What is the purpose of a water-seal trap in plumbing?

The primary purpose of a water-seal trap in plumbing is to block odors. A water-seal trap achieves this by creating a barrier of water that prevents sewer gases from escaping into living spaces. When water is held in the trap, it acts as a seal that stops these harmful and unpleasant gases from traveling through the plumbing system into the home. This is crucial for maintaining indoor air quality and ensuring a safe and comfortable environment.

In terms of the other options, while preventing backflow is an important function of certain plumbing components, it is not the specific purpose of a water-seal trap. Additionally, regulating pressure and measuring flow rate are tasks associated with different plumbing devices, such as pressure regulators and flow meters, respectively, rather than the primary function of a water-seal trap. Thus, blocking odors is the key role of this essential plumbing element.