in occupancy, accuracy of the steady- state assumption, and the representativeness of measurements, ventilation parameters can provide key information to interpret the significance of emission sources and to support engineering controls to reduce exposure.…”

Ventilation rates in recently constructed U.S. school classrooms

“…VRs were determined in each classroom using CO 2 as a tracer gas, classroom‐specific parameters (volume, grade‐level and occupancy), and four methods detailed elsewhere . The first used the “steady‐state” method, the maximum CO 2 concentration (assumed to be the steady‐state level) over the school day, the room volume, and the CO 2 generation rate for the 2 hour prior to the CO 2 peak.…”

“…Actual start and stop times were allowed to vary by ± 1 hour so as to maximize the concentration change over the period. The steady‐state concentration required by this method was determined using both the midpoint method and an implicit method that numerically solves the build‐up and steady‐state equations simultaneously . The implicit method improves stability and better addresses variable occupancy and non‐ideal shapes of the CO 2 build‐up curve.…”

“…VRs were determined in each classroom using CO 2 as a tracer gas, classroom-specific parameters (volume, grade-level and occupancy), and four methods detailed elsewhere. 27 The first used the "steadystate" method, [28][29][30] the maximum CO 2 concentration (assumed to be the steady-state level) over the school day, the room volume, and the CO 2 generation rate for the 2 hour prior to the CO 2 peak. For children, grade level-specific generation rates were based on the Dubois equation, 28 height and weight data from U.S. representative growth charts, and an activity level of 1.4 MET.…”

Building and indoor environmental quality assessment of Nigerian primary schools: A pilot study

“…Natural ventilation usually has a low investment and operational costs, while mechanical ventilation systems are more expensive and consume energy . Although there are several guidelines and standards by reputable organizations such as WHO (World Health Organization) and ASHRAE (American Society of Heating, Refrigerating and Air‐Condition Engineers), as well as national building codes, these standards are not always met: many studies from different countries have shown ventilation to be inadequate in a large proportion of classrooms . The reasons could be related to overcrowding in classrooms, as well as the type and condition of the school building and the ventilation system …”