In terms of safety, which is critical for the operation of natural draft venting systems?

Natural draft venting systems rely on the principle of buoyancy to create a flow of combustion gases from the appliance through the vent to the outside atmosphere. For this flow to be effective, it is essential that there is a difference in density between the ambient air and the flue gases. When the flue gases, which are hot and less dense, rise through the vent, they create a vacuum that draws additional combustion air from the appliance and allows the exhaust gases to exit safely.

This difference in density is crucial for proper venting because if the gases do not rise adequately, there is a risk of backdrafting, which can lead to dangerous situations such as carbon monoxide buildup inside the living space. Thus, ensuring that there is sufficient variation in these densities is a fundamental aspect of maintaining the safety and efficiency of natural draft systems.