What plumbing component helps to equalize pressure in drain lines?

The vent pipe is a crucial component in plumbing systems that helps to equalize pressure in drain lines. When wastewater flows through the drain pipes, it creates negative pressure, which can hinder drainage and lead to slow drains, water backups, or even siphoning of traps. The vent pipe allows air to enter the plumbing system, balancing the pressure inside the drain lines with atmospheric pressure. This air flow provides venting for sewer gases and helps maintain the proper functioning of the drainage system, ensuring that water can flow freely without obstruction and that traps remain filled with water to prevent the escape of sewer gases into the building.

In contrast, the drain trap is designed to prevent sewer gases from entering the living space by maintaining a water seal, but it does not directly address pressure equalization. The water heater, while essential for providing hot water, does not contribute to the pressure dynamics of drain lines. Similarly, an expansion tank is used primarily in systems with water heaters to accommodate the expansion of water when heated, and does not serve the function of pressure equalization in drainage systems. Thus, the vent pipe is the correct answer as it specifically serves to equalize pressure in drain lines.