A combination (combi) boiler is a water heater and a central heating boiler in a single compact unit. Combi boilers work directly from the mains when you turn on a tap, and don’t need a storage cylinder to store hot water or a tank in the roof to provide the cold water supply This makes them simple to install and can often free up space in the home if they replace an older system.
Combi boilers are energy-efficient because water is heated instantly rather than being heated and then stored. Combi boiler are particularly suitable for smaller properties where there is little room for a hot water cylinder and no loft space for a cold water tank and can free up space in a larger property for a loft conversion.
System boilers need a cylinder for storing hot water, but, like Combi boilers, can be fed directly the mains supply. The primary reason for opting for a system boiler, and keeping a supply of hot water is that the demand for hot water in the house surpasses (often through multiple bathrooms) what can be generated on demand by a reasonably-sized Combi boiler.
As central heating systems with system boilers have a hot water cylinders these can also be used to store water heated by solar panels or heated (expensively) with an electric immersion heater.
Regular boilers (sometimes known as traditional, conventional or heat only boilers) are ideally suited to homes that already have a traditional heating and hot water system which is linked to a separate hot water cylinder. These boilers also need a cold water storage tank in the loft to feed the hot water cylinder as well as a tank that maintains the water level of the central heating system.
A regular boiler may be the best option for replacing an existing boiler if the property has an older radiator system, as it might not be able to cope with the higher water pressure that is delivered by system or combi boilers.
Regular boilers are best suited as replacements for homes with existing, traditional heating and hot water systems with separate hot water cylinder and cold water tank in the roof and expansion tank where there is no desire to replace these. This may be the best solution if the property has an older radiator system that may not be able to support the higher pressures of the modern Combi or System boiler.