A natural draft system is designed to remove flue or vent gases under _________ static vent pressure entirely by natural draft.

A natural draft system is designed to remove flue or vent gases under _________ static vent pressure entirely by natural draft.

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

Answer: (B)

Natural draft uses the buoyancy of hot flue gases to rise and exit the vent. For this to happen without any mechanical help, there must be no positive pressure inside the vent that would push back against the flow. A static vent pressure that is zero or negative allows the buoyant force to drive the exhaust upward and out entirely by natural draft. If the vent pressure were positive, that push-back would hinder the flow, and natural draft wouldn’t reliably remove the gases. So the design expects nonpositive static vent pressure.

Natural draft uses the buoyancy of hot flue gases to rise and exit the vent. For this to happen without any mechanical help, there must be no positive pressure inside the vent that would push back against the flow. A static vent pressure that is zero or negative allows the buoyant force to drive the exhaust upward and out entirely by natural draft. If the vent pressure were positive, that push-back would hinder the flow, and natural draft wouldn’t reliably remove the gases. So the design expects nonpositive static vent pressure.