In normal venting conditions, the pressure in the vent is characterized as:

In normal venting conditions, the pressure in the vent is typically more negative than the pressure in the combustion appliance zone (CAZ). This negative pressure differential is essential for ensuring that combustion gases are effectively drawn out of the appliance and vented safely to the outdoors. A negative pressure in the vent helps promote the flow of combustion products away from the appliance, thereby preventing the backflow of gases into the CAZ and ensuring safety for occupants.

This pressure difference is crucial for maintaining proper venting functionality. A positively pressurized vent could lead to the accumulation of hazardous gases within the living space, which would pose serious safety risks. Understanding the relationship between vent pressure and CAZ pressure is fundamental for ensuring that combustion appliances operate safely and efficiently.