In which hot water system would you usually find an expansion relief valve?

An expansion relief valve is typically found in an unvented hot water system. The primary purpose of this valve is to safeguard against the pressure buildup that can occur due to thermal expansion in a closed system. In an unvented system, water is heated in a sealed cylinder, and as the temperature rises, the water expands. If this expansion is not managed, it can lead to dangerously high pressure, which could result in system damage or even failure.

The expansion relief valve operates by opening when the pressure exceeds a safe threshold, thereby releasing water to relieve the excess pressure and maintain safe operational conditions. This is crucial in ensuring the safety and efficiency of the hot water system.

In contrast, vented systems do not typically require expansion relief valves because they are open to the atmosphere and can accommodate water expansion by allowing excess pressure to dissipate through a vent. Combi boilers and gravity-fed systems also have different methods of pressure management and don't utilize expansion relief valves in the same manner as unvented systems. Therefore, the unvented system is the correct context where an expansion relief valve is essential for safe operation.