What does the term "mixed-mode ventilation" refer to?

The term "mixed-mode ventilation" refers to the combination of natural and mechanical ventilation systems in a building. This approach allows for flexibility in how fresh air is supplied and managed, optimizing indoor air quality while reducing energy consumption.

In practice, mixed-mode ventilation might allow natural ventilation to be utilized—such as opening windows or vents—when outdoor air conditions are favorable, while also incorporating mechanical systems, like HVAC units, to meet air quality requirements when natural ventilation is insufficient or impractical. This hybrid method is particularly beneficial in climates where temperature and humidity levels vary significantly, allowing the building to maintain comfort and efficiency year-round.

The other options describe different ventilation methods or systems, but they do not capture the essence of mixed-mode ventilation. For example, ventilation only during night hours is a specific strategy but does not encompass the combination of natural and mechanical systems. Using portable air filters is a separate approach focused more on filtering air rather than on how it enters a space. Full dependence on HVAC systems implies no use of natural ventilation at all, which does not align with the definition of mixed-mode systems. Thus, the combination aspect of natural and mechanical ventilation is what makes the correct answer clear.