What can happen when a plumbing system is not properly vented?

When a plumbing system is not properly vented, vacuum formation can occur within the pipes. Venting in a plumbing system is essential because it allows air to enter the system, which helps maintain atmospheric pressure. If venting is inadequate, the pressure can drop within the pipes, creating a vacuum effect. This vacuum can hinder the flow of water and waste through the drainage system, resulting in slow drainage.

It is important for proper venting to ensure that air can equalize the pressure within the pipes, allowing for smooth and efficient drainage. A well-vented system not only addresses the gravity flow of wastewater but also prevents potential problems such as trap siphoning, which can lead to odors escaping into living spaces. Therefore, the correct answer emphasizes the consequences of inadequate venting, specifically the formation of a vacuum that adversely affects drainage performance.