We all know that when installing unvented hot water cylinders, it is important to ensure that all the safety cutouts and pressure relief systems are correctly specified, installed and maintained. Of course, if this is done, these systems are safe and provide superlative performance.
Those who doubt the importance of these safety systems and interlocks, should read:
Fleur de Lys sank because her hull was damaged by violent rupture of the hot water storage cylinder of the unvented hot water system. The water in the cylinder had overheated and generated steam. This resulted in overpressure which, in turn, led to it rupturing. The cylinder overheated probably because its immersion heater thermostat failed to shut off the electrical supply to the heater. There were no safety devices fitted to prevent overheating and overpressure in case of thermostat failure.
Christian.