What does a negative air pressure indicate in a natural draft system?

A negative air pressure in a natural draft system indicates that the system is functioning correctly under specific conditions. In an efficient natural draft system, negative air pressure helps facilitate the flow of combustion gases out of the appliance and into the venting system. This creates a draft that allows fresh air to enter the combustion chamber, which is necessary for proper combustion.

When the pressure is negative, it signifies that air is being drawn into the system rather than pushing gases out, maintaining a proper combustion process. This is vital for safety and efficiency, ensuring that exhaust gases are vented outside instead of leaking back into the living space.

In contrast, if the system is not functioning properly—for example, if it is over-ventilated—it might exhibit positive air pressure or improper venting conditions, indicating that there are issues that require attention, such as blockages or excess airflow that disrupts the natural draft.