What should all portions of a mechanical draft system designed under positive pressure prevent?

In a mechanical draft system designed to operate under positive pressure, the primary concern is the prevention of leakage of flue gas. This is crucial because any leaks in the system can lead to flue gases escaping into areas where they can pose health hazards, such as carbon monoxide and other harmful emissions. Maintaining positive pressure helps ensure that the flue gases are contained and directed properly through the system, minimizing the risk of exposure to dangerous substances.

The design of such a system must also adhere to safety regulations and standards, which mandate that the entire mechanism operates efficiently and securely. An effective mechanical draft system will feature tightly sealed components to prevent any backflow or leakage that could allow flue gases to escape into the living or working environment.

While preventing air drafts, condensation, and heat loss are also important considerations in system performance and efficiency, the paramount objective in a positive pressure setup is to safeguard against the uncontrolled release of flue gas, making option C the correct answer.