Unvented cylinders heat water, sourced from the mains supply, with immersion heaters to a pre-set temperature. As the water heats, pressure is exerted on the cylinder which in turn provides the force to pump the heated water around the home. Unvented systems are sealed which means they also need an expansion vessel attached or next to the cylinder or boiler. This vessel accommodates the expansion of water as it heats. When fitted and maintained correctly these systems are very safe and an efficient system for household use.