What is the term for the pressure difference that causes a continuous flow of air and products of combustion through the gas passages of the appliance to the atmosphere?

The term that describes the pressure difference responsible for the continuous flow of air and combustion products through the gas passages of an appliance to the atmosphere is known as "draft." Draft is typically generated as a result of the difference in temperature between the hot gases produced during combustion and the cooler air outside, creating a pressure difference that allows air to flow into the appliance and exhaust to exit.

In the context of combustion appliances, maintaining an adequate draft is essential for efficient operation. It ensures that the necessary amount of air enters for combustion and that harmful exhaust gases are effectively expelled, preventing backdrafts or the release of dangerous fumes into the living space.

Ventilation refers to the process of supplying fresh air to replace contaminated air but does not specifically address the movement of combustion gases. Pressure alone doesn't specify the cause or result of the movement of air and gases. Exhaust typically refers to the gases that are expelled from the appliance rather than the pressure differential that causes those gases to flow. Therefore, draft is the most precise term for describing the phenomenon in question.