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:

http://www.dft.gov.uk/stellent/group...ety_504222.pdf

(Summary follows)

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.