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.
Highlights
The ventilation rate (VR) affects indoor environmental quality (IEQ) and the energy consumption of buildings
We present techniques presented in the literature, propose enhancements to the build-up method, extend the transient mass balance method, provide school grade- and age-specific emission rates needed for these methods, and discuss the effect of post-exercise recovery periods on CO2 generation
Because of the shut-down, decay air change rates will not apply to the occupied portion of the day. This applies to most U.S schools where shut-down may occur immediately following the last class, e.g., as early as 14:20 (2:20 p.m.). (24-h time notation is used throughout this paper.) Fourth, variable air volume (VAV) systems will provide less ventilation air if the thermal load diminishes after the space becomes unoccupied, which would have the effect of lowering the VR during the decay period even if the HVAC system is not shutdown
Summary
The ventilation rate (VR) affects indoor environmental quality (IEQ) and the energy consumption of buildings. Ventilation metrics, including the VR (m3 ·h−1 ), VR per person (L·s−1 ·person−1 ), the outdoor air change rate (A, h−1 ) and others [13], can be determined using air flow measurements [14], pulse or constant injections of tracer gases [15,16,17,18], occupant-generated carbon dioxide (CO2 ) as a “natural tracer” gas [19,20,21,22,23,24,25,26], and the comparison of indoor and outdoor concentrations, concentration trends, and sometimes temperatures [27,28]. The paper consolidates methods and experience in the literature, and provides guidance for applications in classrooms, a critical environment with known ventilation issues
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