Does an overheat cut-out in an indirectly heated vented system have to be non-self-resetting in the same way as energy cut-outs do in directly-heated vented systems and in unvented systems?
No, sub-paragraph 3.13a simply requires that in a vented hot water storage system, “for all indirect heat sources, an overheat cut-out to disconnect the supply of heat to the storage vessel in the event of the stored water overheating” is incorporated.
This is because, for indirect heat sources on vented systems, water overheating is more likely to be for operational reasons, for example, on solar-powered systems because of a very hot day and little hot water use. In such circumstances, self-resetting reduces the inconvenience to users, by resuming normal operation as soon as the operational circumstances change, without compromising safety. However, for direct heat sources, for example, electric immersion heaters, overheating associated with such devices is likely to be due to a failure of the device and the use of a non-self-resetting device would draw attention to the problem.