Combination boilers (or more commonly “Combi”) is a “hot water on demand” appliance. It does not store hot water, it heats cold water via secondary plate heat exchanger to give you hot water at your preferred temperature. These boilers are more suited to smaller houses and apartments as hot water flow per minute is generally quite low.System and Heat-only (Regular or Standard) boilers work slightly different. These appliances will require hot water storage (cylinder or heat battery). Generally, system boilers are smaller in KW and more suited for larger properties.