Unvented boiler systems, often referred to as pressurized systems, are supplied directly from the mains water supply. Usually, you will have a large, flat-topped metal cylinder that heats and stores the water in your home.
Because the system is completely sealed, the mains water pressure will directly affect the flow of water through your showers and taps. As the water in the system is stored under constant pressure, a smaller pressure vessel will be situated on top of the cylinder.
Salamander have a number of pumps that can help improve your water pressure in an unvented system, including the CT Force and ESP range.