What does the term "measurable pressure difference" describe in the context of draft?

The term "measurable pressure difference" in the context of draft specifically refers to a phenomenon that is caused by the movement of combustion byproducts through a flue. When combustion occurs, it produces gases that are expelled through the venting system. This expulsion creates a pressure differential between the inside of the flue and the outside environment.

This pressure difference is essential for effective drafting, as it helps to ensure that the combustion gases are properly vented out of the appliance and away from the living space. The effectiveness of draft relies on this measurable pressure difference, which indicates that the exhaust gases are being drawn out efficiently, thereby allowing for safe and effective operation of combustion appliances.

Other options, while related to the overall context of combustion and venting, do not accurately capture the specific meaning of "measurable pressure difference." Temperature difference, unburned fuel, and airflow resistance are related factors but do not define the draft in the same way that the venting of combustion byproducts does.