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Indoor CO2 concentration is considered a metric of indoor air quality that affects the health of occupants. In this study, a hybrid model was developed for forecasting the varying indoor CO2 concentration levels in a residential apartment unit in the presence of occupants by controlling the ventilation rates of a heat recovery ventilator. In this model, the mass balance equation for a single zone as a white-box model was combined with a Bayesian neural network (BNN) as a black box model. During the learning process of the hybrid model, the BNN estimated an aggregated unknown ventilation rate and transferred the estimation to the mass-balance equation. A parametric study was conducted by changing the prediction horizons of the hybrid model from 5 to 15 min, and the forecasting performance of the hybrid model was compared with the stand-alone mass balance equation. The hybrid model showed better forecasting performance than that of the mass balance equation on the experimental dataset for a living room and bedroom. The average MBE and CVRMSE of the hybrid model for the prediction horizon of 15 min were 0.65% and 5.23%, respectively, whereas those of the mass balance equation were 0.99% and 9.30%, respectively. [ABSTRACT FROM AUTHOR]

This study introduces a hybrid model that combines physics and machine learning (ML) models to describe the behaviour of variable refrigerant flow (VRF) systems. The standalone ML model was developed with identical data and conditions for comparison between the hybrid and ML models. A Bayesian neural network (BNN) was used for both the models, and the predictive abilities and uncertainties were investigated. For the experimental dataset, the predictive performances of both models were similar. For example, the predictive performance of the hybrid and ML models showed mean absolute error of 0.73 and 0.78 kW, respectively. However, the epistemic uncertainty of the hybrid model quantified using the BNN was 36.4% lower than that of the ML model. A parametric study showed that the hybrid model combined with the physics model can achieve better generalization performance than the ML model, yielding results that are more reliable and physically explainable. [ABSTRACT FROM AUTHOR]