In a natural draft ventilation system, the static vent pressure must be under what condition?

• A. Positive
• B. Nonpositive
• C. Negative
• D. Zero

Answer: (B)

In natural draft ventilation, the goal is to rely on buoyancy to move combustion gases up and out of the building. The static pressure inside the vent should not exceed ambient atmospheric pressure. Keeping it nonpositive (zero or negative) ensures there is no push from inside the vent toward the living spaces, which helps prevent backdrafting of exhaust gases into the building and keeps the natural draft working safely and reliably. If the vent pressure were positive, it could force exhaust back into the space or hinder the upward flow, especially when wind or other loads interact with the stack. So the best condition is nonpositive.