Unvented DHW
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.
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Manufacturers:
Telford
Albion
Heatrae Sadia, (the market leader whose model Megaflow is used as a synonym for all other makes and models).
Santon.
Gledhill
Elston
IMI