A natural draft system removes flue or vent gases under what type of static vent pressure?

In a natural draft system, the removal of flue or vent gases occurs when there is a static vent pressure that is not positive, which is referred to as a nonpositive static pressure. This system relies on the natural buoyancy of the hot gases, which are lighter than the surrounding cooler air. As the hot gases rise, they create a negative pressure (relative to atmospheric pressure) within the venting system that helps draw in more air and expel these gases.

The concept of nonpositive static pressure in this context indicates that the pressure within the flue is either negative or very close to zero, allowing for the movement of gases without the necessity of mechanical assistance. Natural drafts depend on environmental conditions such as temperature differences and wind action to facilitate gas removal, rather than relying on fans or blowers that would create positive pressure.

Hence, the notion of negative or neutral pressure being more relevant in describing a natural draft system solidifies the understanding that these systems operate effectively under conditions that do not exceed atmospheric pressure, making a static pressure classification of nonpositive entirely appropriate for this application.