Unvented hot water cylinders are now the most popular option for new hot water systems and operate at the same mains pressure as the incoming cold water supply (typically 3 bar or so) and therefore provide excellent water flow rates without requiring an header tank in the loft space or additional pumps. In 'indirect' units the heat source is an external boiler that circulates hot water through a coil heat exchanger within the cylinder. In practice indirect units also have an electric immersion heater as a back-up in case of boiler failure. Indirect cylinders are generally more expensive to purchase and install but usually result in faster reheat times as much more heating power is normally available from a boiler than from electric immersion heaters.