Which fitting would be least appropriate for a high-pressure water line?

The reasoning behind plastic fittings being the least appropriate choice for a high-pressure water line lies in their material properties and structural integrity under pressure. Plastic fittings, while often lightweight and resistant to corrosion, can be less durable compared to metal counterparts when exposed to high pressure. They can deform, crack, or break under stress, especially with temperature fluctuations that can affect their rigidity.

In contrast, soldered fittings create a strong and permanent joint that is capable of withstanding high pressures without leaking, making them a suitable choice for robust plumbing systems. Compression fittings provide a reliable seal that can handle pressure well, while threaded fittings can also maintain integrity in high-pressure applications when designed correctly and employed with appropriate sealing methods. Therefore, for high-pressure applications, durability and strength are key factors, which plastic fittings may not fulfill effectively.