A combi (or combination) boiler provides hot water directly, whenever it is required, and does not need a hot water cylinder. Gas, oil and LPG boilers may be combination. A combi boiler frees up space in a home, because there’s no need for a hot water cylinder (tank) as there is with a conventional boiler. The flow of hot water may be slower than if it was coming from a cylinder so a bath will take longer to run – not too much of a sacrifice!!
Note: A regular boiler is more efficient than a combi at producing hot water, but some heat is inevitably lost from the hot water cylinder, so a combi may be more efficient overall.
Modern boilers generally no longer have 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 operation that takes a few seconds, and involves opening a valve on a pipe below the boiler.