Water heater relief valve discharge lines shall not _________________________________.

The requirement that water heater relief valve discharge lines must not be threaded at the outlet end is rooted in safety and code compliance. The purpose of a relief valve is to prevent excessive pressure buildup within the water heater, and it needs to operate effectively without the risk of leaks or obstruction.

Using threaded connections at the outlet can pose a risk because they are more prone to leaks due to thermal expansion and contraction from the heated water, leading to potential scalding hazards or water damage. Additionally, threads can allow for the accumulation of debris or mineral build-up, which could impede the proper functioning of the valve. Therefore, most plumbing codes specify that the discharge line should have a smooth, unthreaded outlet to ensure a clear and unobstructed flow, enhancing safety and reliability.

In contrast, fastening options like J-hooks are often acceptable as they provide support without compromising the integrity of the discharge line. Discharging into a sink may not always be advisable due to the possibility of back-siphonage or contamination, but it does not inherently conflict with the primary function of the discharge system.