Which of the following statements is true regarding the discharge of water heater relief valves?

The statement that relief valves for water heaters must be discharged positively into an open air location is correct because it is essential for safety and effective operation. Relief valves are designed to release excess pressure or temperature from the water heater to prevent potential failures or explosions. When they discharge into an open air location, it ensures that any released water or steam is directed away from the structure and any personnel nearby, minimizing the risk of scalding or damage.

Moreover, discharging to a safe location reduces the likelihood of creating an unsafe environment within the building and allows for proper dissipation of steam or hot water. This practice is aligned with the standards and safety codes prescribed for plumbing installations, ensuring that pressure relief systems operate effectively without creating hazards.

The other choices do not align with the safety protocols set forth in plumbing codes. Connecting relief valve discharge lines to washing machine drains could potentially block or reverse flow, creating a dangerous situation. Additionally, requiring threads at the outlet end contradicts the standard practice of ensuring that the discharge line remains clear and unobstructed for emergency release, as smooth piping is typically preferred to facilitate this function.