Unvented cylinders must be installed (and repaired and serviced) by G3 qualified and registered installers.
Unvented cylinders and calorifiers sold in this country are perfectly safe because they have to meet some of the highest design standards in the world. Continental standards on this are much less safe!
An Unvented cylinder is a pressure vessel and as such carries the danger that all pressure vessels carry so they are fitted with a number of safety devices.
The greatest potential danger of this system is that the control and high limit stats could fail and the water temperature could rise to above 100 degrees C.
The pics above are typical Temperature and Pressure Relief Valves which are pre-set to open at 7 bar (101.5 psi) OR 90ºC.
If the temperature exceeds 90c it opens and the hot water is released to waste. As a result of this, cold water enters the vessel and cools the water down preventing the water boiling. It continues to release water to a tell tale tundish thus admitting cold water into the cylinder continuously until the temperature falls below 90 degrees C or/and if the heat source is removed.
An Unvented hot water cylinder also has to have an expansion vessel fitted to allow for the water expansion that occurs when heated. This vessel will be pressure tested and certified to 15bar and batch tested to 30 bar.
To prevent expansion vessel failure there are other safety valves fitted.
If the water overheated and turned to steam, the vessel could rupture, and the rapid expansion would cause an explosion. To avoid this happening a pressure reducing valve (pic below) is fitted to the inlet water to limit the pressure to about 3 bar. If both the vessel and Expansion relief valve fails,( usually part of the inlet valve assembly) the valve opens and the pressure is released to outside.
More pics HERE
There are two further safety devices which prevent the water in the cylinder ever getting to near boiling point. One is a thermostat additional to the normal stat on the electrical heater, and the other is a valve to shut off the hot water from the boiler should it get too hot.
To date there has not been a case of an unvented cylinder exploding in this country. However, there was an off shore explosion on the 16th of April 2000 aboard a fishing vessel call the Fleur De Lys.
http://www.maib.gov.uk/cms_resources/fleur-de-lys.pdf
It was found that the cylinder was fitted without the safety devices and the cylinder shell was of inferior quality.
