Where is a full-open valve not allowed to be installed in the water heater piping?

A full-open valve is not permitted in the circulation return line of a water heater system because it can create a significant risk of water stagnation and potentially cause damage to the system or affect water quality. In a circulation system, the return line is designed to allow hot water to flow back from the fixtures to the water heater. If a full-open valve is installed in this line, it may prevent proper circulation, leading to areas of stagnant water, which can promote bacterial growth, reduce system efficiency, and create conditions that could harm the plumbing system.

Conversely, in the supply line, service line, and discharge line, the use of valves serves specific purposes, such as controlling flow and servicing equipment. It’s important to maintain good circulation within the system to ensure consistent water temperature and to prevent potential health hazards associated with stagnant water. This is why the installation of a full-open valve in the circulation return line is explicitly restricted.