To temporarily isolate and drain off hot water from an indirect open vented storage vessel, which valve must be turned off?

To temporarily isolate and drain off hot water from an indirect open vented storage vessel, turning off the service valve from the cold water storage cistern is essential. This valve controls the flow of cold water into the storage vessel; closing it will effectively stop additional water from entering the system, allowing for safe drainage of the hot water already in the vessel.

This technique is important to prevent any potential overflow or spillage when draining the water, as it ensures that the water level is stabilized during the process. It also aids in maintaining system pressure and avoids any unnecessary complications with the hot water system's operation while maintenance or inspection is carried out.

Other valves mentioned, such as the main supply valve or the return valve to the heating system, do not directly control the flow from the cold water storage cistern that feeds the indirect vessel specifically for this situation. The pressure release valve is designed for safety purposes and to release excess pressure within the system, but it does not serve the purpose of isolating water from the cistern for drainage.