What method of venting protects two fixtures from both positive and negative pressure?

The method of venting that effectively protects two fixtures from both positive and negative pressure is dual venting. This technique involves using two vents that work together to accommodate the pressure changes within the drainage system. When one fixture is in use, the other vent helps balance the pressures, preventing the risk of siphoning or backups.

In dual venting, the design allows for the venting of multiple fixtures simultaneously, rather than relying on a single vent that might not be adequate to handle fluctuations in pressure. This setup is particularly advantageous in situations where multiple fixtures are used closely together, ensuring that the overall plumbing system maintains proper airflow and prevents issues such as water seal breakage in traps due to negative pressure or pushback during positive pressure events.

The other venting methods do not specifically provide this dual protection. Single venting may not suffice for multiple fixtures because it can create situations where pressure imbalances occur, leading to potential drainage problems. Direct venting typically refers to a system that vents combustion gases from appliances rather than managing drainage air. Indirect venting focuses on allowing drainage to discharge into a vent while covering specific needs without fully addressing the potential for pressure changes across multiple fixtures. Thus, dual venting is specifically designed for protecting multiple fixtures from both types of