A case study on residential mixed-mode ventilation using the Ventilation Controls Virtual Test Bed

Mixed-mode ventilation uses intelligent switching between natural and (partly) mechanical ventilation modes to find the best possible balance between indoor air quality, user comfort and energy consumption. It applies demand-control at the level of the operating mode depending on the constraints imposed by the building, its users and its surroundings. Although mixed-mode ventilation is said to have the potential to achieve a comfortable and healthy indoor environment while achieving significant energy savings, it is rarely used in practice. Both academia and practitioners state that a lack of dedicated simulation tools, capable of modelling the inherent complexity of mixed-mode ventilation and its control algorithms, constitutes an obstacle. Also, case studies demonstrating the potential of mixed-mode ventilation in a residential context are scarce. A newly developed tool, VCVTB, is used to compare the performance of a generic mixed-mode ventilation system to a number of ventilation systems that are commonly used in the Belgian residential ventilation context. As key performance indicators, the unmet hours for operative temperature, relative humidity and CO2 level while people are present are used, as well as the energy demand for heating and the auxiliary energy use for fans. To test the robustness towards users all systems are tested by a semi-probabilistic virtual user panel. A low-energy house for which long-term measurement data is available constitutes a spatial boundary condition. The case study showcases the possibilities of VCVTB to design and compare advanced ventilation strategies. More importantly, it illustrates that mixed-mode ventilation may be a promising concept for residential ventilation within the Belgian context and in other climate areas with mild winters and cool summers. In the investigated building the main advantage of the mixed-mode strategy is not so much the reduction in auxiliary energy use for fans, but rather the outdoor connection during warmer months. Repeated opening of windows effectively reduces unmet hours by adopting adaptive comfort limits. Over an entire year, the windows are opened 30% of the time when using MMV. This can mainly be attributed to the opening of windows during the summer months. Finally, the case study shows that mixed-mode ventilation can provide a solution that is robust towards user behaviour.