What is a drawback of using outside air in mechanical ventilation systems?

Using outside air in mechanical ventilation systems can lead to unbalanced conditions within a building. When outside air is introduced into the building without proper control or balancing measures, it can disrupt the intended flow of air. This may result in areas of the building becoming pressurized or depressurized, depending on the volume and distribution of outside air brought in compared to the air being exhausted.

Unbalanced conditions can lead to various issues, such as difficulty in maintaining indoor air quality, increased energy consumption due to heating or cooling demands, and potential problems in controlling humidity. It is crucial for a mechanical ventilation system to ensure that the amount of outside air brought in is properly matched with the amount of air being exhausted to maintain equilibrium within the space.

In contrast, while the other options touch on potential issues related to ventilation, they do not specifically address the direct consequence of introducing outside air into the system. For example, excess heat might occur in some scenarios, but it is not specifically a direct drawback of using outside air. Similarly, the requirement for large ducts and problems related to air stagnation may arise from various factors in a ventilation design, rather than being inherent to the use of outside air itself.