The Impact of Simplified Window and Exhaust Fan Assumptions on Model- Based Predictions of Inter-Zonal Air Flow and Contaminant Transport in Multifamily Buildings

In residential buildings, the indoor air quality can be significantly affected by ventilation measures initiated by occupants, including the operation of windows and in-unit exhaust fans in kitchens and bathrooms. The outcome of these measures can be highly variable and difficult to accurately characterize in building simulation frameworks. Consequently, many simulations simplify these factors by disregarding window opening behaviours and using fixed schedules for exhaust fan operation across all residential units. To determine if these simplifications are reasonable and estimate the magnitude of changes in air flow and contaminant transport, this study used coupled CONTAM and EnergyPlus models to simulate airflow, contaminant transport, and controls in multifamily dwellings. The coupled models parametrically varied climate zone, building airtightness, and mechanical ventilation system types. The study focused on two key occupant behaviours: (1) operating kitchen and bathroom exhausts on different schedules in individual dwelling units, and (2) scheduling open windows on ground and top floors. The findings show that the simplified assumptions regarding uniform in-unit exhaust fan operation and window operation had a minimal impact on inter-unit air flow and contaminant transport simulations across a broad range of building airtightness and mechanical ventilation system types. Staggering exhaust fan operation schedules had close to zero effect on average inter-unit air flow with maximum changes of about 1 L/s (2 cfm). For contaminant transport, the changes in concentrations were typically much less than 1%, compared to baseline assumptions. These findings suggest that for buildings with tight construction it is reasonable for most modelling and simulation efforts to ignore the effects of non-uniform exhaust fan operation and window opening.