Why must certain intervals in a drainage stack have relief vents?

Certain intervals in a drainage stack must have relief vents primarily to manage pressure increases and avoid seal failures within the plumbing system. When wastewater flows through a drainage stack, it can create a vacuum or positive pressure due to the movement of liquids and gases. Without proper ventilation, this pressure can lead to situations where traps may lose their water seals, which serve to prevent sewer gases from entering the living spaces.

Relief vents maintain a proper pressure balance within the system, allowing air to enter and equalize the pressure as liquids move through the pipes. This balance is crucial because it helps to ensure that the traps remain filled with water, thus providing a reliable barrier against harmful sewer gases. Maintaining this balance directly influences the functionality and effectiveness of the plumbing system, which is why having relief vents at certain intervals is a fundamental requirement in plumbing design.