The key difference is that combi boilers provide instant hot water. They have the advantage of freeing up space in a home, because there's no need for a hot water cylinder (tank) like there is with a system or regular boiler.  In most cases, heating water instantly is more energy-efficient than 'stored' hot water systems. However, the hot water flow is slower than if it was coming from a cylinder. This is why combi boilers are more suited to smaller properties, where demand for hot water is less. Note that some combi boiler systems do support heating water in a cylinder. Modern boilers generally no longer require tanks in the loft to 'pressurise' the system through gravity. Instead, they are sealed systems, and typically only require manual topping up from the mains water supply when the internal pressure has dropped (usually because of tiny leaks). This is a simple process that just involves opening a valve on a pipe below the boiler.