An Unvented Cylinder is a hot water storage system that has a hot water storage cylinder that does not incorporate a vent pipe to atmosphere.

Balanced pressures at hot and cold taps for showers. Higher water pressures available at the hot taps.
Eliminates risk of contamination as there is no storage cistern. (tank in the loft).
the hot water storage cylinder can be sited anywhere in the house, are very suitable for one storey dwellings, e.g. flats and bungalows.
Quicker to install, no cold storage tanks in the loft, less pipework.
Small diameter pipework may possibly be used.  Gives architects and service designers greater flexibility of design.

Normally there would be no back-up of stored water should the main water be shut down for any reason.
If the mains water pressure is low then all water outlets are affected.
The possible need for long runs of safe discharge pipe with high temperature capacity.
The need for higher level of maintenance compared with conventional systems.
Possible noise problems due to velocity in small pipes.

Unvented Cylinders are covered by section G3 of the Building Regulations which came into force on November 11th 1985
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