To prevent stored hot water from an unvented system entering the cold water supply, which device should be installed?

To prevent stored hot water from an unvented system from entering the cold water supply, it is essential to use a single check valve. This device allows water to flow in one direction while preventing backflow, which is when water flows backward into the cold water supply from the hot water system.

In an unvented hot water system, maintaining the integrity of the water supply is vital for safety and hygiene. A single check valve ensures that any pressure or temperature changes in the hot water system do not force the hot water back into the cold supply lines, thereby protecting the cold water supply and preserving the quality of the water.

While a double check valve could also be appropriate in certain applications, specifically designed to provide a higher level of protection against backflow, the question specifically addresses the functionality of preventing backflow in a more straightforward application, which is served well by the single check valve.

Devices such as pressure reducing valves help manage pressure rather than preventing backflow, and isolation valves are used to shut off water flow for maintenance or repairs. Neither function addresses the specific need to prevent the backflow of hot water into the cold water supply effectively as the single check valve does.