With an unvented water system, both hot and cold water in the house are delivered from the taps/showers at mains pressure. This is achieved by plumbing mains cold water directly into the cylinder where it is heated by an immersion or other suitable heating system. When a hot tap is turned on the mains cold water pushes the hot water out of the cylinder. To accommodate for the expansion of this heated water, an internal or external expansion vessel is installed together with a pressure relief valve in case of vessel failure.

This type of set up is ideal for large houses where multiple showers/baths are required. Fast filling baths will be achieved as well as good showers all through the property.