The impact of mechanical ventilation operation strategies on indoor CO 2 concentration and air exchange rates in residential buildings

Abstract

In Portugal, residential buildings commonly have their ventilation strategy changed after commissioning. This occurs due to the building managers' willingness to reduce shared costs with the electricity needed for fan operation. However, this option is not technically supported, and the effects of such a strategy on indoor air quality-related to human pollutants are yet to be quantified. CO₂ was monitored in 15 bedrooms and air exchange rates were calculated for each room. The air exchange rate values ranged from 0.18 to 0.53 h⁻¹ when mechanical extraction ventilation was off, and from 0.45 to 0.90 h⁻¹ when mechanical extraction ventilation was on, which represents an average increase of 119%. With the current intermittent ventilation strategy, all rooms remain above 1500 ppm for a given percentage of time, and 12 rooms presenting CO₂ concentrations above 2000 ppm. Simulations of theoretical CO₂ concentrations, for a non-interrupted mechanical ventilation strategy show that no rooms would accumulate CO₂ concentrations above 2000 ppm, and only 25% would present CO₂ concentrations above 1500 ppm. Pearson correlations between the monitored CO₂ and human and spatial factors identified two relevant parameters. Those parameters correspond to ratios between CO₂ generation and floor area ( $G_{{\mathrm{CO}}_2}/Ar$ ), and airflow with CO₂ generation ( ${Q/G}_{{\mathrm{CO}}_2}$ ). The proposed ratios could be used as ways to optimise ventilation costs and indoor air quality.