The traditional 'vented' hot water cylinder operates with a header tank and generates lower water pressure than an unvented cylinder. In 'direct' units the water is heated by one or more electric immersion heaters. 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. Direct (all-electric) units come into their own when a gas/oil boiler is either not available or too expensive to install, or in situations where the requirement for annual gas safety checks of appliances, flues and pipework is considered onerous, for example at rental properties.