Measuring Air Change Rates by CO2 Decay




A method and its limitations


The three preceding posts have been about minimum safe levels of air change, types of indoor air pollutants, and ways of estimating air change rates. The last reference was to a method based on monitoring the decay of indoor CO2 levels in a room or building [1]. This method and its results will be examined here in more detail. The concept is straightforward: CO2 is produced by the occupants of a space over a suitable time, and then they leave. Normal ventilation is maintained, and a monitor records the subsequent levels of CO2 in the space and in the incoming air. These data allow calculation of the air changes per hour (ACH).

The exact value of the initial level of CO2 is unimportant. Under a constant level of ventilation, the rate of removal of CO2 will be proportional to its concentration in the space. This defines an exponential decay, tending to equilibrium when CO2 concentration in the space matches that in the incoming air. Monitoring must continue for long enough to establish the time constant of the decay, and illustrative results from another study will be given below.

Two assumptions will be examined; that vacation of the building removes all internal sources of CO2, and that the ventilation rate is constant.

While it will normally be easy to ensure that there are no people or pets in a building, and no active sources of CO2 such as burning fuel, at least one potential source is not easily removed. Soil is both a sink and a source of CO2, and emissions of this and other gases from soil have been the subject of study [2]. There are reports in grey literature of CO2 from soil accumulating in buildings under some conditions (e.g. in houses on former farm land) and this might conceivably affect the measurement method [3].

The variation of air change rate with external conditions has been investigated by Tirfe [4]. The method described also used CO2 decay, but in an empty building. CO2 was released into the building from a cylinder operated by solenoid from a microcontroller, with a fan used to aid mixing. A CO2 monitor was connected to the microcontroller, which turned off the CO2 supply when, after a short time, the preset level was reached. The decaying CO2 levels were then logged, and the microcontroller switched the supply on again when a preset lower limit was reached. Thus successive decays between fixed concentrations were recorded. The decay rates differ with external conditions, which were also monitored.

The building used was a test laboratory with a volume of about 800 m3. During a test session, the internal CO2 level decayed from 1100 ppm to 365 ppm over 24 hours, during which external CO2 fluctuated between 345 to 365 ppm. The initial rate of decay (p.60) indicated a time constant of about 5 hours. In cyclic tests the preset levels used were 1200 and 550 ppm, allowing four cycles to be completed in 30 hours. The times taken for each cycle differed considerably, the shortest taking about 200 minutes, and the longest about 800 minutes (p.86). The reasons for variation are related in the analysis to temperature, wind speed and wind direction, which are documented. The ACH figures over the test period ranged from approximately 0.05 to 0.25.

The relative cheapness of CO2 monitors of reasonable accuracy may encourage attempts to measure the ACH of buildings. If results are viewed in the light of the variability described above, it might be concluded that an ACH figure obtained in conditions of still air and little difference between internal and external temperatures could provide a useful guide to a minimum or base level ACH figure. Considerably higher figures would be expected as wind speeds and temperature differentials increase. The theoretical models of building infiltration discussed by Tirfe [4] might help to predict these higher values.

References are all open access:

[1] Simple and Cheap Air Change Rate Measurement Using CO2 Concentration Decays by Roulet & Foradini (2002)

[2] Greenhouse gas emissions from soils—A review
Cornelius Oertel et al (2016)

[3] Influence of Soil Moisture on Soil Gas Vapor Concentration for Vapor Intrusion
Rui Shen et al., (2013)

[4]  A Novel Approach to Near-Real Time Monitoring of Ventilation Rate and Indoor Air Quality in Residential Houses

Tirfe, A.K. (2017)



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