Like a conventional hot water cylinder fed from a tank in the roof, an unvented cylinder contains hot water (heated by a boiler or immersion heater) which directly supplies the hot taps. Unlike a conventional cylinder, however, the water in the unvented cylinder comes directly from the cold water main and is at (nearly) mains pressure. To contain this pressure the cylinder has to be physically much stronger than in a gravity-fed system. Unvented cylinders are made of thick copper or stainless steel.
Water expands as it is heated. Since the outlet of the cylinder is to DHW taps which are normally closed, and the inlet is from the cold water main which may incorporate non-return (check) valves or other devices preventing expansion back into the supply pipe, measures have to be provided to accomodate the expansion of the hot water which could otherwise give rise to enormous pressure in the cylinder. These take the form of some type of container of gas which can be compressed as the water expands. This may be arranged as a bubble of air in the cylinder or a separate expansion vessel.
Safety of unvented systems
At normal atmospheric pressure water boils at 100C. At higher pressures the boiling point increases so that pressurised water can be heated to over 100C and remain liquid. However if the pressure is released it will rapidly turn to steam, expanding greatly and causing a steam explosion. In the days of steam the explosion of boilers or pipework in locomotives, shipping and industry was rightly feared for the destruction and loss of life it caused. If the water in an unvented hot water cylinder is allowed to rise above 100C then the reduction of pressure when a tap is opened could cause a steam explosion. For this reason unvented systems must have safety systems to control the temperature and pressure of the water, these systems must be checked annually, and their installation and servicing must be carried out by engineers with specific qualifications for these tasks.
The safety equipment would normally consist of the following:
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